Icom IC-761 Instruction Manual

Most Common Service Questions for the Icom IC-761
The information on this page is subject to the terms and conditions set forth on the ICOM America
Service FAQ Index page.
Symptom: Garbled RX/TX, 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 equivalents. Remove excess 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.
Also, replace variable resistor R43 on the PLL unit with a fixed-
value, 220 ohm, 1/2 watt version. (Use a 1.2k resistor if the value of
R43 variable resistor is 2.2k.)
Remarks: While the PLL unit is lifted, we recommend resoldering the 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.
Symptom: Frequency unstable in USB mode.
Probable
Causes:Bad trimmer capacitor C202 in BFO section on the main unit.
Cure: Replace plastic trimmer capacitor with a 30pf ceramic trimmer and
re-align.
Symptom: No or very low RX sensitivity.
Probable
Cause:
Bad components on the 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 C47 are
corrected.
Symptom: Distorted RX on strong signals. BC band RX sensitivity may low or
marginal.
Probable
Cause:Q15 on the RF unit has become leaky.
Cure: Replace faulty Q15 (2SC2878B). If Q15 is leaky, there may be other
problems on the RF unit. See cure for RX problem listed above.
Symptom: Intermittent RX sensitivity. Problem seems mechanical. Banging on
case or switching between RX & TX may temporarily restore
sensitivity.
Probable
Cause:Intermittent contacts in relay on tuner relay unit.
Cure: Replace RL13 (LY2-0-DC12V) on tuner relay unit.
Symptom: Noise in RX, birdies.
Probable
Cause:Voltage regulators on PLL unit are oscillating.
Cure: Replace overheated C128, C129, C132 and C137 regulator filter
capacitors on PLL unit.
Symptom: RX disappears when outer shield of antenna coax is connected.
Inspection reveals 6 volts DC at the center conductor of the antenna
jack.
Probable
Causes:Shorted DC-blocking capacitor C40 on the ANT AW 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
stuck by lightning or sustained a large RF overload through the
antenna jack.
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 (2SC2053). There may be other problems on the RF
unit. Check the DC voltages at D47.
Symptom: The power to the radio flutters on and off when the TX power output

is increased toward maximum. Inspection reveals that the internal
power supply is unable to handle the current. 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 ONLY the edges of the bar before the holes at either end.
Symptom: Distorted/raspy-sounding TX in SSB modes. Average talk power is
low, even at higher mic gain levels. CW, RTTY and FM seems
normal.
Probable
Cause:
No base bias voltage at driver or PA transistors. Measure voltage at
bases of driver and PA transistors in TX SSB mode. Should be
around .67 volts. 0 volts indicates trouble.
Cure: Bad Q6 (2SD880Y) on PA unit. If problem is intermittent, check for
bad solder at the legs of Q6, D2.
Symptom: Tuner won't tune on one or more bands.
Probable
Cause:Presets are out of adjustment.
Cure: Adjust presets.
Symptom: Tuner does not tune on any band. Inspection reveals that only one
motor is turning.
Probable
Cause:Bad motor driven transistors.
Cure: Check motor driven transistors Q1 & Q2, or Q3 & Q4 (depending on
the motor that is not turning). If the motor transistors overheated
before they failed, also check related components for failure before
replacing the bad motor driver transistor (or the new drivers will fail
too).
Symptom: TX oscillation/spurious output, especially on lower bands at lower
power levels.
Probable
Causes:
1) Open parasitic-suppression resistor on PA unit. 2) Burned
components on the ANT SW unit. 3) Bad drivers or finals.
Cure: Check the value of R22 (4.7 ohms) on PA unit. (This resistor may
look OK but still be open.) Inspect the D4 area of the ANT SW unit
and replace any burned components. If these solutions don't help,
disconnect the cable at J10 on the RF unit and see if the problem
goes away. If it does, the there is a problem with the TX/RX
switching on the RF unit. If disconnecting J10 dies not alleviate the

problem, then most likely either the drivers or PA transistors are bad,
especially if the ALC meter deflection is low.
Remarks: Spurious output may 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, however,
since low-pass filter failures are uncommon in the IC-761.
Symptom: Radio seems to be stuck in "scan" mode.
Probable
Cause:Bad IC2 on the logic unit.
Cure: Replace IC2 (RP5G01-007)
Symptom: Intermittent display. S-meter may peg and RX/TX may disappear
when the unit is in the failure mode.
Probable
Cause:
Cracked solder beneath the DP-6 DC-DC converter IC (IC1) on the
display unit.
Cure: Carefully remove the DP-6 converter IC from the display unit and
resolder the pins of the IC to its circuit board. Then reinstall the
DP-6 back on the display unit.
Symptom: VFO tuning is erratic, sometimes skips frequencies in a certain area
if VFO knob travel. Least significant digit in frequency display
sometimes "flutters" back and forth instead of changing
incrementally when the VFO knob is turned slowly.
Probable
Cause:Bad rotary encoder.
Cure: Substitute rotary encoder with a known good one. It is recommended
that the rotary encoder be replaced as a complete assembly if it is
bad.
Remarks: If the unit tunes in only one direction, i.e. either up or down, there
may be a problem on the logic unit.
Symptom: Loud, high pitched squeal emanates from inside radio. May be
intermittent.
Probable
Cause:DC-DC converter transformer has a loose core.
Cure: Replace DP-6 on the display unit.
Symptom: A whine emanates from inside radio in TX only at certain power
levels. Transmitter works ok.
Probable
Cause:Loose core in DC-DC converter transformer inside power supply.
Cure: Replace T2 on reg. unit inside power supply.