TM 11-6625-1633-45
Correction
Probable trouble
Symptom
Check operation of relay A4K1 (fig. 3-
Relay A4k1 or 1500 MC potentiome-
No frequency modulation of rf output
5) and repair or replace defective
ter A4R1 defective.
above 1060 MHz.
parts.
Note: MODULATION switch S2 set
at INT SINE position; INTERNAL
MODULATION
FREQUENCY
selector switch S1 set at any posi-
tion; DEVIATION control knob
fully clockwise.
Mechanical drive loose or defective.
No MICROVOLTS/-DBM dial rota-
Check and tighten mechanical drive
.
tion with ATTENUATOR control
knob adjustment.
ometer R16 to obtain 5.6 volts. Reconnect
3-6. Stage Gain Measurements
P3 to jack A1J3.
Use the techniques outlined in a through e below when
the DEVIATION control knob can not be adjusted for a
Peak-to-peak
300 KHz deviation indication on the deviation meter, or
Test Connection
voltage (volts)
Stage gain
adjusted for a 300 KHz deviation of modulated rf carrier
Pin 1 of V1
1.5
Pin 1 of V2
21.0
V1 gain of 14
output.
Pin 5 of V3
13.0
V2 gain of 0.62
Pin 1 of A1V!
1.15
source.
Pin 1 of A1V2
25.0
A1V1 gain of 22
b. Set the front-panel controls as indicated below:
(1) POWER OFF toggle switch S8 to up position.
3-7. Waveform Analysis
(2) INTERNAL MODULATION FREQUENCY
selector switch S2 to 1,000~.
(3) DEVIATION METER RANGE selector switch
jacks and at other significant points in the circuits of the
S3 to 300 KC.
signal generator by using Oscilloscope AN/USM-140.
(4) MODULATION selector switch S2 to INT
The normal waveforms obtained at the test jacks and at
SINE.
other significant points are either dc or pure sine waves,
(5) Center frequency selector handwheel to 600.
with the frequency determined by the position of IN-
(6) DEVIATION control knob fully clockwise
TERNAL MODULATION FREQUENCY switch S1,
except as shown on the waveform diagram (fig. 3-13).
and as directed in the test procedures.
By comparing observed waveforms with the normal
waveforms, troubles can often be quickly located.
determine the gain of each stage.
b. Before comparing the waveform with the normal
NOTE
waveform, carefully read the note on the waveform dia-
Stage gain is computed by dividing the
gram and duplicate exactly the conditions under which
input voltage applied to the grid of the
the normal waveform was obtained. If an observed
following stage by the input voltage
ap-
waveform is not a dc level, or a sine wave which closely
plied to the stage under test.
resembles the normal waveform, trouble is indicated.
c. A departure from the normal waveform indicates
e. Stage gains are shown in the chart below; ac volt-
trouble between the point at which the waveform is ob-
ages in the chart are peak-to-peak voltages as observed
served to be normal and the point at which the wave-
on the oscilloscope.
form is observed to be abnormal. For example, if a
waveform is observed to be normal at the grid of a stage,
NOTE
and abnormal at the plate of the same stage, it is an in-
Excessively distorted sine waves indicate
dication that trouble is in that stage. When trouble is
a fault in thermistor HR1. Disconnect
indicated in a stage, replace the tube before making any
plug P3 from jack A1J3 and measure the
further tests. If replacing the tube does not correct the
voltage at P3. It should be 5.6 volts peak-
trouble, place the original
to-peak; if necessary, adjust 1 KC potenti-
3-15
