>>I have noticed that the carrier transmitted by
>>my Icom 751A in CW mode will drift just after
>>I have begun sending a "block" or group of
This may be some help
Most Common Failures and Cures for the Icom IC-751 & IC-751A
*Unless otherwise noted, the following failures and cures
apply to both models.
Symptom: Distorted, or no RX/TX on one or more bands.
Problem may be temperature related.
Probable Causes: 1) Bad trimmer capacitors on PLL unit. 2)
Noisy variable resistor on PLL unit.
Cure: Replace plastic trimmer capacitors C78, C88, C97, and
C107 with 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.
Remarks: 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 future failures.
Symptom: 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.
Probable Causes: 1) Cracked solder on RAM unit is causing
logic errors. 2) Program in RAM unit is corrupted. 3)
Burned/overheated R20 on logic unit is causing low voltage
in logic unit.
Cure: Check for burned R20 (10 ohm, 1/2 watt) on logic unit.
If it looks OK, resolder cracked joints at J1 and J2
connectors on RAM unit, then reprogram. If this does not
help, substitute RAM unit with a known good one. If the
substitute RAM unit works properly, 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.
Symptom: No RX audio in any mode except FM. No TX. Unit
is not unlocked.
Probable Cause: PBT oscillator is not running.
Cure: Verify absence of 9.4665 signal at W86 on main unit
in 751A, or W134 in the IC-751. Replace faulty X7 on main
unit in IC-751A, or X5 in the IC-751.
Symptom: Frequency instability in USB mode.
Probable Cause: Bad trimmer capacitor in BFO section on
Cure: Replace plastic trimmer capacitor with a 30pf
ceramic version and realign.
Symptom: 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
Probable Causes: Misadjustment of the BFO or PBT
oscillator, or a faulty crystal filter FL-30 (or FL-80 in
751A) on the main unit.
Cure: Check adjustment of the BFO and PBT oscillator. If
these are OK, then the problem is most likely in the filter.
Remarks: If the problem is intermittent, and the radio is
an IC-751 (not "A" version), then the ground return path for
the -5 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 on the display unit and chassis ground at
the closest mounting***of the main unit PCB. If no
spare pin is handy for the connector at J2, the jumper may
be soldered to the transformer shield of T1 instead.
Symptom: AC hum or buzz in transmit and receive audio when
an internal power supply is installed.
Probable Cause: AC in being induced into the PLL from the
AC wires and power supply case.
Cure: Twist the AC wires that run from the rear panel to
the power switch and route them away from the PLL unit.
Tape them up against the chassis if necessary.
Remarks: In some radios, especially older IC-751s, it is
not possible to completely eliminate all AC hum when using
the internal power supply.
Symptom: No, or very low RX sensitivity.
Probable Cause: Bad components on RF unit, probable result
of RF overload. To verify this, check DC voltages at D47 on
RF unit. Correct voltages are: Cathode side- RX: 8.5, TX:
13.8. Anode side- RX: 9.0, TX: 9.5.
Cure: Replace all of these components on the RF unit: D42,
D44, D45, D46, D47, Q15. Check C174 and C179 for leakiness.
There may be other failures. Unit is not repaired until
listed voltages at D47 are correct.
Symptom: Distorted RX on strong signals. BC band RX
sensitivity may be low or marginal.
Probable Cause: Q15 on the RF unit has become leaky.
Cure: Check and replace Q15. If Q15 is leaky, there may be
other problems on the RF unit. See cure for RX problem
Symptom: RX disappears when outer conductor of coax is
connected to antenna jack. Inspection reveals 6 volts DC at
the center conductor of the antenna jack.
Probable Cause: Shorted DC-blocking capacitor C40 on the
ANT SW unit.
Cure: Replace the capacitor.
Remarks: There are probably other problems on the RF unit
if this capacitor is bad. Check the voltages at D47 on the
RF unit. Radio may have been struck by lightning or
sustained a large RF overload through the antenna jack.
Symptom: RX cuts out randomly when turning VFO knob in IC-
751A with UX-14 installed.
Probable Cause: Incompatibility between UX-14 and 751A is
causing the PLL to unlock.
Cure: Turn off transceive mode on UX-14 (bank S1, switch #3
to OFF position)
Symptom: Hash noise in RX, birdies.
Probable Causes: 1) Voltage regulators on PLL unit are
oscillating, producing noise on VCC line. 2) Overheated
components and/or loose screws are causing the power supply
to run noisy. 3) Noisy DC-DC converter on display unit.
Cure: Replace overheated C129, C129, C132 and C137
regulator filter capacitors on PLL unit. Inspect power
supply for overheated components and replace them as needed.
If the components look OK, tighten the screws that hold down
the reg. unit. Don't forget to tighten the***hidden
beneath the wire bundle where the DC output lines leave the
case. If none of these help, and radio is a IC-751 (not A
version), then the display DC-DC converter may need
modification. Contact Icom Technical support for update
Symptom: No receiver audio at all in IC-751A. Signals can
be seen on S-meter.
Probable Cause: Cracked joint on main unit is causing no
VCC the audio amp IC.
Cure: Resolder cracked joint at L46 on main unit. Repair
damaged foil 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.
Symptom: RX audio seems low in IC-751.
Probable Cause: Unit needs update to audio amp circuitry on
Cure: Contact Icom Technical Support for update bulletin.
Symptom: No TX output in all modes. Problem is traced to
no output at J8 on the RF unit.
Probable Cause: Bad Q14 amplifier on RF unit.
Cure: Replace Q14. There may be other problems on the RF
unit. Check the DC voltages at D47.
Symptom: Distorted/raspy-sounding SSB TX. Average talk
power is low, even at higher mic gain levels. CW, RTTY and
FM seem normal. RX in all modes is OK.
Probable Causes: 1) External Astron power supply needs
additional RF decoupling. 2) No bias voltage to driver or
Cure: Install RF decoupling capacitors on Astron power
supply (if used). If this does not help, measure voltage
at bases of driver and PA transistors in TX SSB mode.
Should be around .67 volts. O volts indicates trouble and
probably a bad Q6. If problem is intermittent, check for
bad solder at the legs of Q6, or D2.
Remarks: The driver and PA idle current was set too high at
the factory in some IC-751s. (not A version). Align the
idle current as per the service manual after repairs are
Symptom: When using an internal power supply, the power to
the radio 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.
Probable Cause: Cracked solder in the current-sensing
circuit of the power supply is shutting it down prematurely.
Cure: Resolder joints at the .0012 ohm resistor, R26 inside
the power supply.
Remarks: 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.
Symptom: TX oscillations/spurious output.
Probable 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.
Cure: Check the value of R22 (4.7 ohms) on PA unit. (This
resistor may 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.
Remarks: 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).
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