Most guides about radio deal primarily with details of electronic theory. It is often assumed that a thorough knowledge of circuit design and theory is all that is needed for troubleshooting. But expertness in troubleshooting does not come automatically from a study of theory--it requires the use of special techniques which, though based on theory, are more extensive than theory alone.
Troubleshooting consists of selection and orderly examination of key test points leading to a logical deduction of the cause of a symptom. This requires a study of the symptom itself, taking into consideration the frequency of the various types of failures, the reliability of test results, and the practicality of using certain kinds of test instruments. The effect that a defective component has on the operation of other parts of the receiver must be understood from previous experience and study. The physical construction of receivers and components leads to variations in the test procedures, and this re quires the mental sifting and sorting of circuits into groups suspected of containing the fault. Then a test procedure must be decided on, which will eliminate entire sections of the suspected circuitry with a minimum of tests.
Theory will not be neglected in this guide where it contributes to the study of the test procedures, but the emphasis will be on the art of troubleshooting radio receivers. The skillful technician, like all highly proficient journeymen, combines his knowledge of electronic fundamentals with his experience and intuition to achieve a blend of techniques which is as unique as his own personality. Symptoms of failure in receivers can be successfully approached in many ways, and the procedures set forth in this guide are basic examples to be expanded by the technician as he grows through experience.
ORGANIZATION OF THIS GUIDE
The sections in this guide are grouped into three parts:
AM Receivers with Tubes; Transistor Receivers; FM and Stereo Multiplex Receivers.
Each part contains sections devoted entirely to troubleshooting as well as sections explaining the necessary theory. A SERVICING CHART which summarizes the isolation procedures described is included at the end of each troubleshooting section.
The following letters and abbreviations used on the charts indicate the types of tests to be used:
E Voltage Measurement
R Resistance Measurement
Sep Oscilloscope Waveform
S Substitution of New Parts
I Signal Injection
Opn Opening the Designated Connection
Numbers on the charts, such as 3-4, indicate the paragraphs in the section where the discussion of the particular test can be found.
The testing procedure starts at the top of the chart and works progressively downward, with each test eliminating those on one side of the horizontal line below that particular test.
To become familiar with the system, turn to SERVICING CHART I at the end of Section 3 and try this example: The symptom is NO SIGNALS, AUDIO FAILURE. The first test is at TEST POINT 1. It is indicated that a signal is injected at the volume control; the numbers 3-1 indicate that an explanation can be found in paragraphs marked 3-1.
The labels on the horizontal line beneath TEST POINT 1 correspond to the only two possible results that can occur and lead to the next step.
If a sound is heard at TEST POINT 1, the symptom is not audio failure, and reference is made to Section 4. If no sound results from TEST POINT 1, then testing proceeds with TEST POINT 2, which again leads to two possibilities.
SUGGESTIONS FOR WORKING ON THE CHASSIS
1. Be equipped with both a good selection of quality tools of convenient size and shape and a set of various lengths of clip leads made of good materials.
2. Keep your soldering iron, or gun, in good condition. A piece of steel wool should be kept in the tool box for this purpose.
3. Have a small flashlight handy.
4. Use a sharp-pointed object as a soldering aid and, when ever possible, unsolder connections instead of cutting leads. However, do not hesitate to cut leads when it seems that unsoldering will damage the terminals due to excessive mechanical pressure or damage the components due to excessive heat.
5. After a connection has been opened for purposes of testing, resolder it at once if the part is not going to be re placed; it is easy to forget to do this later or to forget where the part came from. Do not hesitate to make a sketch when several leads are unsoldered at once in re moving a part for replacement. The toughest repair jobs are those which are the result of an error made in previous testing, since this usually introduces a second fault that was not there when the receiver failed.
6. Unless you have a schematic, do not try to work on a chassis with which you are unfamiliar. Even though the operation of the circuits is understood perfectly, the time wasted looking for things that are supposed to be there is never recovered.
7. When testing at oscillator grids, isolate the scope or VTVM with a resistor or capacitor in series to avoid loading down the oscillator grid circuit. Often, the use of a voltmeter having low internal resistance will stop oscillator operation entirely.
8. Resist the impulse to adjust controls, unless you have a means of observing the results of the adjustment while it is being made. It is seldom that a receiver can be repaired by making adjustments, so wait to take this step until you know the circuits are working. A receiver does not fail because some control has misadjusted itself.
9. Subscribe to at least one of the monthly publications which have information on circuitry of new models, and form the habit of reading this material as soon as it is published. Keep a card file of important articles, or tear out the pages and file them.
10. In many places throughout the text, the directions, "Trace toward B+," are given. This refers to a simple technique used by all technicians after measuring for a positive voltage at some point and not finding it there. The probe is then moved across each component in series with the point, moving toward B+ (Fig. 1-1). If one of the components in this series line is faulty, the voltage appears on the B+ side of the part, and the technician knows that he has just crossed the faulty part. When a capacitor from the line to ground, such as C1 (Fig. 1-1), is shorted, the voltage will appear at the B+ end of R2. When a capacitor is associated with the suspected part of the B+ line, it is faster to check the capacitor by removing one end of it and measuring the voltage again. The possibility of dam age to R3 when the capacitor is shorted must not be over looked.
Fig. 1-1. Tracing toward B+.
11. Sometimes, when testing at the plate of a tube for positive voltage, an interesting phenomenon occurs. Instead of the positive voltage expected, a small negative voltage is found. This is a sure sign that a connection from the plate to B+ is open. The reason for the negative reading on the voltmeter is that the meter completes a circuit from the plate of the tube to ground, or to the cathode. With the cathode heated, electrons are emitted and form a space charge in the tube. Because electrons have left the cathode, it takes on a positive charge with respect to the neutral plate. When the voltmeter completes the path from plate to cathode, electrons flow from the neutral plate to the positive cathode through the meter. This causes the meter to be deflected in a negative direction when the positive probe of the meter is on the plate of the tube.
12. The cathode circuit should always be checked with an ohmmeter instead of a voltmeter. This is because an open cathode will cause the voltmeter to operate as a micro-ammeter and produce a deflection which can be easily mis taken for normal cathode voltage. This erroneous reading is produced by the drawing of electrons from B-, through the meter, and into the cathode.
13. When full B+ is found at the plate of a tube which has a resistive load in series with the plate, then it is certain that the tube is not conducting. With a large resistance in series between the plate and B+, any electron flow from the plate through this resistance causes a voltage drop at the plate, and the reading there should be less-than-normal B+ voltage. The technician then knows he must check the cathode circuit for opens and that he must measure the grid-to-cathode voltage, looking for possible cutoff bias.
It is often helpful to remove the tube while measuring the plate voltage with a voltmeter in order to determine if abnormally low plate voltage is due to conduction through the tube, or to a shorted capacitor connected from ground to someplace along the B+ line.
14. In checking capacitors connected from B+ to ground, such as filter capacitors in the power supply or plate bypass capacitors, a shorted unit is identified by the missing volt age or by the burned components associated with the capacitor. Further testing of the capacitor can be done with an ohmmeter, or one end of the capacitor can be disconnected and another voltage measurement taken.
An ohmmeter does not give a good check for leakage or opens on small-value capacitors. This is especially true in the case where the capacitor is used across a high voltage or handles large peaks of AC. The small voltage supplied by the ohmmeter may not be enough to cause the capacitor to break down in the same manner as it does in the circuit.
When in doubt, disconnect one end of the capacitor, and put a new one in the circuit.