Most Common Service Questions for the Icom IC-761
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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.
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