Men have been killed by very low voltage circuits. As little as 12 volts may be fatal under the proper combination of circumstances.
Haste, heedlessness, attempts to work where there are distracting noises, and attempts to carry on a conversation while servicing lead to material and personnel casualties.
USE THE PROPER METER
Test instruments are delicate and easily damaged. Connect voltmeters across the circuit, in parallel. Connect ammeters in the circuit, in series. Never use an ohmmeter until the power is off.
CHECK SYSTEMATICALLY AND NOT BY HUNCHES
Time is saved in the end.
MANY A RECEIVER HAS BEEN DISMANTLED NEEDLESSLY
Examine the wiring diagram and consult the notes in the instruction pamphlets. Check the easy and obvious things first. Some of these are:
(1) (a) Poor antenna or ground connection.
(b) Loose parts, contacts or connections.
(c) Burned out tube.
(d) Surface defect, such as , dirty band switch points.
(e) Broken grid cap lead, or other broken lead or cable.
(f) Odor, indicating overheated part.
(2) (a) If no tubes light, check power supply to the receiver.
(b) If one or two tubes do not light, check these tubes, and check the filament voltage on the affected sockets before plugging in new tubes.
(c) If the receiver tubes burn out with unusual rapidity, check the voltage the power supply delivers to the filaments or heaters with the tubes in place.
(3) Tune through the various frequency bands. Only one coil or band switch section may be defective.
While checking' through for the obvious defects consider the answers to the following questions:
(1) What happened?
(2) Did the trouble develop suddenly or did it come on gradually?
(3) Has the same trouble been experienced before?
If there is "gradual fading" or "intermittent operation":
(4) Can operation be restored by striking the cabinet, operating the switches, or other unusual method?
(5) When the signal fades out and then comes back, can you hear a click?
(6) Does the signal die out, fade gradually, or does it cut off sharply ?
(7) How long is it before reception becomes possible again? If the receiver is noisy:
(8) How long has the noise been noticed?
(9) Is it intermittent or continuous?
These questions and their answers lead to definite clues to the trouble being encountered, as will be seen later on.
Vacuum tubes should be checked with a tube tester as part of the preliminary routine. In an emergency, it may be desirable to substitute a new tube after the filament or heater voltage has been checked at the socket and found to be normal.
The supply of spare parts is limited. Random substitution of spares without a thorough check may result in the destruction of all spares without the equipment being placed back in service.
IMPORTANT.-If the instruction books do not tabulate the normal operating voltages and currents for all tubes in all receivers on hand, this data should be taken on normally operating equipment at the earliest opportunity. The time spent will be found to be well invested.
The basic principle that underlies the operation of most electric measuring instruments used in radio work is the fundamental law of magnetism which states that like poles repel and unlike poles attract each other. The fundamental meter consists of a permanent magnet between whose poles is placed an electromagnet. Attached to the movable electromagnet is a needle or pointer which moves over a graduated scale when the electromagnet is energized by a current flowing through its coil. The basic meter is thus seen to be a current measuring device, since the interaction of the two magnetic fields of the meter will be
proportional to the amount of current flowing through the coil of the electromagnet.
The basic meter is called an ammeter or a milliameter according to the range of currents measured. A shunt, or resistance in parallel, is used to change the range of a meter. The current divides and only a part of it flows through the meter itself. By proper design a wide variety of scales may be provided. In calibrating an ammeter, a known current is allowed to flow through it and the scale is marked off accordingly.
Similarly, the basic meter can be converted into a voltmeter by placing a resistor in series with it. This series resistor is frequently called a multiplier. Since the resistance of the meter and multiplier will be constant, the amount of current flowing through the combination will be proportional to the voltage applied to the device. In calibrating a voltmeter, known voltages are applied to the device and the scale is marked off accordingly.
Voltmeters and ammeters constructed along the lines indicated above are only useful for direct current measurements. Other types of meters, such as the movable iron and hot wire types, can be used for measuring alternating currents. However, for most radio servicing work, the direct current type is used in connection with a copper oxide rectifier. The rectifier is used to convert the alternating current into pulsating direct current. Such a meter will actually register the average value of the alternating current or voltage, but the scales are calibrated to indicate the effective value, thus avoiding the necessity of conversion.
An ohmmeter is a device for measuring resistance. In its simplest form it consists of a basic meter connected in series with a resistance and a small battery. In adjusting such a device, the terminals are short circuited, and the resistance is adjusted so that the current drawn causes the meter to read full scale. If a resistance is connected across the terminals, the meter will deflect, and it can be calibrated to correspond to the value of the resistor in ohms.
It is impossible to do more than briefly touch on the fundamentals of the basic meters used. For a complete understanding of test instruments a good text book should be consulted.
Meters are rated according to the sensitivity of their movements. In the case of ammeters, the sensitivity is indicated by stating the amount of current required for full scale deflection, that is, a "1 milliampere" meter will require 1 ma of current to read full scale. Voltmeters are usually rated in terms of "ohms per volt", that is, the total resistance of the meter divided by the maximum voltage indication marked on the scale for which the resistance is specified. A common rating for ordinary voltmeters used in radio servicing is "1000 ohms per volt".
USING TEST INSTRUMENTS
Ammeters and milliammeters are always connected in series with the circuit being measured and never across it or in parallel with the voltage source. See Figure 5. Ammeters have relatively low resistance and if they are connected across the circuit, a very heavy current will flow that will destroy the meter.
Voltmeters are always connected in parallel with the voltage source. Since they have a high resistance, they may be connected across the circuit, as shown in Figure 6.
An ohmmeter must never be connected into a live circuit. Great care must be taken to see that all voltage sources have been removed from the equipment under test before using an ohmmeter.
In radio servicing work, the voltages applied to a circuit under actual working conditions are of primary importance. Since most radio circuits include high resistances, any voltmeter connected in must have a high resistance if it is not to draw appreciable current
and thus influence the circuit voltages. Figure 7 illustrates graphically the errors in testing that may be encountered if low sensitivity instruments are used.
EFFECT OF SENSITIVITY ON VOLTAGE READINGS
Test instruments with the highest sensitivity available should be used when voltage measurements are made in order to get readings that are as close to the actual operating conditions as possible.
Figure 8 illustrates the points at which various voltage and current measurements are made.
In Figure 8-A observe that the plate voltage Ep is the voltage actually applied to the plate, and is measured between the plate and cathode. Since a triode is shown, the plate current Ip can be measured at the cathode. In Figure 8-B the points of measurement
for the screen grid voltage Esg and the screen grid current Isg are shown. Figure 8-C shows that the control grid bias voltage Eg is measured between the grid and cathode proper. The grid is usually negative with respect to the cathode. Very often a reading taken between grid and cathode will be incorrect due to the high resistance present in the grid circuit. In such cases the voltage drop across
the cathode resistor should be measured instead. The cathode voltage Ek is measured from cathode to ground as shown in Figure 8-D. The chassis is frequently the ground point but may not be, especially in receivers of commercial design. In Figure 8-E the measurement of currents in a multi-element tube is shown. Care must be taken to pick out the point at which to measure the proper currents so as to
avoid measuring the combination of several currents. In measuring such currents, care must be taken to make the measurement as near the cathode as possible. If the meter is placed near the plate itself, it may be at a high DC potential and there is a chance of getting a bad if not, fatal shock from the meter.
It is obviously impossible to include here all of the information available on measurements and on the precautions to be taken with specific instruments. The instruction book on the instrument at hand should be very carefully read before any attempt is made to use the instrument.
The Model OE Analyzer furnishes examples of the necessity for reading the instructions. With this equipment, voltages are read between the outer pin-jack terminals on the selector block. The inner jacks are open circuit jacks for use only in making current measurements. To measure current, always put a patch cord from the outer jack of the selector block to the meter first. Except in control grid circuits, this will normally be the positive meter terminal. Then put one end of a patch cord in the other meter jack. Finally, plug the other end of this last patch cord into the inner jack of the selector block. In making the measurement, as described, the circuit is not opened in such a manner as to cause a voltage surge with possible meter damage.
The Model LP Signal Generator gives us another example. With this equipment there must never be more than one ground at a time on the system composed of the signal generator and the receiver being tested.
Even the notes given above do not completely cover the precautions necessary to use properly the equipments discussed. The only safe procedure is to read the instruction book before using test equipment.
2.1 Methods of Analysis
There are seven distinct methods by which the search for trouble may be made. These methods are:
Each method has its own advantages, which will be described later. No test method can be useful, however, unless it is used systematically.
(1) Visual Inspection.
(a) Check power supply and fuses.
(b) See that all tubes are in place.
(c) Check antenna, projector, and loudspeaker connections.
(d) Have switches and panel controls in operating position.
(e) Search for burned and charred parts or for odors.
(f) Check variable condensers for shorted plates.
(g) Remove dust and dirt.
(2) Load Resistance Test.-With power off, measure the resistance between rectifier filament terminal and chassis. An indication of between 15,000 and 200,000 ohms should be obtained.
If a rather low resistance is encountered, it indicates a short somewhere in the DC circuit. The cause may be a shorted filter or by-pass condenser. The defective part can be located by point-to-point resistance tests and by consulting the schematic diagram.
The receiver must never be turned on until this trouble has been corrected.
(3) Shock Tests.-A shock test is a quick method for locating trouble by a stage-by-stage process of elimination. It is a static method, and since it may be made without dismantling the receiver, it is normally one of the first tests to be made. In making a shock test, the tubes are removed and then replaced, one at a time; exposed grid caps are tapped ; etc., until the troublesome stage is located. Normal circuits, when thus disturbed, will cause the loudspeaker or phones to emit an audible "thud" or "clunk". Any stage which does not give an audible indication of its normal operation should be suspected of being defective.
Care must be exercised when using this system, because stages delivering any amount of power can produce transient surges of voltage that may damage good circuit components. A further disadvantage is that some experience is necessary in order to interpret properly the "thud", "click", or "clunk" sounds that shock testing produces in the phones or loudspeaker. Nevertheless, the method often saves times and it can be used to advantage, provided care is taken.
When tubes have an exposed grid terminal, the first step is to remove the grid cap and touch it to the grid terminal. Listen for the normal sound.
Tubes which have no exposed grid terminal are simply removed and then reinserted.
Shock Test Procedures.
(a) Turn on set and see if tubes light. Some tubes, such as metal tubes and many of the 1.5 volt tubes, have filaments which cannot be seen while in operation. If dial-lights are lit, one can usually assume that tubes are lit. Metal tubes become warm after a few minutes and can be felt with the hand.
(b) Remove audio tube from socket and reinsert. Listen for sound in speaker.
(c) Remove grid cap, and touch finger to grid of 2nd detector or 1st audio tube. Listen for loud hum.
(d) Proceed toward antenna, touching grids with a screw-driver, or, as mentioned above, merely remove the grid cap and touch it to the grid. RF and IF grids will not produce as loud a noise as the first audio grid, but the click will increase as you progress towards the antenna.
(e) Scratch antenna terminal with antenna lead.
(f) Short out the oscillator tuning condenser and listen for pronounced thump as oscillation stops. It is wise to become acquainted with this noise in a normal receiver.
If it is found that one tube did not produce its proper noise, it is obvious that something is wrong in that particular stage. The trouble can be due to a coil, condenser, or other component in the stage or to the tube itself.
(4) Tube Test.
(a) Test the tube found to be silent in the shock test. If no tube tester is available, replace with a good tube.
(b) Assuming no reaction from shock test, test all tubes or substitute with all new tubes.
The most common source of trouble in sound receivers, amplifiers for motor control, and power stacks has been found to be tubes. Always keep a complete set of tubes adjacent to the equipment. Mark the receiver oscillator tubes before removing so they can be reinserted in the same sockets. Due to variations in inter-electrode capacities, a different oscillator tube may cause the sound receiver to be badly out of alignment.
(c) If set operates, replace new tubes with old, one by one, until defect is found. If operation is not restored, the next method of analysis must be tried.
(5) General Analysis.-A general analysis of a receiver can be made most conveniently with a special test set called an analyzer. An analyzer consists of a volt-milliammeter, a tube socket set, and connecting cables, so arranged that the tube from any stage may be removed, placed in the analyzer socket, and the cable plug substituted for the tube in the receiver. Usually, the normal operation of the receiver may then be continued. By means of suitable leads or switches, the dynamic, or operating, currents and voltages on the particular tube under test may be read on the meter. The values obtained by these readings may be compared with a set of similar readings taken on a normally operating receiver and the defective stage located by the variation in the reading.
The Model OE is an analyzer which is frequently available aboard ship.
An experienced man can sometimes cut short this process by taking voltage readings at the tube sockets. The currents can be estimated by noting the circuit components involved. This method is sometimes referred to as "Point-to-Point Voltage Analysis". Since there is danger from high voltage and the possibility of meter damage, the haphazard use of this method is not recommended. Only thoroughly experienced men should employ it.
The power supply output voltages should be checked before starting the receiver analysis. Check the filament or heater voltages as well as the various voltages supplied by the voltage divider.
General Analysis. Point-to-Point Method.
(a) If set has a tuning eye or indicator, check its operation.
(b) If audio does not function properly in shock test:
1. Measure Ep (of last audio).
2. Measure Esg (of last audio).
3. Measure Eg (cathode to ground).
4. Look for positive voltage on the grid.
5. Measure DC output at the rectifier.
6. Measure DC output at last filter condenser.
(c) If second detector does not function properly in shock test:
1. Measure Ep.
2. Measure Esg.
3. Measure Eg.
(d) Take voltage readings at each tube socket, working towards the antenna.
The preferred method of general analysis is by use of plug and adapter sets. The Model OE Analyzer is commonly available for this purpose. The instructions supplied with this equipment cover all of its uses and limitations. Consult these instructions and assemble the analyzer ready for use with the block receptacle in place on the volt-ammeter.
Procedure.-The usual practice is to check the circuits in the order in which the signal passes through them, starting with the antenna stage and ending with the AF output stage.
(a) Remove the first tube from the receiver. Select the proper adapter, and put the socket unit in the block receptacle, and slip the plug section on the plug. Place the tube in the block socket and put the plug in the tube socket in the receiver. The receiver may now be turned on, and the tubes allowed to warm up.
(b) The tube socket numbering scheme must be determined by checking the tube against the socket diagram.
(c) Voltages on the various tube elements are measured by using patch cords to connect the negative side of the voltmeter to the
cathode and the positive side to the electrode being tested. The exception to this rule is the control grid, which is usually negatively biased. The outer pink jack terminals on the selector block are used in measuring voltages. -
(d) To measure electrode currents, first put a patch cord from the outer pin jack of the selector block to the meter jack. Except in control grid circuits, this will be the positive meter terminal. Then put one end of another patch cord in the other meter jack, finally plugging the other end of this last patch cord into the inner jack of the selector block.
(e) Always start with a high meter scale to avoid the possibility of damaging the meter.
(f) Record the voltages and currents measured at each tube element. For a three-element tube measure the:
For special, multi-element tubes there will be other voltages and currents to be measured. Such tubes may have diode plates, extra grids, or complete sets of additional elements.
(g) Replace the tube, and repeat the process with the succeeding tubes through the receiver, recording the data for each one.
(h) The readings are now compared with normal readings for the receiver. Most new instruction books contain average normal readings. Where such readings are not available, compare the results obtained with those gotten from a normally operating receiver. As has been stated before, it is very helpful to compile normal readings for all equipment on hand.
Notice that there may be a small variation in the readings and the receiver conditions will still be entirely normal. When manufacturing circuit components, a tolerance or variation in the values of the parts is allowed. (Refer to Section 1.1-1.3.) In general, a variation of ±10 percent is allowable. A variation of ±15 percent or more should be regarded as suspicious, especially in control grid circuits which may be extremely critical.
Notice further, that the sensitivity of the meter used must be considered in comparing the readings. If the normal readings given in the instructions were measured on a different type meter, this factor must be taken into account. For example, data taken with a 20,000
ohms/volt meter will be higher than those taken with a 1000 ohms/volt meter.
(6) Point-To-Point Resistance Analysis.-The power must be cut off and all voltages must be removed from a receiver before this type of measurement is begun. An ohmmeter is used to measure the resistances of the various circuit components and the measured values are compared with the original values as shown on the circuit diagram or spare-parts list.
The circuit must be carefully examined before resistances are measured, to make sure that parallel circuits are not being included. It is usually necessary to disconnect one lead of the part being measured to insure getting the right reading.
Any point-to-point analysis is tedious and time consuming. It should not be undertaken until the fault has been traced to a particular circuit. Some of the tests which can be made are listed below:
(a) Test voice coil circuit for continuity.
(b) Measure the following grid to ground resistances in this order: audio stages, second detector, and successive stages toward the antenna.
(c) Test primary and secondary of antenna coil for continuity.
(d) Test oscillator circuit for continuity and shorts.
(e) Test IF transformers for continuity and shorts.
Numerous receiver service notes list the correct point-to-point resistance readings. If no information is available, the material man should be able to trace the schematic diagram and interpret the correct resistances.
(7) Signal Tracing.-If a signal generator is available, a very simple and effective stage-by-stage test can be made. This is similar to the shock test previously described, but is far more accurate.
The object is to allow a signal of correct frequency to be passed through an amplifier or detector stage and observe the output. If there is no output, the stage must be defective.
Two types of signal tracing are commonly used. One method, sometimes called sensitivity testing, is to feed a signal into the input of a stage and measure the output, or amplification, by means of a vacuum tube voltmeter or dynamic analyzer.
The other method is to listen to the signal output in the speaker. Most test oscillators have available a 400 cycle audio signal in addition to the RF signal. This audio signal should be applied to the various stages of the audio amplifier, starting close to the speaker and working in steps toward the detector. The oscillator should next be adjusted to the modulated intermediate frequency, if the set is a superheterodyne, and the various IF sections tested.
The modulated RF signal may next be listened to, progressing stage by stage toward the antenna. As progress is made, an increase in
volume should be noticed and the output of the signal generator will have to be reduced. An output meter may be used, if desired, to indicate stage-by-stage gain.
Any stage which shows a definite loss in power, or no signal at all, must obviously be faulty. Usually the bad part can be located by means of voltage tests or point-to-point resistance measurements. Do not overlook the fact that a defective tube will stop the signal.
The following steps will be useful in locating the faulty stage or section:
(a) Using the audio source, touch oscillator test lead, or prod, to grid of last audio tube and observe output.
(b) Touch prod to grid of first audio tube and observe output.
(c) Place prod on grid cap of second detector, and observe output.
(d) Tune modulated oscillator to IF frequency of set and place prod on grid of last IF stage.
(e) Proceed through IF amplifier, stage by stage towards antenna, observing increase of signal at each grid.
(f) Tune oscillator to convenient radio frequency and place prod on grid of mixer.
(g) Place prod on grid of RF amplifier (s), and observe gain.
(h) Place prod on antenna terminal.
(i) Substitute unmodulated test oscillator for the local oscillator of the receiver.
In the above steps (a to h), the trouble will be found in the stage where the signal disappears.
Assume that no signal was heard in step (a). Touching the oscillator test prod to the plate terminal will indicate whether or not the tube is operating. This could be due to no voltage at one or more socket terminals or to a defective tube. The same procedure may be followed in any stage which fails to produce proper results.
For example, suppose a signal was heard by placing prod on 1st IF grid, but not on the plate of the mixer tube. Therefore the coupling device (IF transformer) must be faulty. One winding may be open, have shorted turns, or possibly one trimmer is shorted.
NOTE No. 1.-In many mixer stages, the actual conversion gain will be quite low, and in many cases a slight loss may be observed.
NOTE No. 2.-Some IF coils are wound to provide a slight gain of from 1 to 1.5, but most will show a slight loss.
Assume that a signal was heard in step (g) and none heard in step (h). The antenna coil therefore must be bad.
Simple signal tracing such as applying the oscillator prod to the exposed grid terminals is very fast and helpful in locating the faulty stage and can be done with the receiver in its cabinet. It is usually easier to locate the actual part, however, by using the voltage and resistance methods.
If trouble is suspected in the local oscillator, the signal generator may be substituted as a check. An unmodulated signal (c. w.) must be used. The proper frequency to use is, of course, the RF dial setting plus the receiver IF frequency. The point to apply the test lead will vary with different receivers, but a good point to try is near the 1st detector grid. The prod should be touched to the insulation quite near the grid, but not to the grid itself, because of the detuning effect.
2.2 Application of Clues and Information Obtained in Steps 1-6
Step (1)-(e).-When burned and charred parts are found or a fuse blown, reason for failure should be determined before replacement is made. Resistors and transformers do not ordinarily burn up of their own accord, but are damaged by excessively high currents or moisture. For example, a charred screen dropping resistor could have been caused by a shorted screen by-pass condenser, or a shorted screen in the tube. Regardless of the cause, excess current would flow, and the resistor would overheat. A burned power transformer in some cases- is caused by internal shorts, but in many cases has been brought about by an overload presented by a shorted filter condenser, bleeder, or by-pass condenser.
Step (2).-If the load resistance is at a, low value (less than 15,000 ohms) it will show too little resistance between B+ and ground. Trouble may be shorted filter condensers, by-pass condensers, or filter choke winding shorted to case. Locate trouble by testing resistance of parts in B+ circuit to ground.
Step (4).-In instances of burned out filaments, measure filament voltage in set.
Step (5)-(b).-Compare plate and screen voltages in last audio, if tube is a pentode. Ep should be slightly less than Esg, due to the voltage drop in the primary of the output transformer. No drop in voltage will indicate a short between the plate and screen. In the other amplifier stages Esg should be somewhat lower than Ep. If there is no Ep or Esg, measure the voltage from plate to ground. Voltage between these two points and none between plate and cathode would indicate an open cathode resistor. No voltage from plate to ground as well as none between plate to cathode can indicate an open load resistance or coupling transformer. Trace circuits back from plate to B+ until voltage is found. Defective part will be found where voltage disappears.
Step (5)-(c).-Lack of voltage drop between cathode and ground will indicate either a shorted cathode by-pass condenser or a shorted resistor.
Step (4)-(b)-4.-If the grid has a positive potential, it will indicate a shorted or leaky coupling condenser, allowing the plate voltage
from the preceding stage to be applied to the grid. Remove coupling condenser or transformer and test for shorts.
Step (5)-(b)-5.-Compare reading of voltage at input and output of filter. If no voltage drop is present, the filter choke or speaker field is shorted. This fault will be accompanied by low output and distortion.
When a lack of voltage is observed on any plate or screen grid, voltage measurements should be taken, proceeding toward B + until the point where voltage appears. Faulty part (s) will be found between points where voltage appears and disappears.
Note. A "B supply" short should have been discovered during the "load resistance test."
Step (6)-(a).-No continuity in voice coil circuit would indicate an open voice coil, output transformer secondary, or both.
Step (6)-(b).-Infinite grid resistance to ground indicates open grid-return. Normal grid resistance in a resistance coupled amplifier should be between 100,000 ohms and two megohms. Normal grid resistance in a transformer coupled amplifier is between 500 and 5,000 ohms. Depending on values of A. V. C. circuit, Rg of RF and IF stages will be between 50,000 ohms and 3 megohms. If no A. V. C. is used, the resistance will be very low.
Normal DC resistance of 465-456 kc IF transformers is from 12 to 15 ohms. Primaries and secondaries should measure within 20% of each other. Replace if greater deviation is observed. Normal DC resistance of RF secondaries is between 1/10 to 10 ohms.
In cases of open grid and plate circuits in RF and oscillator stages, examine the band switch for poor contacts.
Step (6)-(e) .-Partially open RF, AF, and IF primary windings are very common. This is due to corrosion which usually produces noisy and weak reception. Partial opens or high resistance windings will sometimes cause cutting out or intermittent operation due to resistance variations. If the primary measures more than 20 percent higher resistance than normal, the transformer should be replaced.
IMPORTANT.-The above steps are merely rules of thumb which may be applied to the servicing of any radio receiver. The most foolproof and accurate means of locating trouble is to have the operator make a complete chart of voltage, current, and resistance measurements, with his own meter, while the gear is in good condition. Should the equipment develop trouble, a comparison of the voltage, current, and resistance readings with the original readings will help to indicate where the trouble lies.
After the readings have been compared with the normal values, it is necessary to interpret those readings of voltage or current that have been found to be abnormal.
2.3 Typical Circuit Analysis
The correct interpretation of voltage and current measurements can be made only when the basic receiver circuits and characteristics are fully understood. Some of the many possible circuit arrangements are discussed in this section. Typical circuit arrangements are given, and then the effects of faults or disturbances within the circuit are discussed.
2.31 Typical Grid Circuits
The most common arrangement for obtaining negative grid bias is shown in Figure 9. The plate current flows through the biasing resistor R. The voltage drop across R makes the point E, and hence the control grid, effectively negative with respect to point D and the cathode. In this type of circuit the grid voltage or bias can be measured across the resistor R.
When a tube operates as a grid-leak detector, as in Figure 10, it is biased by the signal acting on the grid-leak grid-condenser combination R-C, and the cathode is directly connected to the grid return lead. No normal grid bias can be measured in such a circuit, since it exists only while the signal is actually flowing.
In Figure 11 the fixed resistor R1 is used to insure a minimum amount of grid bias voltage at all times, while the variable resistor R2 is used to vary the bias thereby controlling the sensitivity of the tube.
When several similar tubes are used in an amplifier, frequently a single grid bias resistor is used to furnish the proper bias voltage for all of the tubes. Figure 12 shows such a circuit, where R is the bias resistor.
In many superheterodyne circuits, the oscillator tube is connected as shown in Figure 13. In this circuit the path through the grid coil is blocked by the condenser C5, and the grid return path is completed through the 40,000 ohm resistor R2. A low bias voltage reading should be expected when checking such a circuit with a voltmeter connected from grid to cathode because of the high resistance of R2.
Figure 14 illustrates a method of supplying grid bias voltage that is sometimes found in more modern AF amplifiers where low distortion is desired. The bias in this type of circuit is furnished by fixed bias cells. These cells resemble metal acorns. They are purely voltage devices, can supply no current, and will deliver either 1 or 1.25 volts per cell depending on the type used. Since a bias cell will not stand any appreciable direct current, any means of testing that causes current to flow through the cell should be avoided.
INTERPRETING GRID VOLTAGE READINGS
A few of the usual symptoms of faulty grid circuit operation and their probable causes are described below. Other causes of faulty grid circuit operation may be found in the Trouble Chart, Section 2.8.
Trouble.-No grid bias.
Symptom.-Choked, choppy, or distorted signals. Ip may be higher than normal and Ep lower than normal.
Cause.-In the circuit of Figure 15, the cause might be R2 open, R3 shorted, C1 shorted, or C partially shorted.
If R3 were open, there would be no Ep, since R3 completes the plate circuit. An ohmmeter can be used to check R3-C1 for a short, and R2 for an open.
With transformer coupling, as in Figure 12, it is likely that the transformer secondary is open.
In Figure 9, C or R would be shorted.
The bias cells will be found to be defective or R opened in Figure 14.
Trouble.-Low grid bias.
Symptom.-Low, distorted signal. Ip higher than normal. Ep less than normal.
Cause.-Cathode by-pass condenser leaky or shorted, in the circuits of Figures 9, 11, 12, 13, or 15. Changed value of bias resistor, any circuit. The meter current flowing through the high resistance in the grid circuit of Figure 13 would cause apparently low grid bias.
Trouble.-Cathode voltage but no grid bias.
Symptom.-No signals or weak signals. Ip higher than normal, Eg zero.
Cause.-Transformer secondary open in the circuits of Figures 9, 11, or 12.
Trouble.-No bias on first analysis with analyzer.
Symptom.-Oscillation, distortion, or poor low frequency response.
Cause.-Open by-pass condenser in circuits such as Figure 15, which is an AF stage.
Trouble.-No bias on first analysis with analyzer.
Symptom.-Oscillation or instability.
Cause.-Open by-pass condenser in RF or IF stages such as those of Figures 9, 11, or 12.
Trouble.-Positive grid bias.
Symptom.-Ig will flow, Ip usually high, Ep low.
Cause.-Coupling capacitor leaky or shorted, as in Figure 15. Transformer primary shorted to the secondary as in Figure 12. In either circuit, Ep will reach the grid of the following tube.
2.32 Plate Circuits
TYPICAL PLATE CIRCUITS
Common plate circuit arrangements are shown in Figure 16. As far as the voltage supply is concerned, there are two types of plate circuit, the series fed, and the parallel or shunt fed. In series fed circuits, the DC plate voltage is applied to the plate load, as in Figures 16-B and 16-C. In A and D of that figure, the DC voltage is fed to the plate through a circuit which parallels the plate load.
The exact manner in which the tube is being used should be determined before undertaking to interpret an analysis. Particular care
should be taken to note special arrangements such as those of Figure 11, where R2 is in both plate and grid circuits. Maximum plate voltage will be obtained when R2 is set at a minimum.
INTERPRETING PLATE VOLTAGE READINGS
Trouble.-No plate voltage.
Cause.-An open bias resistor will remove Ep from the circuits of Figures 16-A, B, C, and D, since the resistor R forms a part of the plate circuit. An open bias resistor R in the circuit of Figure 12 will remove Ep from all tubes.
Similarly, an open choke, Figures 16-A and D, or an open transformer primary, as shown in Figure 16-C or in Figures 9, 10, 11, and 12 will cause the same result. Another possibility is that of an open voltage divider in the power supply unit. (See Sec. 2.34)
Trouble.-High plate voltage.
Symptom.-Distortion, or increased output, or both. Ip and Esg may be higher than normal.
Cause.-Decreased plate load, caused either by a lowered resistor value, as in Figure 14, or by a partially shorted transformer, as in Figures 10, 11, or 12. High Ep may also be caused by abnormal grid or power supply circuit conditions. (See Secs. 2.31, 2.33, and 2.34)
Trouble.-Low plate voltage.
Symptom.-Decreased output, or distortion, or both. Ip may be higher than normal.
Cause.-Increased resistance in the plate circuit, caused by a plate resistor, as in Figure 16-B, or a transformer primary, as in Figure 16-C. When the Ep of the stage having the highest normal value is low, and all corresponding voltages are low throughout the receiver, a leaky RF by-pass capacitor will usually be found. Since such a capacitor may break down only when loaded, it may be necessary to test all the by-pass capacitors in turn by disconnecting them one by one and turning on the power to the receiver after each one is free. If Ep increases after some particular one is disconnected, that one will usually be found to be faulty.
2.33 Screen Grid Circuits
TYPICAL SCREEN GRID CIRCUITS
The troubles occurring in screen grid circuits are usually so intimately connected with other circuits that no type diagrams are given separately. Typical circuits are those analyzed below.
It should be noted that, on the average, the screen grid current is roughly one-third of the plate current. Usually the screen grid voltage is from one to two-thirds the value of the plate voltage. Exceptional circuits employing pentodes are sometimes found in which the screen voltage may be equal to or greater than the plate voltage.
INTERPRETING SCREEN GRID CIRCUIT READINGS
Voltage Dividing Screen Supply Systems.-Figure 17 is a typical plate, screen, and grid bias circuit of two pentode amplifier stages employing a voltage dividing resistance system.
The screen grid voltage for tube VI is obtained through resistor R1, which is really a tap on a resistor composed of R1, R2, and R3. V2 is similarly supplied through R4, R5, and R6.
Note that sections R3 and R6 are the grid bias resistors for the tubes concerned.
The currents flowing through the various circuits are shown by the arrows on the figure. The voltage drops across the various resistors will depend on the total currents through them.
Trouble.-Eg low, Isg high, no Ep or Ip, Esg low.
Symptom.-No signals or very weak signals.
Cause.-Primary of L1 open, thus eliminating Ip, decreasing the drop across R3, and reducing the bias on the control grid. Lower Eg allows higher Isg to flow, increasing the current through R1. The
higher drop across R1 will reduce Esg. V1 will operate somewhat like a triode with the screen grid as the plate.
Trouble.-Esg and Isg zero, Ip high, Eg low, Ep normal or high.
Symptom.-Distorted, reduced output.
Cause.-R1 open, removing screen supply. Bias reduced by removing Isg and Ir2 from R3.
Trouble.-Ep equal to Esg, greater than Ip, Eg high, Ip low. Symptom.-Distorted or no signal.
Cause.-R1 shorted, placing B voltage on screen. The higher Isg flowing through R3 will reduce Ip and increase Eg.
Trouble.-Eg low, Ip and Isg high, Esg high, Ep nearly normal.
Cause.-R2 open, removing Ir2 from R1 and R3. Decreased drop in R1 and R2 will cause high Esg and low Eg.
Trouble.-Ep and Ip normal or slightly low, if power supplied is good, otherwise low. Esg low, approaching Eg. Eg high, Esg low. Symptom.-Distortion, weak signals.
Cause.-R2 shorted, increasing current through R1, decreasing Esg, and increasing Eg due to increased drop across R3.
Trouble.-Ep, Esg, Eg, Ip, and Isg all zero.
Cause.-R3 opened, removing B voltage from V1.
Trouble.-Ip high, Ep nearly normal, Esg nearly normal or low.
Symptom.-Distorted, loud signals.
Cause.-R3 or its by-pass capacitor shorted, resulting in zero bias and increasing Ip. Increased current flowing through R1 tends to lower Esg.
It should be observed that when a voltmeter reads zero it means that there is no difference of potential between the points to which the meter is connected, and is not a positive indication of an open circuit. In Figure 17, the meter will indicate that Eg is zero if R3 is either opened or shorted, there being no difference of potential between the grid and cathode in either case.
In a case of this kind it is necessary to determine which condition exists. This may be done by checking the plate and screen voltages and currents, for they are affected differently by an opened or shorted grid bias resistor R3.
The screen circuits for two or more tubes are exactly the same as for V1. For instance, the screen circuits for tube V2 in Figure 17 may be analyzed by checking R4, R5, and R6 which correspond to R1, R2, and R3.
Series Dropping Resistor Screen Grid Circuits.-Figure 18 illustrates a simple series type screen circuit, the amplifier stages having a series connected voltage-dropping resistor R2 in the common screen
circuit. The combined screen currents flow through this resistor and produce a voltage drop across it. This is perhaps the most common method for obtaining screen voltages.
Trouble.-Esg and Isg zero, Eg below normal, Ip high.
Symptoms.-Distorted, reduced signal.
Cause.-R2 open, removing screen supply. Eg low because drop caused by Isg flowing through R1 is part of bias. Ip high because of low bias. The general condition and symptoms are the same as in Figure 17 with R1 open.
Trouble.-Esg equals Ep, Isg greater than Ip, Eg high, Ip lower than normal.
Symptoms.-Distorted, or no signal.
Cause.-R2 shorted, placing B voltage on screen. The high Isg flowing through R1 will increase Eg which in turn will reduce Ip. This is similar to the action of Figure 17 with R1 shorted.
Trouble.-Eg, Ep. Esg. Isg, and Ip of V2 zero. Ep of V1 normal. Ip of V1 low.
Cause.-R3 open, removing plate and screen voltages from V2. Esg and Isg of V1 probably high because of the low drop in R2. The high drop in R1, due to high Isg, would increase Eg of V1 if the combination is such that Ip of V1 does not go too low and counteract the effect of high Isg. An open R1 in this figure would cause the same general effect on V1.
Notice that any defect or variation in R2 in Figure 18 affects all of the tubes to which it supplies screen grid voltage. Another series type of screen grid circuit is shown in Figure 19. This is also a very common circuit with the feature that the series dropping resistors R3 and R4 affect only the tubes to which they are connected.
Complex Series Resistor Screen Grid Circuits.-Some circuits are designed to use different voltages on the several tubes, as shown in Figure 20. In this circuit R2 is a bleeder resistor across the power supply (for the action of this bleeder see Section 2.34 on Power Supplies) . R1 reduces the voltage to that required by the amplifier and detector tubes. Resistors R3, R4, and R5 drop the voltage still more for the various screen grids. The analysis of such a circuit is similar to that of other series arrangements.
Trouble.-Ep of all tubes but last one zero, Ep of AF power tube about normal. Esg of tubes fed through R1 zero.
Cause.-R1 open. Note that if R6 feeds screen grids, those tubes may act as triodes as explained for the circuit of Figure 17.
Shunt Connected Screen Grid Circuits.-A shunt or parallel system for supplying screen grid voltages is shown in Figure 21. In this
circuit arrangement, the screen grid is connected to a tap on the voltage divider of the power supply unit. The combined plate, screen, and bleeder currents flow through the section which reduces the voltage to the proper screen value. This circuit differs from the voltage dividing arrangement of Figure 17 in that only the screen and plate currents flow through the grid bias resistor. More information on this type of circuit will be found in Section 2.34 on Power Supplies.
2.34 Power Supplies
There are two types of filter circuits used in radio receivers, namely:
(1) Choke input filters, as in Figure 22-A;
(2) Capacitor input filters, as in Figure 22-B.
The operating characteristics of the two types are:
Good regulation under variable load.
Poor regulation under variable load.
Low DC output for a given AC input.
High DC output for a given AC input.
Low peak and surge currents.
High peak and surge currents.
The circuits of Figures 20, 23, 24 are typical power supply arrangements. Each of these circuits contains a resistance which acts as a bleeder. In Figure 20 it is R2, in Figures 23 and 24 it is the combination of R1, R2, and R3.
A bleeder is a resistance put across the terminals of a rectifier-filter system to discharge the capacitors when the power is removed and to improve the voltage regulation of the supply by providing a minimum load. It may also serve as a voltage divider, as in Figure 23.
If a power supply unit has no bleeder, or if the bleeder should open up, the filter capacitors may contain an extremely dangerous high voltage charge long after the line power has been removed from the unit. If any doubt exists as to the presence of a bleeder or its condition, the filter capacitors should be discharged to ground with an insulated-handle grounding stick or screw driver.
Figure 23 shows a very common type of power supply unit. The field coil of a dynamic speaker is often used as one of the chokes in such a circuit.
The most common casualties in power supply filters are shorted capacitors and open voltage dividing resistors.
Symptoms.-Set wholly inoperative.
Cause.-Power transformer or line circuit defective. Burned appearance or odor shows if transformer is burned out. If not, check line voltage and continuity of plug, cord switch, primary, and fuse.
Trouble.-Filament or heater voltages but no Ep or very low Ep.
Symptoms.-Low or no signal; rectifier plates may be red hot.
Cause.-Shorted capacitor, especially C1. If C2 shorts, L1 will heat up. If rectifier tube heats up, a short is indicated. A leaky capacitor will lower plate voltage but not always wholly remove it. Notice that the plate by-pass capacitors are across the power supply, and if one of these is defective the symptoms will resemble those for defective filter capacitors.
Trouble.-No plate or screen voltages (except for last stage in circuits like Figure 24).
Cause.-Chokes L1 or L2 open, check for continuity. Rectifier will not be hot.
Cause.-Open filter capacitor or open half of transformer high voltage secondary. In full-wave rectifier, also may be defective rectifier tube.
Trouble.-Transformer hot, low Ep and Esg.
Cause.-Partially shorted high voltage secondary. Primary current high.
Trouble.-Ep only on last stage.
Trouble.-No Ep on detector, high Ep on RF and AF stages.
Trouble.-Instability, low volume, poor tone, all voltages slightly high.
Symptoms.-Poor stability, distortion.
Cause.-R3 open. Notice that even with R3 open voltage readings will be obtained at all taps.
Trouble.-Ep of RF and AF tubes the same, but low.
Symptoms.-Reduced output, probably distorted.
Cause.-R1 shorted. Voltages slightly low due to higher current and increased drop across chokes.
Trouble.-Ep of all tubes low.
Symptoms.-Reduced output, distortion.
Cause.-R2 shorted. Voltages low due to increased drop in chokes and resistors.
Figure 24 shows a special type of filter known as a graded filter, because the voltages for the various stages are selected from different points on the system and have different grades of DC purity.
The push-pull AF stage has its plates supplied from the rectifier output, since most of the hum or ripple will be cancelled out. The RF stages are not too critical as to ripple and get voltage from an intermediate point. The detector is supplied with very pure DC from the end of the filter. Such a system is used where cost or weight may be a factor. Since low current flows through the chokes, L1 may be lighter and smaller than usual and L2 very small.
2.4 Point-to-Point Resistance Analysis
Circuit networks.-A thorough knowledge of series, parallel, and series-parallel circuits is necessary to interpret correctly the readings taken during point-to-point resistance analysis, as most modern receivers employ rather complicated resistance networks. For rapid servicing, the material repairman must be able to analyze such circuits at a glance. For this reason, the fundamentals of DC circuits will be reviewed briefly.
Series circuits.-The total resistance R of a series circuit is equal to the sum of the individual resistances. In Figure 25, for example,
R = r1 + r2 + r3 = 100 + 20 + 480 = 600 ohms.
In a series circuit all of the current will flow through each resistor, and there will be a voltage drop across each one due to the passage of the current. The voltage drop, or fall of potential, across a resistor is the voltage necessary to force the given current through it.
The sum of the voltage drops across the resistors of a series circuit is equal to the voltage applied to the circuit. In Figure 25, for example,
A voltmeter connected across r1 would indicate 1 v.; connected across r2, it would show 0.2 v.; and across r3, 4.8 v.
The series circuit does not have to consist of resistors alone. In Figure 26, for example, a choke, a coil, and a resistance are shown connected in series.
As far as DC is concerned, Figure 26 might have its resistance represented by the circuit of Figure 25.
Parallel circuits.-A circuit consisting of three resistances in parallel is shown in Figure 27.
In such a circuit the voltage across each resistor will be the same, and will be equal to the voltage applied to the circuit. The current which flows through each resistor will be a portion of the total current, and the sum of the three parallel path currents will be equal to the total current supplied by the battery. The total resistance of the circuit can be found from the formula:
As a check, the total voltage and resistance values give
I = E/R = 6/15.38 = .39 amperes.
The parallel circuit may also consist of other parts such as a coil, a choke, and a resistor. Figure 28 might have its DC resistances sketched as in Figure 27.
In the special case where all the resistances are equal, the total current divides equally among the various units, and the combined resistance of all the paths is equal to the value of one resistance divided by the number of resistances.
If there are only two resistances in parallel, then the product of the resistances divided by the sum of the resistances is equal to the total resistance.
General notes.-Before beginning a point-to-point analysis, the receiver must be disconnected from the power supply line and the tubes removed from their sockets.
Although many measurements can be made on receivers using less flexible equipment, the ohmmeter used should be capable of measuring resistances from 1/2 ohm to 5 megohms, with several scale arrangements to cover this range of resistance values. The OE Analyzer, which has a maximum resistance range of 10 megohms, is an example of a satisfactory unit. An experienced man can measure resistances from point to point directly at the sockets or leads. A less experienced man can save time by using the adapter set which is a part of the OE equipment, because the block has the tube elements numbered on it. This makes it easier to pick out the right tube elements and it is not necessary to study the tube base diagrams so carefully to insure reading the right circuits.
Tube base diagrams, which give the arrangement of the tube elements as brought out to the socket, are included in the OE kit and in Sec. 4.6 of this pamphlet.
When using the adapter set, the seven prong plug with the proper adapter attached is inserted in the socket of the tube being tested. The resistances from element to element or from element to ground can then be measured by using patch cords between the ohmmeter and the adapter block. By reference to the circuit diagram, the measured value can be checked against the normal values of the components in the circuit.
Notice carefully that ohmmeter errors, errors in reading, and the usual 10 percent tolerances must be allowed for in making comparisons. Suppose the total theoretical resistance in a circuit is 6,190 ohms. Ordinarily, any reading between 5,571 and 6,809 ohms should indicate satisfactory circuit conditions.
The fact that the circuit elements have usually cooled off by the time measurements are made should not be overlooked, since a resistance may have a different value when heated. This change due to heating is frequently the cause of intermittent circuit operation. Usually the receiver can be turned on, warmed up, and the measurement made as soon as the power is shut off. Sometimes it is necessary to use artificial heat to keep the component warm enough to detect the fault.
The most common fault, encountered in making resistance measurements is the failure to go through the wiring diagram thoroughly enough to determine exactly what parts are in a given circuit. This is especially true when parallel circuits are encountered.
Actual networks.-The effects of series and parallel circuits in receivers can be more clearly understood by considering some actual circuits. In some of the diagrams the by-pass capacitors have been omitted since only the DC circuits are involved in the networks being measured. Notice, however, that care should be observed since a remotely located capacitor may block off a part of the apparent DC circuit in some receivers.
Figure 29-A is a typical IF amplifier stage. If an ohmmeter is connected from the plate of the tube to the chassis, or ground, a reading of 25,600 ohms would be obtained since the plate coil winding (100 ohms), the screen voltage dropping resistor (15,000 ohms), the screen bleeder resistor connected between the screen and cathode (10,000 ohms) , and the cathode bias resistor (500 ohms) are all connected in series.
A simplified sketch of this circuit, as measured, is shown in Figure 29-B. The resistance between points B and G would be 10,500 ohms.
Suppose, however, that the tube circuit of Figure 29-A is connected across the power supply unit shown in Figure 30. This is
the schematic diagram of a power supply unit having the 7,500 ohm speaker field connected directly across the circuit, and not used as a choke. If the same check is now made with the ohmmeter by connecting it again from the plate to the chassis, or ground, the reading will not be 25,600 ohms as before because another circuit, consisting of the second filter choke in the power unit and the speaker field in series, is in parallel with the series circuit from point C to G of Figure 29-B.
The series-parallel circuit which now exists is shown in Figure 31. The ohmmeter will indicate approximately 6,190 ohms if it is connected between the plate and the ground (points A and G).
As another illustration of the caution which must be observed when checking resistances, see Figure 32.
The grid bias for the tubes is obtained in the conventional manner by a resistor in the cathode circuit of the tubes.
If the resistance in the grid circuit of each tube is to be checked, an ohmmeter connected between points A and G should read 51,000 ohms. Making the test in this particular circuit is simple since the grid circuit is isolated.
Now consider the circuit shown in Figure 33. The grid bias is not obtained from the conventional cathode resistor, but from the voltage drop across the 6,000 ohm resistor connected across the speaker field.
The actual circuit network which exists between points A and G in circuit 33-A is shown in 33-B.
When the ohmmeter is connected between points A and G, the reading will be about 5,000 ohms, as there is a series-parallel combination involved in this circuit.
The resistance at the terminals of the 6,000 ohm bias resistor, which is shunted by another resistor of 10,000 ohms in series with the 2,000 ohm speaker field, is about 4,000 ohms.
This resistance added to that of the input transformer secondary winding, results in a total resistance of about 5,000 ohms between points A and G.
Figure 34 shows other examples where precautions must be observed when checking resistance values between various points in a receiver. Figure 34-A shows a grid bias resistor shunted by a variable volume control resistor, and Figure 34-B shows an AF transformer secondary shunted by a similar resistor.
Suppose trouble is suspected in this part of the receiver and the resistance from the cathode C to the chassis, or ground, is to be checked in the circuit of Figure 34-A.
The reading of an ohmmeter connected between these two points will depend upon the setting of the variable volume control resistor. If the control happens to be set at the "minimum resistance" position, a zero ohms reading will be obtained. If the control is set at the half-way position (assuming the resistance element is not tapered), a 2,500 ohm reading will be obtained. In such cases, the circuit should be checked with the variable resistance in the maximum position, and the series-parallel combination taken into account.
Trouble shooting by the point-to-point resistance analysis method is tedious and requires great care and constant thought if useful results are to be obtained. Care must be taken to:
(a) Read the wiring diagrams correctly and take into account all series units and parallel paths.
(b) Read the tube base diagrams carefully so that the proper circuits will be measured.
In some cases it will be necessary to disconnect one lead of each component and measure the resistance of each part of the circuit separately.
2.5 Obscure Receiver Troubles
The most perplexing troubles encountered in making radio repairs are those which do not manifest themselves by abnormal current, voltage, or resistance readings. The analyzer readings may be normal, the tubes test within their accepted tolerance, and yet the receiver may not operate or may operate poorly or intermittently.
Cases of this kind are especially common in the superheterodyne type of receiver, because many troubles and misadjustments may exist even though the tubes and voltages are normal.
Since each main section of a radio receiver may develop certain individual troubles that are characteristic of that part, the sections will be discussed separately.
OBSCURE TROUBLES IN R.F. AND I.F. AMPLIFIERS
The symptoms which result from obscure troubles in receivers are:
1. Lack of sensitivity.
6. Intermittent reception.
3. Images and double-spot tuning.
8. Dead spots.
1. Lack of sensitivity.-Lack of sensitivity, resulting in weak signals, may be due to:
(a) Defective coupling between stages.
(c) Faulty connections.
(d) Defective tubes and parts.
(a) Defective coupling between stages.-Figure 35-A shows a common inductive coupling arrangement, and Figure 35-B a capacity-coupled circuit. In the circuit 35-A, if the primary coil opens at x the receiver will be inoperative or extremely insensitive. If the coil opens at y the receiver will be insensitive but will probably operate. In both cases the small amount of coupling' remaining after an open occurs in the coil will be due to the capacity between the coils. The effect of this capacity on the circuit is shown in 35-C. An open in the leads to the coil will usually make the receiver inoperative.
Similarly, in 35-B an open in the coupling lead or the coupling capacitor will make the receiver insensitive or inoperative.
In some circuits, an open in the coupling capacitor or an open coupling transformer will not be discovered on making the usual voltage-current tests. An ohmmeter check on the coil will usually show that it is open. It is ordinarily best to use a capacity meter or analyzer to check the coupling capacitor, although the movement of the meter needle under the charging voltage surge may sometimes serve as an indication when an ohmmeter must be used. An open capacitor will not allow the meter to register any indication.
Once the tubes have been checked and found to be good, the coupling arrangement should be suspected when there is a lack of receiver sensitivity. This is especially true of inductively coupled circuits when the sensitivity is somewhat greater at the high frequency end of the tuning band than at the lower. This is because the transfer of energy through an inductively coupled system is largely due to the magnetic coupling at low frequencies, but at the higher frequencies is partly due to the capacity coupling between the coils.
(b) Misalignment.-(1) In tuned radio frequency receivers or stages, a decrease from normal sensitivity is frequently due to misalignment of the tuned circuits. Aging, absorption of moisture, shock, and other factors can cause a receiver to get out of alignment. Insensitivity and broad tuning are signs of this type of defect.
(2) Insensitivity in superheterodyne receivers may be caused by defects in the IF amplifier, RF amplifier, or oscillator.
Generally speaking, insensitivity in superheterodynes is marked by low signal levels and excessive noise rather than by broad tuning. Most of the selectivity of superheterodynes is due to the IF stages. These stages are tuned to a fixed frequency whose value is determined by the signal frequency and the oscillator frequency.
If the alignment of the oscillator remains fixed and that of the IF amplifier drifts, the amplification of the IF stages will be reduced. If the oscillator alignment shifts, the frequency produced will shift, and the output of the first detector or mixer to the IF stages will not be the right frequency for maximum amplification.
The procedures for realignment are covered in Section 2.6.
(c) Faulty connections.-A high resistance joint in the primary or secondary of the tuned circuit will result in low sensitivity. A high resistance joint is usually caused by poor soldering, such as soldering on a dirty surface or by "cold" soldering. Although this high resistance joint may have little effect on the voltage readings, it can offer considerable resistance to radio frequency signals.
A point-to-point test with a good ohmmeter is the best method of determining whether or not such a condition exists, although high resistance joints cannot always be located by visual inspection or ohmmeter tests. The resistance of the joint may depend upon the temperature of the receiver. The contact may have a high resistance when the receiver is hot or cold, and have a low resistance under the opposite condition.
(d) Defective tubes and parts.-It is not always easy to be certain which tube or part is defective. For example, poor sensitivity in superheterodynes may be caused by poor oscillator tubes which test satisfactorily in a tube tester. In spite of this, such tubes may
oscillate at only certain frequencies, often widely separated, may not produce strong oscillations, or may not oscillate at all. This condition may usually be remedied by inserting a new tube, but occasionally the cathode resistor must be reduced in value so as to increase the oscillator plate current.
As an example of the action of a defective part in an apparently devious way, notice that an external or internal open in a grid-bias bypass capacitor network can reduce the sensitivity of the receiver considerably.
Consider the circuit of Figure 36 where it is assumed that the amplitude of the voltage across the transformer secondary L is less than the
grid bias. This alternating signal voltage causes the potential of the grid of the tube to become alternately more and less negative once each cycle.
When the grid becomes less negative, the plate current increases. Since this plate current flows through the bias resistor R, the voltage drop across R will increase if C is not present or if C has an internal open, thus tending to cause the grid to become negative. The reverse action takes place when the signal polarity tends to make the grid more negative. When the voltage across R thus fluctuates a half cycle out of phase with the signal, there is a loss of sensitivity which is said to be due to degeneration or degenerative action.
It is the function of the by-pass capacitor to maintain sufficient charge so that the voltage across R remains substantially constant during each signal voltage cycle, thus preventing degeneration.
Note that a push-pull amplifier requires no bypass capacitor across its bias resistor, because the plate currents of the tubes are always a half cycle out of phase. One decreases by the same amount and at the same time that the other increases, and the net fluctuating current through the bias resistor is always zero. Consequently, no bypass capacitor is necessary to steady the bias voltage since it is already steady.
2. Oscillation.-(a) Oscillation in general.
(1) Tuned radio frequency receivers.-Oscillation in a tuned radio frequency receiver may be recognized by the audio frequency beat notes set up between the signal tuned in and the oscillations in the receiver. Thus, if a 1,000 kc station is tuned in and the receiver is oscillating at a frequency of 1,001 kc, a 1,000 cycle note will be heard in the phones. Oscillations of this form may be verified by moving the tuning dial slowly. As the station is approached, the beat note will decrease in pitch nearly to zero, and the note will start to increase in pitch as the dial is tuned further.
This test is an important one, for the presence of an audible note is not always an indication of oscillation. Interference between two powerful nearby stations will cause an audible heterodyne note, but turning the tuning dial will not change the pitch of the note.
Oscillation in a tuned radio frequency receiver may also be detected by placing a finger lightly on the control grid terminal of each of the RF amplifier tubes. At one of the tubes the beat note will stop, and the signal will come through clearly. This is almost always a certain indication of oscillation in the receiver and in the stage corresponding to the tube which is touched.
(2) Antenna loading.-Almost all stages in a receiver are regenerative to a certain extent, but ordinarily this regenerative action is not sufficient to sustain oscillation.
In the case of the first RF stage of a receiver, the antenna forms a part of the load on the circuit. The removal of the antenna may decrease the loading so that enough energy will be fed back from the plate to the grid circuit to overcome the other circuit losses and the stage will oscillate. Sometimes a small antenna will not load the stage enough.
In either case, it is simple to check for oscillation due to improper antenna loading by shortening the antenna and ground terminals with a screwdriver or jumper. If the oscillation stops, then it is probably caused by improper antenna loading.
(b) Oscillation due to interstage plate circuit coupling.-Open by-pass capacitors in plate or screen grid circuits frequently cause receiver oscillation. An example of plate circuit by-passing is shown in Figure 37. Capacitors C1, C2, and C3 by-pass the high frequency currents around the chokes L1, L2, and L3 in the plate circuits.
Suppose C2 were to open. The B+ line carrying all the currents for the plates of the other RF tubes would be at a different RF potential than the bottom of the primary P2. Now current flows in a closed circuit whenever there is a difference of potential, so, if the choke L2 is not large enough, any RF current present in the B+ line will flow through P2.
Thus the plate circuit of V2 will have RF energy that comes from some other stage. This circulation of energy between one stage and another is called interstage-coupling. Oscillation can be caused by this transfer of energy from one stage to another by an abnormal path.
C2 is designed to have a low RF reactance and so pass the RF energy directly to ground and not allow RF current to flow in the B+ line. L2 has a high RF reactance so as to assist in this action. The combination is designed to reduce or eliminate RF voltage on the B + line.
If L2 opens the tube will not have any plate voltage on it. If L2 is shorted, the plate voltage will slightly increase, and the tube may break into oscillation if the RF reactance of C2 is not small enough.
The action of the other tubes in such a circuit will be similar for the same type of failure.
The plate chokes can be replaced by resistors with the advantage that the impedance from plate to line will not change with frequency as it does when chokes are used. The use of resistors, however, will reduce the plate voltage by the amount of the drop across the resistors.
(c) Oscillation due to interstage screen, grid circuit coupling.-Consider the circuit of Figure 38, where the screen grid voltage is fed to the screen grids through the voltage dropping resistor R. Each screen grid has a separate capacitor to by-pass RF to ground.
The high frequency screen currents in the successive tubes of the amplifier are 180 electrical degrees (one half cycle) out of phase with each other, i. e., when the screen current of V1 is rising, that of V2 is falling and so on. This phase shift is due to the internal action of the vacuum tubes.
Now suppose that there were no screen by-pass capacitors at all, or that they were too small, or were open. Then an increase in screen current due to V1 would cause an increase in the voltage drop across R. This would reduce the screen voltage on V2, causing a drop in the screen current of V2. But the drop in the screen current of V2 is greater than the increase in the screen current of V1 because the signal voltage on V2 is greater than that of V1 by virtue of the amplification of the preceding stage.
Without considering V3, then, the net result is a decrease in the current through R, which means increased voltage applied to the
screen of V1. Increased voltage on V1 means more screen current and more screen current means more voltage drop in R, more voltage drop in R means less voltage on the screen of V2, and so on. If these fluctuating currents are not by-passed to ground, oscillation due to inter-stage coupling is sure to result.
On the other hand, the phase of the screen current in V3 is the same as Vi, so that any increase in screen current in V1 will lower the screen voltage on V3. Since the screen current in V3 is in the process of increasing in phase with V1, the net result is a smaller variation in current, and inter-stage coupling is minimized.
The same thing happens if the plate circuits are fed by a common resistor to drop the higher B voltage to the proper value required for the plates of the tubes in some of the stages. In this case, a 4 mfd capacitor usually must be connected to the plate side of the common resistor. This large capacity will maintain the voltage applied to the plates essentially constant during high frequency changes, preventing the cumulative action described.
The action of each by-pass capacitor in Figure 38 is similar in effect so that if one of them becomes open there is a possibility of regeneration and oscillation.
Since the IF and RF current variations in the screen and plate circuits of successive amplifier tubes are a half cycle out of phase, the plate circuits of alternate tubes may be connected through a common resistor without by-passing of the resistor or use of plate circuit chokes without danger of oscillation. Screen grid circuits may be similarly arranged, as shown in Figure 39.
(d) Miscellaneous Causes of Oscillation.-A common cause of oscillation is the changing of the magnetic shielding circuits when high resistance contacts are formed in shield can mounts or shielded lead coverings. The shielding effect is reduced when such a sliding contact becomes poor, and there is a possibility of oscillation due to the presence of the stray fields.
Excessive plate or screen voltage can cause a particular stage to break into oscillation. Analysis usually detects such abnormal voltages without any particular difficulty.
In some of the older receivers a fixed suppressor resistor of about 1,000 ohms was connected in series with the grid of the RF tubes to prevent oscillation. At first glance such suppressors appear to serve no useful purpose and they are sometimes removed. When this has been done the stage usually breaks into oscillation. Figure 40 shows this circuit arrangement.
3. Images and double-spot tuning.-"Image interference" is peculiar to the superheterodyne type of receiver. The image, or unwanted signal, is due to the fact that any signal that can combine
with the oscillator to produce a signal at the fixed frequency of the IF amplifier will be heard in the receiver.
Suppose that the receiver is tuned to 550 kcs as shown in Figure 41.
The 550 kc signal will combine with the 1000 kc oscillator signal to send a 450 kc output to the IF amplifier. If at the same time a 1450 kc signal finds its way into the first detector, it will also combine with the 1000 kc oscillator output to send a 450 kc signal to the IF amplifier. The IF amplifier will pass both signals on to the succeeding stages and interference will result.
"Double-spot" reception can be regarded as a type of image interference. It is marked by the reception of the same station at settings of the controls that are supposed to represent different frequencies. Suppose that the receiver input circuits are not very selective and that a very strong 1450 kc signal is being received at the proper setting. If the tuning is changed to the 550 kc setting, the 1450 kc signal may still be received. This possibility is illustrated by Figure 42.
When the receiver is set at 1450 kc, as in 42-B, the oscillator generates a 1900 kc signal which combines with the input to send a 450 kc signal to the IF stages. When the receiver is tuned to the 550 kc setting, the oscillator will produce a 1000 kc signal which will also combine with the 1450 kc signal to send a signal of 450 kc to the IF amplifier.
It is characteristic of this type of double-spot tuning that the dial points are always separated by twice the IF of the receiver. Thus, in the case above 1450-550=900 = (2)(450) = (2) (the IF).
There are other possibilities for multiple-spot reception. Many oscillators generate unwanted harmonics. Suppose that the oscillator produces a 2000 kc harmonic as well as the required 1000 kc signal. Then it is possible to get four stations simultaneously with the receiver tuned to 550 kcs. The four would be 550 and 1450 kc signals beating with the 1000 kc oscillator output and 2450 and 1550 kc transmissions combining with the 2000 he oscillator harmonic.
It is evident that image interference and multiple-spot, multiple-frequency reception can be eliminated if only one signal is allowed to reach the first detector. To do this, one or more selective tuned circuits are built ahead of the first detector. This circuit system is sometimes called a pre-selector circuit. In some receivers this pre-selector circuit is made up of one or more stages of tuned RF amplification. In others a type of band-pass or band-rejection circuit is used.
In Figure 43-A is shown a conventional tuned stage, and in 43-11 one type of filter arrangement.
If the interfering signal is exceptionally strong, it may leak through a pre-selector system and cause interference even though the receiver is adjusted correctly. In such a case a tuned trap may be connected to the antenna circuit to eliminate this strong signal.
The usual reasons why a receiver is troubled with interfering signals are:
1. Insufficient selectivity in the pre-selector stages.
2. Poor shielding of the RF, mixer, and oscillator circuits.
3. Excessively strong interfering signal.
4. Incorrect adjustment of "image frequency", band-pass or rejector circuit trimmer, or defective circuits.
5. Incorrect tracking of RF oscillator and mixer circuit tuning condensers.
6. Coupling between antenna or ground lead and the mixer or oscillator circuits.
7. Incorrect adjustment of trimmers on IF transformers.
8. Excessive control-grid bias on RF and mixer tubes.
4. Hum.-By "hum" we mean the "singing" type of interference caused by 60 cycle, 120 cycle, or higher frequency alternations getting
into the receiver output. Such interference usually comes from the AC power supplied to the set. In many cases it is hard to locate the source of the hum.
Most frequently, hum originates in the power supply unit. It may be caused by the failure of the filter system to eliminate ripple. In this case it will be caused by an open filter capacitor, a shorted choke, or a combination of the two. In a poorly designed unit, or in a well designed one where the power supply lines have been disturbed, the hum may be fed into any amplifier stage by inductive coupling due to the AC
wiring being too close to a tube grid circuit. Failure to twist together AC leads after repairs are made may result in the coupling of excessive hum into the circuit.
Figure 44 illustrates a circuit in which filament type tubes are used. If the center-tapped filament resistor opens up, the balancing action is upset and hum may result. Some receivers use a rheostat instead of a tapped resistor so that exact balance and hum elimination may be obtained by adjusting the rheostat. If the rheostat arm is accidentally moved, the circuit may become unbalanced. Such a circuit should be readjusted for minimum hum while no station is being received.
Defective heater-type tubes frequently cause hum due to poor insulation between heater and cathode. Low emission or gassy rectifier tubes may cause hum.
If a filter choke becomes partially shorted or the core air gap is changed by mechanical damage, hum will result due to the decreased inductance.
The laminations of a power transformer or choke may be loosened by vibration. Such a hum source can be detected by feeling the unit or touching it with a screwdriver. Bolted laminations may be tightened by taking up on the through bolts.
After a few of the more obvious causes have been eliminated, the most effective way of locating the source of hum is to check through the receiver stage by stage, starting with the last AF or output stage. By shorting out the stages in succession, an individual stage causing hum can be located.
If an electro-dynamic speaker is used, the voice coil (or output transformer secondary) should be shorted as indicated in Figure 45, so that signals cannot reach the speaker. Any hum now heard will be due to excessive ripple voltages in the speaker field. To eliminate this, the current supplied to the field must be more adequately filtered. If the hum is eliminated or reduced to a negligible amount, then the stage-by-stage test must be continued.
The next step is to remove the short on the speaker system and short out the last stage grid circuit as indicated in A of Figure 46.
If the hum is still noticeable, the output stage is at fault. If this stage happens to be of the push-pull type, the shorting test is made as indicated in C of Figure 46. In such a stage an incorrectly located center tap on the transformer may be responsible. Any other unbalanced feature, such as unmatched tubes, center-tapped filament arrangement, etc., which results in failure of the push-pull feature can also cause hum. If the output stage is a single tube, the tube may be defective or the trouble may be in the power supply unit.
If the hum is eliminated or reduced by the shorting of the output stage, remove the short and continue.
Short out the control grid of each stage in succession by the methods indicated in Figure 46.
When a circuit is reached in which the shorting produces no reduction in hum, the trouble, or part of it, will lie in that stage.
Section 2.8 on troubles should be consulted for possible causes. Substitution of by-pass or filter capacitors may be used to check suspected parts.
5. Fading.-True fading is caused by transmission conditions in space and is affected by the behavior of the ionosphere, the frequency used, the time of day, and the type of transmitting antenna used. Fading cannot ordinarily be controlled at a receiving position. It can sometimes be counteracted by the use of automatic volume control devices.
6. Intermittent reception.-The most difficult faults to locate are those that cause intermittent reception. Intermittents may effectively turn the receiver off or on sharply, or they may take hold gradually and the effect will resemble fading.
A few of the points to be watched for are covered below, and others are listed in Section 2.8. The possibilities in this sort of defect are so great that they cannot be fully described in the space available.
Intermittents are caused by voltage failure, failure due to heat, or mechanical failure.
As much information as possible on the way the break in reception occurs should be obtained first. When no information is available, the receiver must be turned on and allowed to run until it fails.
Voltage failures may be due to the intermittent breaking down of capacitors, resistors with creeping currents, momentary shorts and opens on transformers, leaky capacitors, and loose strands of wire which are grounded. High resistance contacts may cause intermittent operation by reduction of voltage.
Low signal level circuits, such as RF amplifiers, are susceptible to changing resistances at such small values as to be hard to detect and measure. Any moving part should be particularly suspected.
Heat failures may occur due to resistors or other parts opening up when hot, or by expansion causing parts to move when heated, and then touch and ground out. It should be noted that a receiver out of its cabinet will not reach as high a temperature as when in its normal position. It is sometimes necessary to cover up the chassis so as to get it warm enough to operate the fault. Quick work in testing may be necessary because of the small heat range near maximum at which the failure occurs.
Typical mechanical failures are broken leads, strands of wire shorting to shields, rigid but poor connections between shields and ground, rubbing capacitor plates, and broken heater leads.
Poorly soldered joints are especially apt to cause trouble of this type. Frequently all joints must be carefully resoldered. The worst places for cold-soldered joints are RF coil lugs and voice coil lugs.
Leads, lugs, and connectors should be prodded gently but firmly with a blunt insulated tool while the receiver is running in order to detect poor contacts.
When reception can be brought back by snapping the power switch off and on, leaky or intermittently open by-pass capacitors should be suspected.
Do not forget to turn off the automatic volume control, since the automatic feature may reduce and mask the intermittent action.
Any one of the many possible causes listed in Section 2.8 may be the one responsible. When checking for intermittents, take nothing for granted, and check everything patiently and systematically.
2.6 Receiver Alignment and Sensitivity Measures.-When receivers are built it is expected that there will be small differences in the parts used, a portion of which will be due to the tolerances allowed the manufacturers. In addition, tube manufacturers are not able to make each tube the duplicate of every other one of the same type. Age, heat, and vibration all introduce changes in the circuits or in the parts. As a result of all these factors, when receivers are first built, and periodically afterwards, they must be adjusted to operate with peak performance.
This tuning up process is known as alignment.
There are two general methods available to the designer by which he may provide for alignment. The most common method is to place small variable capacitors in parallel or in series with the main circuit capacitors. These small capacitors are usually called "trimmers" or "tracking condensers". Some manufacturers call the parallel units trimmers and the series units padders. These small capacitors allow for small variations of the tuning of the individual stages and so permit alignment of the various circuits.
The second method is to provide some means for varying the individual circuit inductances, usually by an arrangement for changing the coupling, or by varying the position of a powdered iron core within the coil.
While the need for realignment is indicated by a decrease in sensitivity, reduced volume of output, and broad tuning, alignment should not be undertaken until the receiver has been put in good operating condition. Every receiver that is operating poorly requires servicing but it does not follow that every receiver that needs servicing will require alignment.
In the first place, necessary repairs will frequently require the replacement of components and the disturbing of wiring, making alignment necessary after the repairs. -Do not align the set until after the parts are mounted and connected. The time and effort spent in alignment beforehand will be wasted.
In the second place, haphazard attempts at alignment by inexperienced and incautious men may do more harm than good and put a receiver wholly out of commission when a relatively minor repair carefully made would obviate the necessity for any further work.
Before alignment is attempted, the instruction book or the manufacturer's service notes on the receiver should be carefully examined.
There will be included in such instructions some indication of the general routine to follow in alignment, the specific frequency points to be used in checking, the IF frequency for superheterodynes, and
other data necessary for quickly and effectively accomplishing the work.
An indicating device and a standard signal generator such as the Model LN or LP are required. The signal generator may be checked with a crystal calibrator or standard heterodyne frequency meter to make sure that its calibration is accurate. When a standard output meter with a built-in DC blocking capacitor is not available, an AC voltmeter and 0.1 to 1.0 mfd capacitor may be used, with the capacitor in series, as shown in figure 49.
The standard signal generators are provided with dummy antennas which must be connected in the generator circuit ahead of the antenna connections when the final check is being made, so that the antenna circuit will be operated at nearly normal conditions.
To prevent DC from being fed back into the signal generator and damaging the output circuits, whenever the dummy antenna is not in the circuit, a capacitor of from 0.01 to 0.05 mfd must be connected into the ungrounded lead from the signal generator to the receiver.
A non-metallic screwdriver or wrench must be used for making adjustments on the trimmers. It is difficult, if not impossible, to use metallic tools for this purpose because of the detuning of the capacitor due to the presence of metal.
The chassis of the receiver should be well grounded to avoid hand or body capacity effects as well as to insure stable operation of the equipment.
Any shielding that has been removed and that need not be off to make the alignment adjustments, must be replaced in its permanent position.
Finally, the instruction book on the signal generator must be checked to avoid violating any of the rules applying to the particular equipment in use. For example, in using the Model LP equipment the entire system consisting of the receiver and generator must not be grounded at more than one point.
The receiver and signal generator must be turned on for at least 15 minutes, and preferably for 30 minutes, before aligning so as to allow all parts to reach their working temperatures.
SPECIAL PRECAUTIONS WITH SERVICE RECEIVERS
When aligning commercial broadcast receivers without beat frequency oscillators, it is necessary to use a modulated signal.
Because service receivers are often very selective, some difficulty may be encountered in using a modulated wave. It will be recalled that a modulated wave consists of the carrier and two sidebands, one above and one below the carrier frequency. In a typical generator these sidebands will differ from the carrier by a kilocycle.
When the receiver is very selective it will be difficult to receive the modulated wave with the carrier and both sidebands in true proportion, an essential for accurate alignment or measurement. This will be especially true of frequency bands below 500 kilocycles.
For service equipment, it is therefore safer to use a pure CW signal.
In commercial equipment it is customary to align with the volume control set for the maximum signal. In service receivers there is often more sensitivity than is useful and at maximum volume the amplified inherent noise of the receiver may be sufficient to saturate the detector or AF stages. Accordingly, it is safer to set the sensitivity or volume control so that not more than 60 microwatts of noise is present in the output when there is no signal input.
As will be noted below, sketches are given showing several possible ways of connecting the output meter. Usually it is best to connect the meter across the proper output load as indicated in Figure 47. Output meters, or volume indicators, are incorporated in some of the newer receivers.
Whether the meter is built in or not, the proper load for all newer Navy receivers is 600 ohms and for all older receivers it is 20,000 ohms. Due to their inductive nature a pair of head telephone receivers should not be used as the load. A non-inductive resistance should be used.
For receivers having an installed meter with a zero level of 6 milliwatts, a -20 decibel reading will indicate the proper noise level. When the meter has a zero level of 60 microwatts, a reading of 0 decibels will indicate the proper noise level setting.
When it is necessary to use a voltmeter as an output meter, 0.19 volts across the 600-ohm load will indicate the 60 microwatt noise setting, and 1.1 volts across the 20,000 ohm load will show that the noise level is likewise set at 60 microwatts.
Caution. Each receiver of a different type is in itself a special problem in alignment. There may be differences in the proper signal generator connections, output meter connections, differences in methods by which alignment is accomplished, and differences in the results that may be expected. The instructions given here outline general methods. Each receiver encountered should be studied for its peculiarities and the basic methods modified to meet special cases.
There are two general connection problems to be solved. The first is the connection of the signal generator and the second is the connection of the output meter.
Figure 47 illustrates the connections to be made to Navy receivers for final check, and for output meter connections under all conditions.
Navy receivers have two types of output circuits, 600 ohm, and 20,000 ohm. To insure proper results, the receiver should be aligned while it is operating normally. This requires that it operate with the designed output load. In case the instruction book is not available, the headphones can be examined, or both high and low impedance phones can be tried to determine whether the output load should be 600 or 20,000 ohms. Older equipment uses the higher value; most new receivers use the lower. The headphones themselves should not be used as the load. This connection is shown in Figure 47-B.
For the final check, the input circuit must also be properly loaded. It is usual to provide signal generators with a standard dummy or substitute antenna unit, consisting of resistance, inductance, and capacity. This dummy antenna is connected in the ungrounded lead from the signal generator to the receiver. In an emergency, a 300 ohm non-inductive resistor or a 100-200 mmfd fixed mica capacitor may be used in place of the dummy antenna. The usual connection is shown in Figure 47-A.
Figure 48 illustrates the methods which may be used in connecting the output meter to non-standard or commercial receivers. In A of the figure the meter is connected across the plate circuit. In B another possibility is indicated, the connection being made across the speaker voice coil.
CAUTION.-Standard output meters have a built-in capacitor in series with the meter to prevent damage from DC. If an ordinary AC voltmeter must be used as an output meter, a capacitor must be connected in as shown in Figure 49. A good capacitor must be used with a working voltage higher than the plate voltage of the receiver.
In aligning superheterodyne receivers, and occasionally others that are badly out of alignment, it is necessary to connect the signal generator to the mixer or to some other stage within the receiver. In making this connection, some care must be used in order not to upset the receiver operation or burn out the attenuator on the signal generator.
Frequently the simple connection of Figure 50-A can be used in which the grid circuit is not disturbed, a capacitor is connected in the high side of the signal circuit, and the ground side is connected to the chassis. Sometimes for mechanical or other reasons, it is necessary to connect directly to the grid cap and grid lead. In this case a resistor must be included to provide a DC path to the tube grid as shown in Figure 50-B. This resistor will permit the tube to be normally biased while alignment is in progress.
ALIGNING TUNED RADIO FREQUENCY RECEIVERS
The signal generator, receiver, and output meter are arranged on a suitable work bench. The signal generator is connected to the receiver as indicated in Figure 47. The central conductor of the special lead is connected to the antenna terminal of the receiver. The lead shielding is usually used as a ground connection although sometimes a separate lead may be used.
The output meter is connected as shown in Figure 47, 48, or 49, the exact connection depending on the receiver. The output meter switch is set to a high range to insure against possible danger to the meter. The generator and receiver are connected to the power line, tuned on, and allowed to warm up.
While the equipment warms up, check the instructions and circuit diagram. Locate the trimmer capacitors, which are usually found on the bottom, top, or side of each section of the ganged tuning capacitor unit in receivers with only one frequency range band. In multiple-band receivers, they are usually mounted at the transformers and inside the shield cans.
If the receiver is an unusual one of this type, it may contain an automatic volume control feature. This must be rendered inoperative, either by properly disconnecting it or by using very low signal inputs so that the AVC action will not operate. (See next section for details.)
Set the receiver volume control to the position for maximum signal output. For most receivers of this type the maximum may be used. Adjust the signal generator to produce the signal frequency equal to the highest in the receiver range.
CAUTION.-The output meter readings are relative in these adjustments, the only requirement being that the maximum be readable during any tuning operation. The meter should not be allowed to read more than 1/3 full scale at any time. During alignment, increase or reduce meter range to maintain the meter readings near this value. Watch the meter during alignment. Carelessness in this respect may result in the destruction of the meter.
If the receiver is not equipped with a beat frequency oscillator, adjust the signal generator to produce a modulated signal. If a beat frequency oscillator is used, leave the signal generator unmodulated and turn on the oscillator.
Now tune the receiver cautiously to the signal frequency, watching the output meter. As the resonance point is approached, adjust the generator output to lower levels or increase the meter range or both, so that the meter never reads more than about 1/3 scale.
When the tuning point which gives maximum output has been reached, take a non-metallic tool and vary the trimmer on the capacitor section that tunes the detector stage. Watch, the output meter. Reduce the signal input if necessary. Set the trimmer on the point that results in maximum output meter indication.
Now retune the receiver carefully, rocking the control slowly across the maximum point to insure getting a control setting that gives the absolute maximum response. Then retune the trimmer for maximum indication.
Without moving the receiver or generator frequency controls, adjust the next trimmer, which will be the one tuning the stage just before the detector, so as to get maximum meter indication.
In reducing the output to keep on the proper part of the meter scale, turn down the signal generator attenuator, and do not change the receiver volume control.
Continue back through the stages, without changing any of the controls except the signal general output, until all of the trimmers have been adjusted for maximum meter indication.
In some receivers this will be all the adjustment possible. In others, the end plates of the tuning capacitor sections will be slotted to provide further adjustment at other frequency settings. Turn the tuning control until the first separated section of the slotted plates just meshes with the capacitor stator.
The signal generator is then retuned until a frequency is reached at which the meter again reads maximum. Starting with the detector section, the meshed plate sections are carefully bent towards or away from the stator plates until a maximum meter indication is again observed.
The tuning control is then varied until two slotted sections are just meshed and the process of bending is repeated. This process is repeated in the manner indicated for the first section until all available frequency points have been tuned, by varying all the split or slotted sections.
There is seldom a need for aligning any but simple commercial broadcast receivers of the TRF type. Consequently the procedure given above is very general. Any special shipboard receivers of this type will have alignment procedures included in their instructions.
ALIGNING SUPERHETERODYNE RECEIVERS
Superheterodyne receivers are usually complex, covering several bands of frequencies. Such receivers must be carefully studied and the instructions consulted before any alignment is attempted.
If the set has automatic volume control, this action must be disabled. In some receivers all that is necessary is to switch off the AVC. In others the circuit must be made inoperative by special means. The AVC tube or tubes must never be removed during alignment.
When the AVC action is controlled by a separate tube, the easiest way to stop its action is to remove the tube, and wrap a slip of thin, tough paper around one of the heater or filament prongs. The tube is then replaced and will be inoperative without disturbing the rest of the circuit to any appreciable extent.
When the AVC action is handled in a combination tube, or when the method described above disturbs the circuit, still other means must be used. When the controlled tubes have a fixed minimum bias arrangement, the lead supplying AVC voltage to the controlled tubes may be broken, or the lead to the AVC tube control grid may be removed.
When the controlled tubes do not have minimum bias resistors, a minimum bias must be supplied when the AVC action is stopped. This may be done by connecting a high resistance potentiometer across a
dry B battery and connecting the plus terminal of the battery to the chassis or ground of the set. The potentiometer arm is then connected to the disconnected AVC line and the resistance varied until a suitable minimum bias is applied to the controlled tubes, preventing abnormal plate currents from flowing while the AVC is disconnected.
If it is not convenient to disable the AVC, the following method may be used. Turn the receiver volume control on full. Connect the test oscillator or signal generator in the usual manner, being sure that its attenuator is at a low setting. By having just enough signal to operate the output meter, the AVC action can be kept to a minimum. As alignment proceeds keep reducing the signal input as much as possible, but not to the point where the noise level interferes.
The three basic steps in actual alignment are:
(1) Align the IF amplifier.
(2) Align (or "track") the oscillator.
(3) Align the tuned RF circuits.
Every superheterodyne displays one special problem that is peculiar to its operation, that is: The frequency generated by the local oscillator of the receiver must always bear a fixed relationship to the signal being received.
For proper operation of the receiver, the RF stages must be accurately tuned to the desired signal. This signal must combine with the local oscillator signal in the mixer so that the resulting difference frequency is exactly the intermediate frequency. This fixed relationship must hold for every frequency in the range of the receiver.
The adjustment necessary to insure this frequency difference being constant is usually called "tracking" the oscillator. Figure 51 shows arrangements for tracking that will illustrate the general principles. These figures show the tuned circuits that are often used to control typical oscillators.
The simplest circuit is shown at A. C is the ganged tuning capacitor for the oscillator and C1 is the small trimmer. C1 is most effective at the highest tuning frequency, the minimum setting of C, since then the small capacity C1 will be an appreciable part of the total. This means that this circuit can be accurately adjusted only at the high frequency end of its tuning range.
The fundamental circuit most generally used is shown at B. C is again the ganged tuning capacitor and C1 is its small trimmer. C2 is a large variable capacitor, which will have little effect on the circuit at high frequencies, but will have control at low frequencies.
Now C2 must be large, but will not have to have a large variation,
therefore C2 is frequently split up into parts as shown in C of the figure. Here C2 is a large fixed capacitor and C3 is a small trimmer in parallel with it.
This whole adjustment is also sometimes called "padding" the oscillator but it is more accurately described as "tracking". A capacitor which functions as C1 in Figure 51-C is usually referred to as a high frequency trimmer, while that corresponding to C2 is normally called the low frequency or series padder.
The general procedure for aligning service receivers will be given below. The importance of studying the set and the instructions before attempting to align any particular receiver cannot be overemphasized.
Take all the general precautions outlined in the first part of this section, making sure all tubes are in their sockets, grid leads are connected, and shields are in place.
I. F. AMPLIFIER
Connect the output meter as illustrated in Figure 47. Connect the signal generator as shown in Figure 50, taking the precautions noted to insure that grid bias is maintained on the first detector or mixer tube.
Disable the oscillator to prevent it from interfering with the alignment of the IF amplifier. This may be done by shorting the oscillator grid to its cathode or the oscillator section of the ganged tuning capacitor may be shorted out. Do not remove this tube to stop its oscillations unless the instructions specifically direct you to do so.
Turn on the signal generator and receiver and allow them to warm up. While the equipment is warming up, recheck the instructions and circuit diagram, and find the intermediate frequency. Most commercial receivers now use an IF of 455 kcs. Some of the older ones and some service receivers use an IF of 175 kcs. Various other receivers use other values, depending somewhat on the frequency range of the set.
Turn up the receiver volume control until the output meter indicates a 60 microwatt noise level or the voltage equivalent, or until maximum volume is reached. Turn on the beat frequency oscillator of the receiver. Reset the output meter on a high scale to make sure it will not be damaged.
Set up the signal generator for unmodulated CW signals and adjust it to the IF of the receiver. (Note that modulated signals must be used for receivers that do not have beat frequency oscillators, such as ordinary broadcast receivers.) Watch the meter and adjust its scale or reduce the signal strength by varying the generator attenuator so as to get 1/3 or less scale readings.
Figure 52 is an example of a typical IF circuit arrangement. The transformers T are tuned by the trimmers C1 to C6 which are, mounted inside the transformer shield cans.
Referring now to this figure as an example, locate C6 which is in the grid circuit of the second detector. Take the non-metallic aligning tool required, which may be either a screw driver or a socket wrench, and slowly adjust C6 until maximum output meter reading is obtained. Watch the meter. It may be necessary to shift the meter range or reduce the signal used.
Next adjust the primary trimmer C5 of the same transformer for maximum output. Then recheck C6 to insure absolute maximum, and
finally recheck the adjustment of C5. This recheck is necessary since the circuits have an effect on each other.
Repeat this process in the next stage back towards the first detector, adjusting C4 and C3, and then rechecking both adjustments. Watch the meter.
Continue back to the first detector with the same adjustments. In the example given this will mean adjusting C1 and C2. When the first detector has been reached, the IF amplifier will have been aligned. It is advisable to repeat the IF alignment a second time, especially if the original adjustments were very far off.
Disconnect the signal generator, and reconnect across the receiver input as shown in Figure 47. Do not disturb the output meter. Remove the short or otherwise restore the oscillator to operation.
Ordinarily the instructions give two frequencies on which RF stage alignment and oscillator tracking is undertaken for each frequency band. In broadcast receivers where these are not specified, it is assumed that they are 600 and 1400 kcs.
Sometimes strong broadcast stations will interfere if the recommended frequencies are used. If so, use a frequency slightly higher or lower than that specified. For example, use 1500 or 1600 kc for the high end, and 550 or 650 kc for the low end of the broadcast band.
There are two methods of adjusting the oscillator depending on how badly out of alignment the receiver may be. First it will be assumed the misalignment is not too bad. It is assumed that it is possible to force some signal through the receiver on any frequency.
Pick the higher of the two frequencies of adjustment for any band. Set the tuning control of the receiver very accurately. Set the signal generator to the same frequency. The signal level should be increased slowly to get an indication on the output meter.
Now adjust the high frequency trimmer of the oscillator for maximum meter reading. Decrease the signal input rather than increase the meter range if the output is too high, since caution must be used to avoid overloading the RF stages of the receiver.
Sometimes the receiver will be too badly out of alignment to permit the first method being used, and when set up as above no output can be obtained.
In this case the second method must be used. Set the signal generator on the exact frequency. Then tune the receiver to this signal regardless of the point of reception. It is now necessary to align approximately the RF stages so that exact alignment may be undertaken.
For example, suppose the specified frequency is 1400 kc. The tuning control might now be set on 1300 kc to get a signal. Start
approximate alignment by changing the receiver tuning towards 1400 kc until the signal just registers on the output meter. Adjust the RF and high frequency oscillator trimmers. (See procedure below.)
It may have been possible to set the tuning control on 1345 kc for this preliminary check. Now again shift the tuning towards 1400 kc. Readjust the trimmers for maximum output.
Repeat this process until the tuning control can be set exactly on 1400 kc by the receiver dial markings, and some of the 1400 kc signal registers on the output meter.
The alignment for the now frequencies is usually accomplished by adjusting the padding condenser after the RF section has been aligned.
R. F. AMPLIFIER
It is usually necessary to align the RF stages only on one frequency for each band, and this frequency is specified in the instructions.
Set the signal generator on the specified frequency. Tune the receiver to this frequency by setting the control on the designated point. Again watch the meter, reducing the signal input from the generator if necessary.
The trimmers should be adjusted, starting with the one in the grid circuit of the mixer or first detector, then working back to the antenna circuit. When the adjustment has been made once, rock the tuning control gently across the setting point to insure getting the optimum point which may differ slightly from that marked on the dial. If it is found to be more than a slight amount off of the marked point, the trimmers may be readjusted while the control is being rocked slowly so as to get the absolute maximum.
If the best tuning point is very different from the marked point on the dial, it is usually best to reset the tuning control on the mark and recheck the trimmers, starting with the mixer.
There are so many possible combinations of RF stages and pre-selectors that it is not practicable to refer specifically to a. typical circuit.
There are some general rules that apply to all circuits. Always start from the mixer and work back to the antenna. It should be assumed that every coil has its trimmer unless the instructions and the circuit show definitely that it does not. The entire adjustment should be repeated at least once after the first trial to avoid misalignment errors due to the influence of one circuit on another.
Having aligned the oscillator and RF trimmers at the high frequency end of the band, the low frequency adjustments should now be made. Set the signal generator to the proper low frequency setting and tune the receiver dial to give maximum output at this frequency.
Next turn the padder adjustment slightly to one side and retune the receiver dial. If the output meter indicates a higher reading, continue the adjustment. If a lower reading is obtained, turn the padder in the opposite direction.
Continue adjusting the padder slightly as the tuning dial is slowly rocked back and forth. The operation is complete when that combination of dial setting and padder adjustment has been found which produces the highest possible reading on the output meter.
The signal generator and receiver dials should now be returned to the high frequency setting previously used. Adjust the oscillator, first detector, and RF trimmers in the order named. Note that when the low frequency padder is changed, the high frequency trimmer must be readjusted.
After the alignment has been completed, whenever it is possible, the receiver sensitivity should be measured to obtain data for comparison later in determining how much it has gotten out of alignment and to make sure that the sensitivity compares favorably with similar receivers in good condition and proper alignment.
The sensitivity of a receiver, that is, the measure of its effectiveness in taking a minimum strength of signal input in order to deliver a useful value of signal output, is of great interest in determining whether it is operating properly.
The methods used in measuring sensitivity, definitions, and the sensitivity to be expected of receivers will vary with the progress made in designing and building radio communication equipment. In this pamphlet, the current definitions will be used and the simplest procedure for sensitivity measurement outlined.
As a general rule before making sensitivity measurements it is best to consult the equipment instruction books, the Manual of Engineering Instructions, the Bulletins of Engineering Information and other official sources. This will insure that the data taken are best suited for use by the ship and will be most useful to those interested.
Sensitivity measurements should always be made on receivers after they have been aligned to determine the adequacy of the alignment. They should be made on receivers from time to time in order to determine the need for servicing or alignment.
Never attempt to align a receiver unless the need for it has been definitely established, and then only when the proper instruments and tools are available.
The standard definition of shipboard receiver sensitivity is:
The number of microvolts of RF input, modulated 30 per cent at 1000 cycles and introduced through a prescribed standard antenna, that will result in an output of 6 milliwatts when the receiver gain is adjusted to give an output noise level of 60 microwatts or less.
In other words, sensitivity is measured and expressed in terms of the microvolts input for 6 milliwatts output with a signal-to-noise ratio of 10:1 in volts or 100:1 in power. Most modern receivers are designed to deliver the output power across a 600 ohm load resistance.
The required ratios will be obtained when a 600 ohm non-inductive load resistance is used, the noise voltage across the resistance is 0.19 volts, and 1.9 volts is used as the standard output signal level.
Stated differently, sensitivities should be expressed in the number of microvolts needed to deliver 6 milliwatts of output when the receiver gain has been adjusted so that the noise level of the receiver alone is 60 microwatts or -20 decibels.
Ordinarily, 10 microvolts is the minimum sensitivity that should be tolerated, when it is measured under the standards described above. If the receiver should fall under this figure it is usually necessary to realign it.
The equipment is connected as shown in Figure 47. Notice that sensitivity measurements may be made with the receiver in its operating position.
While the receiver and signal generator are warming up, check the instructions for the particular equipment being used. These instructions normally include frequencies at which the sensitivity is to be measured on each frequency range band.
Set the receiver controls as specified in the instructions. Adjust the signal generator to the first frequency to be used. Tune the receiver very carefully to this frequency, watching the output meter to avoid damaging it. Reduce the signal input or receiver gain as necessary to get a good safe reading on the meter.
After the receiver has been accurately tuned to maximum reading, adjust the antenna compensator, or trimmer, until maximum output is obtained. Most shipboard receivers have such a trimmer, sometimes described by other names.
Interrupt the signal by throwing the plate switch of the signal generator to OFF. Adjust the receiver sensitivity control until the standard noise level of 0.19 volts is indicated by the output meter. This main control setting must not be changed until the value of sensitivity has been read and recorded.
If the standard noise level cannot be obtained it is a sign of either misalignment, poor tubes, or both.
Now throw the signal generator plate switch to ON and adjust the signal modulation as prescribed in the instruction for 30 percent modulation. It may be necessary to adjust the generator attenuator in order to avoid overloading the output meter.
After the modulation has been adjusted, vary the signal generator attenuator until the output meter reads exactly 1.9 volts. The reading of the attenuator in microvolts is the sensitivity of the receiver at this particular frequency, and must be recorded before proceeding to the next frequency.
The entire procedure must be repeated for each frequency of each band, taking all the necessary precautions. Any large discrepancy between the readings for a given band as well as any value greater than 10 microvolts indicates that the receiver needs alignment on that band.
2.7 Noise and Interference
Properly speaking, anything that reduces the ability of the operator to understand the desired signal is interference. However, it is convenient to list the causes of interference under two headings. In this system, interference is the result of definite radio transmissions, intentional or unintentional. Noise is the result of incidental radio transmissions or the result of a circuit defect or characteristic.
Receiver noise may be due to:
(a) Atmospheric disturbances, usually called "static".
(b) Electronic phenomena.
(c) Defective circuits or parts.
(d) Man-made "static", from electrical devices.
Receiver interference may be caused by:
(a) Transmissions on the working frequency.
(b) Interstation interference.
(c) Key clicks or thumps.
Disturbances in the atmosphere, or static, may originate from the action of the charged particles of dust, water, or snow in the air. Such particles can cause charges to build up on masts, superstructures, or whole aircraft by motion or impact. Aircraft in flight pick up relatively enormous charges by this means. Leakage or discharge currents from such charges cause static noises. Clouds may become highly charged by fraction between dust particles or water droplets and the surrounding air. Two cloud banks, or a cloud bank and the earth, then form the plates of a giant capacitor. When a considerable charge has been built up, the insulation properties of the air break down and an electric discharge takes place. In its most severe form such a discharge is called lightning, although it is not necessary to have lightning to have static, since it often occurs as a result of intermittent leakage currents that flow between clouds. No practical method of eliminating static has been discovered as yet.
There are two electronic phenomena that cause receiver noises. One is the noise caused by the random flow of electrons in a vacuum tube,
which is called the "shot effect". The other is caused by the random movement of electrons in the conductors of the receiver input, including the coils of the first stage grid circuit. This movement is said to be due to "thermal agitation", since it is heat that causes the effect. These electronic noise sources are especially important in receiver input circuits because they limit the receiver sensitivity. If great amplification is used, these inherent noises will be amplified so much that weak signals cannot be raised above the noise level. Once a receiver is built, there is little that can be done about such noises.
Noise may originate in any receiver circuit or part that is defective, including almost everything listed in the Trouble Charts of Section 2.8. A partial list of most probable causes is given below.
1. Defective tube.
2. Shorting or grounding leads.
3. Loose or poorly made connections.
4. Defective resistor.
5. Defective power supply.
6. Defective speaker.
7. Insulation break down.
8. Dirty or peeling gang-capacitor plates.
9. Dirty gang-capacitor rotor wiping contacts.
10. Poor connection to antenna or ground.
11. Pad fuse connection, switch, or power line defect.
12. Run down battery.
Methods of locating and correcting many such faults will be found elsewhere. A few additional items will be given here.
Tubes, or tube connections at the socket, are frequent offenders. Each tube should be lightly tapped with the finger while the receiver is operating. If the tube contains loose elements or there are loose socket contacts, a loud noise will be heard. The tube should be replaced if moving it in its socket shows that the trouble is in the tube and not in the socket.
A quick visual inspection before and while jarring the set may reveal a loose connection.
If the trouble has not been located, then turn on the receiver. Remove the first RF tube. If the noise stops, check that stage. If it does not stop, remove the second RF tube or mixer and check. Continue until the offending stage is located. When the noise is still heard after all but the last audio tube has been removed, the possibility that the trouble is in the power supply should not be overlooked.
A detector plate by-pass capacitor will often become noisy, breaking down intermittently under the plate voltage or signal surges. An ohmmeter continuity test will not necessarily show such a defect, since high voltage is necessary for breakdown. The capacitor may be disconnected to check this source of noise.
In power or other biased detectors, the cathode by-pass capacitor sometimes fails. When leaky, this capacitor will cause annoying and irregular frying or sizzling sounds.
Another source of noise in audio circuits is the breaking down of insulation under high voltage, causing a spitting or buzzing noise. Often the noise is loud enough to indicate the location of the faulty part. Unless the breakdown is inside a transformer, a visual inspection in the dark will usually find it.
If the breakdown point cannot be seen, the high voltage leads should be traced out, separating them from each other, from other leads, and from the chassis.
Because the plate voltage may be high, the primary of an audio transformer is apt to break down, although either the primary or secondary may do so. An insulation test can be made to check the conditions between the case and the windings. Often a check can be made by substituting a good transformer for the suspected one. Sometimes it is easier to convert temporarily the coupling to the resistance-capacity type as shown in Figure 53.
For the primary side, disconnect the plate lead as shown in A, and connect in a plate resistor and coupling capacitor. The circuit may now operate at reduced volume but it should operate quietly, if the source of the noise was in the primary. The secondary can be checked similarly, as indicated in B.
It will be found that the plate circuit resistor in a resistance coupled circuit often becomes noisy. This can be checked by the substitution of a new resistor of the proper value.
In addition to the very common defect of open filter capacitors, there are several items that might be mentioned in connection with the power supply. Often a voltage dividing resistor will "creep" or "spark" across one or more sections. This sparking is due to leakage paths along the insulation or to imperfections such as metallic particles in the cement or enamel covering. Usually such sparks may be seen in a dark room.
The high voltage secondary winding of the power transformer may break down intermittently. The sound of the sparking frequently leads to the identification of this fault.
The line switch or fuse mounting may get noisy as the contacts wear or become corroded. Such action is especially likely when the set is subject to vibration.
When there is dust or flaky, peeling electroplating between the plates of a variable tuning capacitor, the alternate shorting and clearing will cause noise. A pipe cleaner will usually serve to clean out the heavier matter and the rest of the dust can be blown out.
Above all, corroded or poorly soldered joints cause noise. When tracing noisy conditions it is often the best policy to clean all tube socket contacts and to resolder every joint that looks at all suspicious.
Man-made interference may enter a receiver:
(a) Through the antenna system.
(b) Through the power supply line.
(c) Through the receiver circuits themselves picking up the direct or reradiated interference.
(d) Through a combination of the above.
The first step in combating this type of noise is to find the path through which it is entering the receiver.
Some noises are tunable, that is, they can be tuned in and out like any regular signal. Those that are tunable are usually the result of radiation, those that are not are generally due to direct conduction.
Tune the receiver to a frequency point where no signal is heard and increase the volume or gain control to a maximum. Remove the antenna lead from the receiver antenna terminal, and see that it is several feet from this terminal.
If there is a decrease in noise, there is interference outside of the receiver and a part of it at least is being picked up by the antenna.
If there is no decrease in noise, short the antenna terminal to the ground connection with a short piece of wire and listen again. If the noise drops off to an insignificant amount the antenna circuit is picking it up.
If the noise remains, remove the ground lead to eliminate the possibility that the ground lead is feeding in noise, and listen again.
If the noise is still apparent, it is most apt to be reaching the receiver through the power supply line, assuming that it is not due to a defective receiver circuit.
It is difficult to isolate the receiver from the power supply line and locate the source of noise from this cause unless another power source is conveniently near. Where dual motor generator sets or other power supply equipment is used for the receivers, the alternate supply should be cut in to eliminate the possibility of a defective supply.
Once the general path by which the noise enters the receiver is determined, then the search for the cause can begin.
In searching for an interference source, it is helpful to have some idea as to the type of device that creates each type of noise. For that reason it saves time to check the nature of the noise and try to classify it by its characteristic sound. The condensed table below is not complete, since much space would be required for such a tabulation, but it will serve to start the search.
Noise characteristic table
Crackling, Scraping, Sputtering, or Short Buzzes
Loose bulbs in light sockets.
Loose or corroded connection in power circuit.
Loose connection at antenna transfer panel.
Antenna grounding intermittently.
All types of switches.
Controllers for motors, cranes, etc.
Heater control thermostats.
Relays or contactors.
Transmitter harmonics or key clicks.
Antenna or ground lead parallels power line.
Rushing and Buzzing
Boat or plane ignition.
Moving picture machine.
Diathermy, X-ray, or other medical apparatus.
Machine Gun Fire or Rattling
Boat or plane ignition.
Annunciators and call bells.
Vibrating rectifiers, or vibrators.
Sewing machine, dental laboratory, or other small motor.
Whistle or Squeal
Nearby receiver oscillating in first stage.
Heterodyne with another signal.
Other small commutating motor.
Electric razor. Gun director circuits.
The most effective and simplest method to track down interference is to station one man at the receiver to note any change in interference, while another starts and stops each motor or other possible source.
After a source has been found, the next step is to eliminate the interference. The first thing to do is to check the particular piece of equipment to make sure it is operating correctly and does not require overhauling. Almost any defect covered in the section on power supply equipment will produce noise.
Once the device has been put in good condition, the next step usually requires the use of a filter, which should be placed as near the equipment as possible. There are two methods of reducing or eliminating apparatus-produced radio noise. One is the use of low impedance shunt filters. The other is the use of high impedance series filters. The method must be suited to the particular noise source.
Low impedance shunt filters are simply capacitors of proper size and rating. The most effective arrangement, shunting both lines and the pair of lines to ground, is shown in Figure 54-A.
The capacitors used may range from 0.1 mfd to 2 mfd, with 1 mfd frequently proving a satisfactory value. The voltage rating of the capacitors should be twice the nominal line voltage to take care of
voltage surges. In circuits carrying heavy loads or subject to fluctuating loads it is good practice to connect a suitable fuse between the capacitors and the lines as a safety measure.
The importance of having the filter close to the source and of having the filter connecting leads as short as possible cannot be overemphasized. The closer the filter is to the noise source, the less chance that noise will be coupled into adjacent circuits. The shorter the leads, the less chance that the lead inductance will cause the filter to be resonant, thus increasing its effectiveness at some particular frequency at the expense of the whole range of frequencies it should cover. As an example, in a certain case it was found that a filter with 2-foot leads was almost entirely ineffective. The leads were shortened to 3 inches, and interference was cut out up to 16 megacycles. With one lead lengthened to 6 inches, the arrangement was only satisfactory up to 1.5 megacycles.
Figure 54-B shows the simple arrangement of high impedance series choke coils. Such an arrangement may be used on high voltage lines, provided properly insulated wire of the right size is used in the coil. There cannot be too high an inductance in chokes used in AC lines, but in extreme cases iron cored chokes have been used in DC lines.
Figure 55-A illustrates a combination type of filter used in some cases. In B a suppressor is illustrated. This type of small filter is particularly effective on small devices such as vibrating contactors. The lowest value of resistor has the greatest spark suppressing action, but the arc may burn the points. Thirty-ohm, 3-watt resistors together with 0.5 mfd capacitors have been used very successfully with bells and buzzers.
When constructing a simple choke for noise reduction, the wire size selected must have sufficient current carrying capacity and insulation so that it will not break down under the circuit voltage. Wire specifications give data about the insulation, and a wire table will give the current carrying capacity. The following short table covers a few of the common sizes that may be available.
B & S wire gauge
Safe current capacity in amperes
A simple choke might consist of 75 turns of the selected wire wound on a 2-inch form, covered with a piece of empire cloth, over which 75 more turns are wound, to total 150 turns. The coil should be taped to hold the turns in place.
Commutator type motors can radiate a considerable interference field in the vicinity, and can feed noise into the line that will travel long distances. Shunt capacitor filters from the brush connections to the grounded frame nearby should be tried first. In bad cases the more complicated arrangement of the combined filter shown in Figure 56 may be tried. A reduction in the noise level due to the motors of the average ship can be made by seeing that all commutators are cleaned, turned down if worn, brushes properly fitted, and checked to insure against defective coils. Often an overhaul will render a filter unnecessary.
Thermostats, contactors, bells, and buzzers will frequently be cured by the suppressor type filter of Figure 55-B. If it does not and the contacts have been adjusted to open properly, then the schemes of Figure 57 can be tried.
Two 0.5 to 1 mfd capacitors C-C should be connected across the line, as shown. In cases where the contacts break considerable current, two additional capacitors C1-C1 may be required. In very severe cases the line chokes L-L may be helpful.
Blinker light circuits sometimes cause a very annoying interference, because they not only feed noise into the line but also run close enough to receiving antennas to radiate a field which couples into them.
While the suppressor type of series capacitor and resistor filter is often effective, sometimes it is necessary to use more complicated arrangements such as those of Figure 58 with 2 mfd capacitors.
The electric refrigerator is occasionally overlooked as a source of noise. When new they cause little disturbance but as the starting contacts become worn the noise of starting increases. The motor and contacts should be checked first. If the trouble is not cured, then shunt and series filters should be tried.
Refrigerators with belt driven compressors sometimes cause noise for an unsuspected reason in dry weather. The belt collects a static charge which leaks off intermittently. A simple remedy for this type of hash is to ground the motor and compressor to the box frame or other large metal mass nearby.
Shielding a device to eliminate noise is sometimes done, but it is worthwhile only in extreme cases because of the difficulties involved. Complete shielding is necessary, which means the entire unit must be enclosed in carefully grounded copper mesh. This makes the equipment very hard to service. A line filter must also be installed and it must be inside the shield. This is because the shield only eliminates the direct radiation, which does not ordinarily travel very far. Thus to be effectively silenced, the noise must be drained off the line before the line passes out of the shield.
Only a few of the steps necessary to eliminate noise from man-made static have been touched on. A detailed study would require more space than can be given to the subject here.
It should not be forgotten, when hunting for hard to find noise sources, that tenders and many yards have been furnished noise locators. These locators are essentially small portable receivers which can be carried around to possible noise sources and thus locate offending devices.
Interference due to transmissions on the working frequency is beyond the control of a ship. The only thing that can be done is to log carefully the data on the interfering station and see that the proper
reports are turned in. However, care should be taken to see that such transmissions are actually being made on the frequency and are not due to something local. For example, leaving a shield-can off may not disable a receiver, but it may allow an "image" signal to get into the circuit that ordinarily would not do so.
There are several types of interstation interference. One is image interference, which has been covered in Section 2.5. Another is adjacent channel interference, which results from stations, close together in frequency, transmitting simultaneously. It is this type of interference that produces the heterodyne whistling noise in broadcast
receivers, for example. This tone noise is characteristically continuous, and is often termed "peanut whistle."
There is also internal cross modulation, which is usually due to poor design or to the use of certain tubes. This type of trouble will not often be found in ship equipment.
A type of interference that is sometimes baffling is that due to external cross modulation. This type occurs because an undesired rectifier is formed in a metallic system, or in an antenna. This rectifier takes the form of a corroded joint or contact, copper being particularly apt to form such joints when oxidized. The rectifier converts an unwanted and otherwise remotely located signal into one that feeds into a receiver on a working frequency setting. An antenna may feed such a rectified signal directly into a receiver. An adjacent object, such as a copper pipe, may reradiate the converted signal and so couple it into the working antenna.
Key clicks and thumps are often very troublesome. In most modern transmitters, the keying circuits and associated relays have been designed so that when the circuits are operating properly, no clicks or thumps are created. When this type of interference is experienced, the first thing to check is the transmitter itself to make sure the trouble is not caused by defective relays or keying circuits.
When the interference is due to the key itself, it may be necessary to install a suppressor type filter as illustrated in Figure 55-B. In aggravated cases, the extreme remedy of filters such as Figure 55-A or Figure 58 may be required.
Any receiver may be required to receive the weakest possible signal at any time. If it is to do this, the ship's interference level must be low. A low noise and interference level can only be obtained by careful elimination work and unceasing vigilance.
2.8 Trouble Shooting Chart For Common Receiver Troubles
1. Antenna too close to power lines.
2. No ground wire.
3. Antenna near that of an oscillating receiver.
4. Pickup in ground wire.
1. Gassy AF power tubes.
2. Unmatched AF power tubes.
3. Center-tap connection open.
4. Tubes with low emission.
5. Leakage, cathode to heater.
Power Supply Unit
1. Open filter capacitor.
2. Loose laminations of power transformer.
3. Shorted filter choke.
4. Loose laminations in filter choke.
5. Shorted filter choke by-pass capacitor.
6. Open filter choke by-pass capacitor.
7. Electrolytic filter or by-pass capacitor dried up.
8. Open line-voltage supply buffer capacitor.
9. Defective rectifier.
1. Charger operating while receiver is in operation.
1. Exhausted battery.
Receiver Circuits Proper
1. Open center-tapped resistor or hum control.
2. Hum control or balancer out of adjustment.
3. Push-pull input transformer secondary unbalanced.
4. Open AF secondary winding or grid resistor.
5. Open or leaky line supply by-pass capacitor.
6. Shorted bias resistor or by-pass capacitor.
7. Open screen or cathode by-pass capacitor.
1. Sensitivity of receiver inadequate.
2. "Dead-Spot" for reception.
3. Line voltage too low.
1. Antenna or ground disconnected.
2. High resistance leaks or grounds.
3. Antenna too short.
4. Antenna too close to grounded object. 5. Shorted lightning arrester.
1. Tubes with low emission.
2. Wrong type.
3. Loose elements.
5. Control-grid cap not soldered to lead properly.
1. Weak or gaseous rectifier tubes (filament type).
2. Weak or exhausted rectifier tube (gas type).
3. Open voltage-divider section.
4. Voltage divider changed value.
5. Transformer winding partially shorted.
6. Leaky or shorted by-pass capacitor.
1. Battery exhausted.
2. Corroded battery terminals.
3. Charger has not been functioning.
4. Dead cell.
1. Battery exhausted.
2. Battery terminals (intermediate and high) reversed.
Receiver Circuits Proper
1. Tuned stages out of alignment.
2. Open RF coil.
3. Open AF transformer.
4. Open plate or grid resistor or suppressor.
5. Open or leaky by-pass capacitor.
6. Open, leaky, or shorted coupling or isolating capacitor.
7. Antenna binding post grounded.
8. Shorted by-pass condenser.
9. Open bias resistor.
1. Speaker out of adjustment.
2. Spider on cone worn.
3. Voice coil winding partially shorted.
4. Field coil shorted.
5. No field coil voltage supply.
6. Field coil open.
7. Defective rectifier for speaker field supply.
8. High-resistance connection.
1. Natural static.
2. Man-made static due to electrical devices.
3. Nearby regenerative receiver.
4. Loose lamp fixtures.
5. Loose wiring in building.
6. Loose line fuses or lamps.
1. Antenna too long.
2. Antenna too short (noise within building).
3. Loose or corroded connections.
4. Antenna or lead-in too close to power lines or line-supply cord.
5. Antenna or lead-in near electrical devices.
6. Antenna grounding to nearby antenna or grounded object.
7. Corroded lead-in strip.
8. Break in antenna circuit.
9. Defective lightning arrester.
1. Loose elements in tubes.
2. Shortening elements.
3. Corroded tube pin terminals.
4. Tubes with low emission.
5. Poor oscillator tube.
Power Supply Unit
1. Sparking, porous voltage divider.
2. Punctured filter or by-pass capacitor arcing.
3. Noisy carbon resistors.
4. High-voltage winding of power transformer arcing over to shield.
5. Loose or corroded line switch, or fuse contacts.
6. Carbonized rectifier socket.
7. Leaky line buffer capacitor.
1. Battery sulphated.
2. Terminals corroded.
3. Charger operating while receiver is in operation.
1. Noisy carbon resistor.
2. Wire-wound resistor arcing across section.
3. Noisy AF transformer primary.
4. Noisy volume control resistance element or contacts.
5. Capacitor gang plates peeling or burred.
6. Dirty or corroded condenser gang rotor wiping contacts.
7. High-resistance, poorly soldered, connections especially in RF circuits, chassis soldered grounds and grid connections.
8. Leaky by-pass condenser.
9. Corroded tube socket contacts or prongs.
10. Inadequate shielding of receiver.
1. Speaker out of adjustment.
2. Snapped spider.
3. Scraping voice coil.
4. Poorly soldered connection.
5. Unfiltered field supply.
6. Loose connection.
7. Loose apex.
8. Torn or worn cone.
9. Loose armature.
10. Loose mounting nuts or bolts.
1. Receiver incorrectly wired.
2. Receiver incorrectly connected.
3. Distress signal on the air.
4. Receiver not turned on.
5. Station not transmitting.
6. No power supply voltages.
1. Tubes burned out.
2. Tube shorted or paralyzed..
3. "Flat" oscillator tube.
4. Faulty tube prong contacts.
5. Series-connected pilot lamp burned out, so other tubes in set do not light.
Power Supply Unit
1. Not connected to power supply.
2. Fuse blown.
3. Rectifier inoperative.
4. Line plug reversed (DC).
5. Filter choke open.
6. Open voltage-divider section.
7. Open bias resistor.
8. Shorted filter capacitor or by-pass capacitor.
9. Rectifier tube socket fused.
10. Loose connection.
11. Shorted power transformer winding.
12. Open high-voltage winding, or section of power transformer.
1. Battery exhausted.
2. No water in battery.
3. Corroded battery terminals.
4. Dead cell.
1. Open RF coil (primary or secondary) .
2. Open AF transformer (primary or secondary).
3. Open plate or grid resistor.
4. Open voltage-divider section.
5. Shorted by-pass capacitor.
6. Open or shorted coupling or isolating capacitor.
7. Shorted tuning, compensating, or neutralizing capacitor.
8. Line switch open.
1. Speaker disconnected.
2. Voice coil open or shorted.
3. Field coil windings open or shorted.
4. Open or shorted output transformer secondary.
5. Open or shorted output capacitor.
6. Open output choke.
7. Open hum-bucking coil.
1. Improper tuning.
2. Weather conditions unsatisfactory.
3. Two stations transmitting at or near same frequency.
4. Nearby oscillating receiver.
5. Poor modulation of transmitting station.
1. Antenna too long.
2. Insufficient antenna.
3. No ground wire.
1. Gassy tubes.
2. Wrong type tubes.
3. Cathode-heater leakage.
4. Weak AF power tubes.
1. Defective bias resistor.
2. Shorted bias resistor by-pass capacitor.
3. Leaky or open coupling or isolating capacitor.
4. Open AF transformer secondary.
5. Tuned circuits adjusted too sharply.
6. Plate or screen voltage high.
7. Bias voltage too high or too low.
8. Push-pull input transformer secondary unbalanced.
9. Open plate, screen, or cathode by-pass capacitor.
10. Pilot light socket or wiring shorting against chassis.
11. Dirty wiping contact on gang-capacitor rotor.
12. Loose or dusty coil or tube shields.
1. Speaker out of adjustment.
2. Spider on cone snapped.
3. Voice coil rubbing on pole piece.
4. Armature not centered.
5. Cone out of round or warped.
6. Cone too soft or too stiff.
7. Speaker overloaded or not matched to output.
8. Insufficient field coil energizing voltage.
9. Poor rectifier in field coil supply.
SYMPTOM-INTERMITTENT RECEPTION OR FADING
1. Poor operation of transmitting station.
2. Intermittently interrupted line supply.
3. Natural atmospheric fading.
1. Antenna lead or terminal shorted to chassis.
2. Loose connection.
3. Corroded joint between antenna and lead-in, or at any other joint or connection.
4. Faulty contact, ground lead, or antenna lead-in terminals.
5. Lead-in wire snapped in middle.
6. Antenna touching or rubbing against conducting or semi-conducting surface.
1. Gassy tubes.
2. Cathode-heater leakage (heater type).
3. Inter-element short.
4. Poor prong contacts at socket.
5. Cold joint, prong to wire lead.
6. Faulty weld at element support, moved by heating.
7. Emissive coating flaked and fallen between elements.
8. Heater cathode buckle.
Power Supply Unit
1. Fluctuating line voltage.
2. Poor contact in line switch.
3. Poor contact or burning contact at fuse block.
4. Corroded line switch terminals, or fuse clip contacts.
5. Leak between transformer winding and core.
6. Improper contact between slider and voltage divider.
7. Faulty contacts under filter capacitor cover.
8. Broken turn on divider, only apparent when breaking down or hot.
9. Current "creeps" between divider taps.
10. Eratically open filter choke.
11. Leaky filter capacitor.
1. Loose connection.
2. Battery run down.
3. Renew acid.
1. Tube checks but does not oscillate or oscillates intermittently.
2. Tuning capacitor plates flake and partially short.
3. Breakdown in mica trimmer capacitor.
1. Cold soldered joint at lug.
2. Poor but rigid connection between shield can and mounting.
3. Cross-wire, winding to form.
1. Inter-winding leak.
2. Bad insulation-winding to core. 3. Lead riveted but not soldered.
1. Rubbing and dirty plates.
2. Poor connection-rotor to chassis.
3. Peeling plates-high resistance contact.
4. Rotor "wipers" ridged or dirty.
5. Poor insulation on trimmer.
1. Open or short at inside connection.
2. Leak-elements to grounded case.
1. Shorting to one another, warped, or moved.
2. Intermittently open.
3. Loose terminal connection.
4. Cracked carbon rod.
5. Contact with other parts.
6. "Creeping" current between turns, wire wound type.
1. Loose terminal wire-at inside lug.
2. Dirt and wear under moving contact. 3. Loose wire.
1. Loose connection to voice coil.
2. Open or shorted voice coil or field coil.
3. Armature stuck.
4. Loose apex.
5. Scratched voice coil.
1. Leads shorting to one another or chassis intermittently.