Icom IC-751 Common Questions page 5

This resistor appears as a metal bar soldered to the bottom PC board.
Remarks:
Solder the edges of the bar BEFORE the holes at either end.
TX oscillations/spurious output.
Symptom:
1) Open resistor or faulty electrolytic capacitors on PA unit. 2) PA
Probable
unit needs modification. 3) Bad drivers or PA transistors. 4) Burned
Causes:
components on the ANT SW unit.
Check the value of R22 (4.7 ohms) on PA unit. (This resistor may
Cure:
look OK but still be open.) Replace C18 and C25 on the PA unit if
they appear swollen or overheated. Modification for old IC-751:
Install a metal film, 220-ohm 1/2 watt resistor parallel to C6 on PA
unit if there isn't one installed already. Inspect the D4 area of the
ANT SW unit and replace any burned components. If none of these
solutions solve the problem, disconnect the J10 on the RF unit and
see if the problem goes away. If it does, then there is a problem with
the TX/RX switching on the RF unit. If disconnecting J10 doesn't
help, then most likely either the drivers or PA transistors are bad,
especially if the ALC meter deflection is low.
The spurious output may also be caused by incorrect band switch
Remarks:
voltage to the low-pass filter unit, or even faulty/burned components
on the filter unit itself. The above causes should be ruled out first
since low-pass filter failures are uncommon in the IC-751(A).
Intermittent low FM TX output power in IC-751.
Symptom:
Probable
Bad relay RL1 on FM unit.
Causes:
Replace bad RL1 (221-D009-M)
Cure:
No TX output at all in RTTY or CW modes. SSB and AM are OK.
Symptom:
Probable
Bad carrier generator crystal on main unit.
Causes:
Replace bad X1 on main unit.
Cure:
No frequency display, only decimal points. Problem may be
Symptom:
intermittent or temperature related.
Probable
Display DC-DC converter has noisy output.
Causes:
Replace all of the following dried-out electrolytic capacitors on the
Cure:
display unit: C17, C18, C20, C21, C22, C23, C24, and C25. (These
capacitors are light yellow when they are new. Capacitors that have
turned brown are old and dried-out).
While service the DC-DC converter section, be on the look out for
Remarks:
cracked solder joints, as this section of the radio tends to run hot.
Also, it would be a good idea to reprogram the RAM card after