Icom IC-751 Common Questions

Most Common Service Questions for the Icom IC-751 & IC-751A
The information on this page is subject to the
Service FAQ Index page.
Distorted, or no RX/TX on one or more bands. Problem may be
Symptom:
temperature related.
1) Bad trimmer capacitors on PLL unit. 2) Noisy variable resistor on
Probable
PLL unit.
Cause:
Replace plastic trimmer capacitors C78, C88, C97, and C107 with
Cure:
ceramic types. Remove excessive wax from around replacement
trimmers to prevent wax contamination. We suggest using a 12pf
trimmer for C97 instead of a 7pf 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 2.2k
type, replace it with a fixed-value 1.2k resistor.
While the PLL unit is lifted, we recommend soldering all joints in
Remarks:
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 future failures.
Erratic display i.e., frequency listed will be out of the normal
Symptom:
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.
1) Cracked solder on RAM unit is causing logic errors. 2) Program in
Probable
RAM unit is corrupted. 3) Burned/overheated R20 on logic unit is
Causes:
causing low voltage in logic unit.
Check for burned R20 (10 ohm, 1/2 watt) on logic unit. If it looks
Cure:
OK, resolder cracked joints at J1 and J2 connectors on RAM unit,
then reprogram. If this does not help, then the RAM unit must be
replaced. If the substitute RAM unit does not cure the problem, there
may be noise from the DC-DC converter causing the logic unit to
glitch. Follow instructions for "No frequency display" problem.
No RX audio in any mode except FM. No TX. Unit is not unlocked.
Symptom:
Unless otherwise noted, the following applies to both models.
terms and conditions set forth on the ICOM America