ICOM IC-735: Most Common Failures, Symptoms and Cures

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ICOM IC-735: Most Common Failures, Symptoms and Cures

Post by JohnPoet » Tue Jan 12, 2016 6:50 am

Most Common Failures and Cures for the Icom IC-735

Symptom: Erratic display, segments missing from frequency
display. Radio seems to work OK otherwise.
Probable Cause: Bad solder at IC1 display driver on display
Cure: Resolder pins on IC1.
Remarks: IC1 is a surface mount IC.

Symptom: Intermittent RX/TX. Intermittent high SWR. Seems
like antenna is intermittent.
Probable Cause: Cracked solder joint at antenna connector.
Cure: Resolder joint at antenna connector where it connects
to the filter unit. We recommend replacing the thin wire
between the filter unit and antenna connector with a thicker
wire to prevent future failure.

Symptom: Intermittent RX. Seems like antenna is
disconnected in receive mode. TX is OK.
Probable Cause: Cracked solder at external RX phono jack on
main unit.
Cure: Resolder phono jack to main unit.

Symptom: No TX, or low TX output on one or more bands with
high current draw from power supply. Other bands are OK.
Probable Cause: Damaged components in low-pass filter.
Cure: Replace all obviously burned/overheated components on
filter unit. Also inspect PA for overheating.

Symptom: Power drops off as unit warms up. ALC meter
deflection remains high.
Probable Cause: APC out of alignment.
Cure: Follow APC alignment procedures in the service
manual. (R262 on main unit.)

Symptom: Distorted RX/TX, or no RX/TX on one or more bands.
Distorted SSB audio sounds raspy and rough. Problem may be
temperature related.
Probable Cause: Bad trimmer capacitors on PL unit, HPL VCO
Cure: Replace plastic trimmer capacitors C37, C45, C52, and
C59 with ceramic equivalents. Also replace the reference
oscillator trimmer C138.

Symptom: Frequency instability.
Probable Cause: Unstable plastic trimmer capacitor C138 on
PL unit.
Cure: Replace C138 with a ceramic equivalent. We also
recommend replacing the HPL trimmers at this time.

Symptom: Intermittent mic audio.
Probable Cause: Dirty mic gain control.
Cure: Spray-clean control with Blue Showers solvent.
Remarks: Using a cleaning solvent other than Blue Showers
may cause damage by removing a substance inside the control
that allows smooth movement of the slider.

Symptom: VFO tuning is erratic, sometimes lags or skips when
changing frequencies or bands. Sometimes least-significant
digit in frequency display "flutters" back and forth instead
of changing incrementally when the VFO knob is turned
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 frequency tunes in only one direction, i.e.
either up or down, there may be a problem on the logic

Symptom: Mic gain is low when using Heil microphone
Cure: Change value of R235 on PL unit to 200k.

Symptom: Radio trips/shuts down power supply when power
connector is plugged in, even when radio is turned off.
Probable Cause: Shorted PA transistors.
Cure: Replace shorted PA transistors. The pre-driver and
driver transistors are probably bad too. Check electrolytic
capacitors C25 and C29 on PA unit for swelling and replace
if needed.

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. USB/LSB RX frequency response
may be equalized, to a degree, by adjustment of the front
panel PBT control. TX frequency response remains poor.
Probable Causes: Misadjustment of the BFO and/or PBT shift
oscillator, or bad FL-30 on main unit.
Cure: Realign the BFO and PBT shift oscillator. If this
doesn't help, replace the FL-30.
Remarks: Some filters are better than others so some test
selection may be needed to satisfactorily alleviate the
problem. The better-performing (but more expensive) FL-80
filter can be substituted for the FL-30.

Symptom: "Chirp" in CW operation.
Probable Causes: 1) Low power supply voltage in TX. 2)
Astron power supply needs additional RF decoupling. 3)
Early production IC-735 needs factory update.
Cure: Check power supply voltage at pin 7 of either
accessory jack in TX mode. It should not vary excessively
from RX standby voltage or drop below 12 volts. If an
Astron power supply is being used with radio, install
additional RF decoupling capacitors to its output terminals.
If the IC-735 is an early production unit, it may need a
factory update to prevent chirp. Call Icom Technical
Support for update information.

Symptom: Harsh-sounding or extremely rough-sounding mic
audio when used on either antenna or dummy load. Average
SSB output seems low; must increase mic gain to a high level
for proper TX output. RX and FM TX are OK.
Probable Cause: Driver and PA idle current is low.
Cure: Align driver and PA idle current to specs: 100mA for
driver, 600mA for PA. If the PA idle current will not
adjust up to specs, Q4, D2, or D3, may be bad. If the PA
idle current is erratic, there is probably a cracked joint
at Q4 or D2. If driver idle current will not quite adjust
up to specs, install field mod: Decrease value of R18 on PA
unit from 120 ohms to 68 ohms, or simply, piggyback another
120 ohm resistor across R18.

Symptom: Raspy-sounding mic audio when used on antenna.
Radio sounds good when transmitting into dummy load. Power
output seems normal. May be complaints of RFI.
Probable Causes: 1) Station ground is in adequate. 2)
Astron power supply used with unit needs additional RF
decoupling. 3) RF feedback in mic circuit.
Cure: Check station ground. If an Astron power supply is
being used with radio, install additional decoupling
capacitors across output terminals. Short pins 6 & 7 in
microphone connector to remove RF feedback.
Remarks: If nothing helps, radio should be checked for
spurious output. (see below)

Symptom: Spurious TX on some bands. SWR reading may be
high when transmitting into a dummy load. Power is low on
some bands. Ic may be high.
Probable Cause: Bad components on PA unit caused by over-
voltage or overheating.
Cure: Check and replace as needed: C25 & C29 (1000uF, 16v)
and R25 (4.7 ohm, 2 watt) on PA unit. Also look for
overheated components and solder joints on the L4 assembly.

Symptom: No display, no RX/TX. Inspection reveals PLL
Probable Causes: 1) Low power supply voltage. 2) Bad IC8
on PL unit.
Cure: Measure power supply voltage at pin 7 of either
accessory jack. Should be not less than 12 volts for
reliable operation. If OK, replace IC8 on PL unit.

Symptom: No RX/TX when cold or warm. May have dead spots
in tuning range of each band.
Probable Cause: PLL unlock.
Cure: Align LPL lock voltage on PL unit.

Symptom: Frequency unstable in TX on 10 meters, especially
after warm-up.
Probable Cause: PL unit needs factory update.
Cure: On PL unit, change value of R163 from 3.3k to 1.5k,
and spread coil L36.

CJR 2/96
John Poet
"A treasonous voice of dissent"
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