TM 11-6625-486-14&P
Table 6-3. Troubleshooting-Continued
Malfunction
Probable Cause
Corrective Action
Choke L303 open.
With power off, check resistance of
L303 (para 6-9), and replace if
open.
Coupling capacitor C302 or C304.
Replace capacitor.
19. With FUNCTION SW set to
Diode CR302 defective.
Check forward and reverse resistance
CHK, and RANGE SW in A
of diode, and replace diode if
position, only weak beat notes can
reverse resistance is not at least 10
be obtained.
times greater than forward
resistance. Make forward resistance
check with common test prod of
multimeter on cathode side of diode
and positive test prod on anode of
diode. For the reverse resistance
check, use positive test prod on
cathode and common test prod on
anode of diode.
Insufficient diode biasing voltage (in
With vtvm, measure bias voltage on
AN/USM-159 only).
TP302 (mixer board). If 0.09 volt is
not present, check resistor R308
connected from TP302 to contact 5
of S703 (fig. FO-6).
20. With FUNCTION SW set to
Diode CR301 defective.
Unsolder one end of R302. Check
OPER, and RANGE SW in range
forward and reverse resistance of
A or B, little or no harmonic output
diode and replace if defective.
signals appear on ANT. jck.
21. With FUNCTION SW set to
Diode CR303 or CR304 defective.
The two diodes are connected back-
OPER, and RANGE SW in C
to-back and the voltage measured
position, harmonic output signals
on TP301 (junction of diodes)
are weak.
should be anything from 0 to 0.06
volt. The voltage may be either
positive or negative with respect to
the meter chassis depending on
which diode is conducting the most.
If the diodes are suspected, un-
solder one diode from the board
ground and check forward and
reverse resistance. If the voltage
exceeds 0.06 volt, one diode is not
conducting properly.
22. No audio tone on noise heard in
Headset or wiring.
Check headset and wiring repair;
headset when AUDIO GAIN
repair if necessary.
Control is turned fully clockwise.
Coupling capacitor C410 open.
Replace C410.
Incorrect voltages on Q403.
With AUDIO GAIN control fully
ccw, measure the base and collector
voltages of Q403. When AUDIO
GAIN control is advanced, the B-
voltage on the base and collector
should increase.
Measure the B- voltage at TP603 on
the power supply circuit board. If
present, trouble is in audio am-
plifier. (Refer to following steps).
Resistor R402 defective.
Replace R402.
Coupling capacitor C406 open.
Replace C406.
Resistor R409 defective.
Replace R409
Q403 defective.
Replace Q403
No voltage on Q402.
Check voltage on Q402.
Resistor R407 defective.
Replace R407.
Resistor R405 or R406 defective.
Replace R405 or R406 if defective.
Q402 defective.
Replace if necessary.
6-11
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