TM 11-6625-486-14&P
Table 6-3. Troubleshooting-Continued
Malfunction
Probable Cause
Corrective Action
23.
Noise output signal louder than
Capacitor C403 or resistor defective.
Replace capacitor or resistor if
audio tone.
defective.
Incorrect voltages on Q401.
Check voltages according to figure
FO-2 or FO-3 and replace Q401 if
necessary.
Capacitor C409 open.
Replace C409.
Audio output stage Q403 not per-
Check base and collector voltages and
forming the proper noise limiting
perform forward and reverse
action.
Resistance measurements. Replace
Q403 if necessary.
24.
Noise present but no audio signal.
No output signal from mixer circuit.
Check mixer circuit and perform the
signal tracing procedures in
25.
When FUNCTION SW set to
Modulated signal not being applied to
Set up conditions for test according
MOD, filmstrip is lighted but no
vfo supply voltage because switch
to figure 6-7. On vtvm, set
audio tone can be heard in
S701A or Zener diode Z602 is
FUNCTION to AC and RANGE to
frequency meter headset.
defective.
3V. With ac probe, connect ground
clip to chassis and touch the probe
to TP604 on power supply circuit
board.
If the signal indicated in figure 6-7
cannot be obtained, make con-
tinuity checks through switch S701
to Zener diode Z602. Be sure that
the cathode side of Z602 is not
grounded when switch S701 is set
to MOD. The modulated signal is
applied through Z602 to test point
TP604 which is the vfo supply
voltage point.
The forward resistance of Z602 should
be approximately 800 ohms and the
reverse resistance about 1,500
ohms. Replace Z602 if necessary.
26.
When FUNCTION SW is set to
Audio oscillator Q501 not oscillating.
Refer to the test setup in figure
MOD, no aduio tone can be heard
in frequency meter headset and
AC and RANGE to 10V. With ac
filmstrip is not lighted.
probe, connect ground clip to
chassis and touch probe to TP501
on modulator circuit board.
No B- voltage on collector of Q501.
With vtvm for measuring dc, measure
the collector voltage at TP501.
Primary winding of T501 open.
Replace T501.
No voltage on base of Q501.
Check resistance of R501 and R502
and replace the defective resistor.
Perform forward and reverse
resistance measurements on Q501
and replace Q501 if necessary.
Capacitor C501 defective or open.
Replace C501.
Capacitor C504 shorted.
Replace C504 and Q501.
Capacitor C505 defective.
Replace C505.
27.
When FUNCTION SW is set to
Neon lamp DS701 or DS702 defec-
Replace lamp (para 6-23).
MOD, audio tone heard in
tive.
frequency meter headset, but
Current limiting resistor R702 or
Check and replace defective resistor
filmstrip is not illuminated.
R703 defective.
Transformer T501.
Measure resistance of T501 (table 6-2)
and replace transformer if
necessary.
6-12
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