at your test bench under as close to normal conditions as possible. In some cases,
you may have to take a few measurements at key test points. This operational test
is important because it frequently indicates the general location of trouble. For
example, in receiver-transmitter sets, it will frequently indicate whether trouble
is in the receiver or transmitter circuit.
It may often indicate the defective
stage.
5-4.
TROUBLESHOOTING BY SIGNAL SUBSTITUTION
a. What
is
Signal
Substitution?
Signal
substitution
is
a
method
of
troubleshooting.
Its object is to localize trouble to a specific stage of your
receiver, or to the coupling between stages. All you do is to inject a signal at
the output, then the input of each stage starting at the speaker, and listen for
the signal from the speaker or headset. The applied signal is a substitute for the
signal normally present at the test point. An audio signal is applied to all test
points from the speaker through the output of the detector.
A modulated
intermediate frequency, equal to the IF of the receiver, is applied to all test
points from the input of the detector through the output of the mixer.
To check
the mixer and the RF stages, select a frequency from the center of the band for
which the receiver is designed. This frequency should be unmodulated. This will
also enable you to check the operation of the oscillator.
Another way to see
whether you are getting an output is to connect an output meter (ac meter) between
plate and ground of the final audio stage. If you use this method, connect a 0.1-
microfarad, 600-working-volt-dc capacitor in the test lead going to the plate.
This will protect the meter. By using an output meter you can accurately check the
value of the output signal going to the speaker or headset.
b. Use of Equipment Manual in Signal Substitution.
In the troubleshooting
section of the equipment manual you will find a list of the tools and equipment you
will need for signal substitution.
You will also find information such as the
following to help you along:
(1) How to set the receiver controls before starting.
(2) At what frequency to set the signal generator for each stage you are going
to check.
(3) What value of input signal you are going to inject.
(4) Test points to be used.
(5) What signal or meter indication you should get if operation is normal.
(6) What components are possibly faulty if the signal does not pass through
the stage.
c. Signal Substitution Procedure. Let's take a typical AM receiver and see how
to use the signal substitution method for locating a trouble. All you have to do
is substitute the signal, following the order and at the points shown in figure 5-
2. Remember, signal substitution is used to localize trouble to a stage. Once you
have located the stage, you must test the tube in the stage and take voltage and
resistance measurements to isolate the trouble to a component.
322 L5
40
Previous Page
