Icom IC-751 Common Questions page 4

No receiver audio at all in IC-751A. Signals can be seen on S-meter.
Symptom:
Probable
Cracked joint on main unit is causing no VCC the audio amp IC>
Causes:
Resolder cracked join at L46 on main unit. Repair damaged foil
Cure:
trace, if necessary. Re-route ferrite beads so they will not press down
on L46 (or any other components) when the top cover is reinstalled.
RX audio seems low in IC-751.
Symptom:
Probable
Unit needs update to audio amp circuitry on main unit.
Causes:
Contact Icom Technical Support for update bulletin.
Cure:
No TX output in all modes. Problem is traced to no output at J8 on
Symptom:
the RF unit.
Probable
Bad Q14 amplifier on RF unit.
Causes:
Replace Q14. There may be other problems on the RF unit. Check
Cure:
the DC voltages at D47.
Distorted/raspy sounding SSB TX. Average talk power is low, even
Symptom:
at higher mic gain levels. CW, RTTY and FM seem normal. RX in
all modes is OK.
1) External Astron power supply needs additional RF decoupling. 2)
Probable
No bias voltage to driver or PA transistors.
Causes:
Install RF decoupling capacitors on Astron power supply (if used). If
Cure:
this does not help, measure voltage at bases of driver and PA
transistors in TX SSB mode. Should be around .67110 volts. 0 volts
indicates trouble and probably a bad Q6. If problem is intermittent,
check for bad solder at the legs of Q6, or D2.
The driver and PA idle current was set too high at the factory in some
Remarks:
IC-751s (not A version). Align the idle current as per the service
manual after repairs are made.
When using an internal power supply, the power to the radio flutters
Symptom:
on and off when the TX power output is increased toward maximum.
Unit runs fine when the internal power supply is substituted with an
external supply.
Cracked solder in the current-sensing circuit of the power supply is
Probable
shutting it down prematurely.
Causes:
Resolder joints at the .0012 ohm resistor, R26 inside the power
Cure:
supply.