TM 11-6625-2629-14&P
c. Intermediate Frequency Amplifiers.
7-10. Signal Substitution
(1) Connect the rf output of the sweep
a. General Signal substitution procedures
generator to the if input of the modulation meter.
help to localize troubles to a section or stage in the
(2) Connect the sweep output of the sweep
modulation meter. An externally generated signal
generator to the external sweep connections on
is substituted for the signal normally present in
the oscilloscope.
each stage. The test equipment required is listed
(3) Set the sweep generator frequency to 12
in table 7-1. In the following tests, ground one
MHz and the sweep from 10.5 through 13.5 MHz.
side of the external generator to the modulation
(4) Use the oscilloscope to observe the
meter chassis and the other side to the point
bandpass of the if amplifiers. Start at pin 4
indicated in the test procedures below.
(output) of integrated circuit V1. If no bandpass
b. Oscillator Substitution. If the oscillator is
is present at this point, check at the cathode of
suspected of malfunctioning or nonoperation, the
V2. Check all the components of the faulty stage.
am generator maybe used to apply an if signal to
d. Discriminator Substitution. If a modulated
the if amplifier strip. Proceed as follows:
signal input gives an indication on the CARRIER
(1) Disconnected the rf cable at the if input
SHIFT meter, but does not indicate deviation,
mixer connection (fig. 7-1).
the audio circuit is probably faulty. Use the audio
(2) Connect the am generator output to this
oscillator to simulate the discriminator output,
cable. (Be sure that the modulation selector
(1) Disconnect pin 3 of the discriminator can
switch is in. the OFF position. )
(T9) an connect the lead to the audio oscillator
(3) Tune the am generator to 12 MHz and
output .
increase the output until the LIMITING meter
(2) Set the audio oscillator frequency be-
indicates in the black area.
tween 50 and 70,000 Hertz (Hz)
(4) Check the indication on the CARRIER
(3) Monitor the audio oscillator level with
SHIFT meter. It should read nearly zero with
the vtvm. Set the voltage at 0.16 volt rms.
TUNE-FINE TUNE switch in FINE TUNE
(4) Use the oscilloscope and observe the
position. If it does not read zero, tune the am
waveform in the audio circuit.
generator until it does.
(5) If LIMITING and CARRIER SHIFT
7-11. Audio Circuit Stage Gains
meter readings are indicated, the oscillator-
Follow the procedure in paragraph 7-10 d to check
probable is faulty.
the gain of each audio stage. The gain for each
(6) If the LIMITING meter reading cannot
stage is shown in table 7-4.
be indicated, the trouble is probably in the if
amplifiers. If the if amplifiers are suspected o f
being fault y, use the test method given in c
below.
Table 7-4. Audio Amplifier Stage Gain
Pins
2
(grid) and 1 (plate) of V1OA
0.16
0.11
0.7
0.07
0.11
Pins
7
(grid) and 6 (plate) of V1OB
0.07
(grid) and 5 (plate) of Vll
3.6
0.07
Pine
1
52
9.7
3.6
Pins
1
(grid) and 5 (plate) of V12
2.7
9.7
Pins
2
(grid) and 1 (plate) of V13A
9.2
0.94
9.2
8.7
0.94
Pine
7
(grid) and 6 (phtd .of V13B
7-12. Isolating Trouble Within Stage
tester, or by substituting a similar type tube
which is known to be operating normally.
When trouble has been localized to a stage, either
b. Take voltage measurements at the tube
through operational checks, troubleshooting
sockets (fig. 7-4 and 7-5) and other related points
to the stage in question.
the following techniques to isolate the defective
c. If voltage readings are abnormal, take
part:
resistance readings (fig. 7-4 and 7-5) to isolate
WARNING
o p e n and short circuits. Refer to the dc
The 6661 WA tube contains radioactive
resistances of transformers and coils (table 7-5).
material. H a n d l e carefully to avoid
d. If signals are weak and all checks fail to
breaking.
a. Test the tube involved either in a tube
7-11
