Quoted directly from Icom America's web site, Knowledge Base article number 5B9F593A21
Issue
My 751/751A has distorted, or no RX/TX on one or more bands. Problem may be temperature related.
Resolution
Probable Cause:
1) Bad trimmer capacitors on PLL unit.
2) Noisy variable resistor on PLL unit.
To fix:
Replace plastic trimmer capacitors C78, C88, C97, and C107 with equivalent ceramic types.
Remove excessive wax from around replacement trimmers to prevent wax contamination.
We suggest using a 12 pF trimmer capacitor for C97 instead of a 7 pF as listed in the service manual.
This will allow you to properly adjust the HPL lock voltage for that band.
Modification:
If variable resistor R43 on the PLL unit is the 470 ohm type, replace it with a fixed-value 220 ohm version.
If it's the 2.2 K type, replace it with a fixed-value 1.2 K resistor.
While the PLL unit is lifted, we recommend soldering all joints in the regulator section as this area runs hot. Also, inspect electrolytic capacitors C128, C129, C132, and C137 in the regulator circuit for discoloration caused by overheating. Replace all overheated capacitors to prevent any future failures.
Other Issues:-
--------------------
Knowledge Base Article 5B9F62421A
Issue
My 751/751A has an erratic display i.e., frequency listed will be out of the normal operating range of the radio. Rotating the VFO knob may cause strange characters to appear in the display. Radio does not work properly. Cycling the power switch a few times may restore operation temporarily. Problem may be temperature related.
Resolution
Probable cause:
1) Cracked solder on RAM unit is causing logic errors.
2) Program in RAM unit is corrupted.
3) Burned/overheated R20 resistor on logic unit is causing low voltage in logic unit.
To fix:
Check for burned R20 resistor (10 ohm, 1/2 watt) on logic unit. If it looks OK, resolder cracked solder joints at J1 and J2 connectors on RAM unit, then reprogram. If this does not help, then the RAM unit must be replaced.
--------------------
Knowledge Base Article 5B9F66138A
Issue
My 751/751A has no RX audio in any mode except FM. No TX. Unit is not unlocked.
Resolution
The PBT oscillator is not running. Verify absence of 9.4665 MHz signal at W86 test point on the main unit in 751A, or W134 test point in the IC-751. Replace faulty X7 crystal on the main unit in IC-751A, or X5 crystal in the IC-751.
--------------------
Knowledge Base Article 5B9F676739
Issue
There is frequency instability in USB mode on my 751/751A.
Resolution
There is a bad trimmer capacitor in BFO section on main unit. Replace plastic trimmer capacitor with a 30 pF ceramic trimmer capacitor and realign.
--------------------
Knowledge Base Article 5B9F695171
Issue
My 751/751A is experiencing a drastic change in audio frequency response between USB and LSB., i.e. USB audio has too much bass while LSB audio has too much treble. TX frequency response is also poor.
Resolution
There is a misadjustment of the BFO or PBT oscillator, or a faulty crystal filter FL-30 (or FL-80 in 751A) on the main unit.
To fix:
Check adjustment of the BFO and PBT oscillator. If these are OK, then the problem is most likely in the filter.
If the problem is intermittent, and the radio is an IC-751 (not "A" version), then the ground return path for the -5 V supply between the display and main units may be poor. To cure this, connect a short wire jumper between the unused pin 2 at J2 connector on the display unit and chassis ground at the closest mounting screw of the main unit PCB. If no spare pin is handy for the connector at J2, the jumper may be soldered to the shield of T1 transformer instead.
--------------------
Knowledge Base Article 5B9G62191A
Issue
On my 751, the RX cuts out randomly when turning VFO knob in IC-751A with UX-14 installed.
Resolution
There is an incompatibility between UX-14 and 751A is causing the PLL to unlock. Turn off transceive mode on UX-14 (bank S1, switch #3 to OFF position).
--------------------
Knowledge Base Article 5BA93A169A
Issue
No receiver audio at all in my 751A. Signals can be seen on S-meter.
Resolution
There is a cracked solder joint on the main unit causing no VCC to the audio amp. Resolder the cracked solder joint at L46 coil on the main unit. Repair damaged foil trace, if necessary. Re-route ferrite beads so they will not press down on L46 coil (or any other components) when the top cover is reinstalled.
--------------------
Knowledge Base Article 5BA9475961
Issue
When using an internal power supply, the power to my 751/751A flutters 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.
Resolution
There is a cracked solder joint in the current sensing circuit of the power supply is shutting it down prematurely. Resolder the joints at the R26 resistor (0.0012 ohm) inside the power supply.
This resistor appears as a metal bar soldered to the bottom PC board. Solder the edges of the bar BEFORE the holes at either end.
--------------------
Knowledge Base Article 5BA94A3586
Issue
There are TX oscillations/spurious output from my 751/751A.
Resolution
Possible causes:
1) Open resistor or faulty electrolytic capacitors on PA unit.
2) PA unit needs modification.
3) Bad drivers or PA transistors.
4) Burned components on the ANT SW unit.
To fix:
Check the value of R22 resistor (4.7 ohms) on PA unit. (This resistor may look OK but still be open.) Replace C18 and C25 capacitors 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 capacitor 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 connector 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 connector 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 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).
--------------------
Knowledge Base Article 5BA9554454
Issue
No TX output at all in RTTY or CW modes from my 751/751A. SSB and AM are OK.
Resolution
There is a bad carrier generator crystal on the main unit. Replace X1 crystal.
--------------------
Knowledge Base Article 5BA9575393
Issue
No frequency display, only decimal points on my 751/751A. The problem may be intermittent or temperature related.
Resolution
The display DC-DC converter has noisy output.
To fix:
Replace all of the following dried-out electrolytic capacitors on the 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 servicing the DC-DC converter section, be on the lookout for 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 servicing the DC-DC section.
--------------------
Knowledge Base Article 5BA95A434A
Issue
My 751/751A seems to be stuck in "scan" mode.
Resolution
There is a bad IC2 chip on the logic unit. Replace the IC2 integrated circuit (RP5G01-007).
--------------------
Knowledge Base Article 5BA9635A39
Issue
Loud, high pitched squeal emanates from inside my 751/751A
Resolution
The DC-DC converter transformer has a loose core. Replace T1 transformer (TO-9) on the display unit.
--------------------
Knowledge Base Article 5BA9656882
Issue
On my 751/751A, the VFO tuning is erratic, sometimes skips when changing frequencies or bands. The display does not increment smoothly when VFO knob is turned slowly.
Resolution
The rotary encoder is dirty. Remove the white plastic cover from the back of the rotary encoder and spray clean the carbon track with Blue Shower solvent. Spin VFO knob vigorously while spraying.
Do not let solvent saturate the display unit or allow it to come into contact with the plastic display lens. Stuff a paper towel between the rotary encoder and display unit to catch spray run-off.
--------------------
Knowledge Base Article 5BA9685218
Issue
There are missing segments or mode indicators on my 751/751A display. the problem may be intermittent.
Resolution
There is a bad connection between logic and display units. Check the connectors J5 and J13 on the logic unit where the flex cables to the display unit are attached. If they look OK, and none of their connector spring contacts flattened, replace the flex cables.
--------------------
Knowledge Base Article 5BA96A3A75
Issue
Low mic gain when using Heil mic element on my 751/75A.
Resolution
Resolution
Install modification: In 751, change the value of R264 resistor on main unit to 200 K. In 751A, change the value of R21 resistor on AF-VR unit to 47 K.