TM 11-6625-486-14&P
Table 6-3. Troubleshooting-Continued
Malfunction
Probable Cause
Corrective Action
6. Frequency meter inoperative in all
No voltage on either vfo.
On power supply circuit board check
ranges and functions, but noise can
voltage on TP604. If no voltage
be heard when headset is plugged
exists, check R603 and replace if
into PHONES jack.
necessary.
Range switch wafer S703A not
With power off, check continuity
completing circuit.
through S703A contacts 9, 10, 11,
and 12 and replace switch if
necessary.
7. No frequency meter output signal
No voltage in low frequency vfo
On low frequency vfo circuit board,
at ANT. jack when FUNCTION
circuit.
use vtvm and measure dc voltage
SW set to OPER, and RANGE SW
on test points TP101 and TP102
is in either A or B.
(fig. FO-2 or FO-3). If voltages are
not as indicated, make resistance
measurements between test points
and ground. Be sure that vfo
supply voltage is present at test
point TP604 on power supply
circuit board (fig. FO-2 or FO-3).
If no voltage exists at TP101,
Replace C104 if shorted.
capacitor C104 may be shorted.
If no voltage exists at TP102,
Replace CR105 if shorted.
capacitor C105 may be shorted.
No voltage on buffer amplifier Q102.
Check collector, base, and emitter
voltages with vtvm. If voltages are
not as indicated, remove power and
make forward and reverse
resistance measurements on Q102.
Replace Q102 if necessary.
No voltage on Q101.
With vtvm, check collector, base, and
emitter voltages, if voltages are not
as indicated in figure FO-2 or FO-3,
make resistance measurements and
replace Q101 if necessary.
NOTE
If base bias voltage of
Q101 changes ap-
preciably, replace diode
CR101.
8. No frequency meter output signal
Voltage supply through coil-box
With vtvm, check voltage on TP101
at ANT. jack when FUNCTION
Z101 not being applied to collector
(fig. FO-2 or FO-3). If voltage is
SW set to OPER, and RANGE SW
of Q101.
normal, check collector voltage of
in A position. Operates normally in
Coil L102 open, or S703 not making
Q101. If voltage is not present,
B or C position.
contacts.
Turn power off. With the
multimeter, check resistance
between TP101 and collector of
Q101. If an open circuit exists and
trouble is not due to switch S703,
replace coilbox Z101 (para 6-20).
9. No frequency meter output signal
Coil L103 in Z101 open.
Check resistance between TP101 and
at ANT. jack when RANGE SW is
collector of Q101. Replace Z101 if
in B position.
necessary.
Q101 not oscillating.
Perform low frequency vfo signal
tracing procedures given in
Check emitter bias resistor R103 and
replace if necessary.
Aging of coil L103 in Z101.
Replace Q101 if necessary.
10. Insufficient rf output signal in A
Peaking coil L105 (on collector of
Replace L105 (fig. FO-2 (2)) or (in
only.
Q102) in AN/USM-159 or resistor
AN/USM-159A and AN/USM-
R102 in AN/USM-159A and
159B) R102 (fig. FO-3 (2)).
AN/USM-159B defective.
11. CORRECTOR control does not
Capacitor C705 defective.
Replace C705 (fig. FO-2 or FO-3).
correct vfo to crystal checkpoint on
A and B.
6-9
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