| Symptom: |
Erratic display,
segments missing from frequency display. Radio seems to work OK
otherwise. |
| Probable
Cause: |
Bad solder at IC1
display driver chip on display unit. |
| Cure: |
Resolder all pins on
IC1 chip. |
| 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
power 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 unit
for overheating. |
|
|
| 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
inadequate. 2) External power supply used with unit needs additional
RF decoupling. 3) RF feedback in mic circuit. |
| Cure: |
Check station ground.
If a 3rd party 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 capacitors (1000 µF, 16v) and R25 resistor
(4.7 ohm, 2 watt) on PA unit. Also look for overheated components
and solder joints on the L4 assembly. |
|
|
| 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 trimmer pot 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
Causes: |
Bad trimmer capacitors
on PL unit, HPL VCO section. |
| Cure: |
Replace plastic trimmer
capacitors C37, C45, C52, and C59 with ceramic equivalents. Also
replace the reference oscillator trimmer capacitor C138. |
|
|
| Symptom: |
Frequency instability. |
| Probable
Causes: |
Unstable plastic
trimmer capacitor C138 on PL unit. |
| Cure: |
Replace C138 trimmer
capacitor with a ceramic equivalent. We also recommend replacing the
HPL trimmer capacitors at this time. |
|
|
| Symptom: |
Intermittent mic audio. |
| Probable
Causes: |
Dirty mic gain control. |
| Cure: |
Spray-clean control
with Blue Shower solvent. |
| Remarks: |
Using a cleaning
solvent other than Blue Shower 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 displays
"flutters" back and forth instead of changing
incrementally when the VFO knob is turned slowly. |
| Probable
Causes: |
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 unit. |
|
|
| Symptom: |
No display, no RX/TX.
Inspection reveals PLL unlock. |
| Probable
Causes: |
1) Low power supply
voltage. 2) Bad IC8 chip on PL unit. |
| Cure: |
Measure power supply
voltage at pin 7 of either accessory jack. Should not be less than
12 volts for reliable operation. If OK, replace IC8 chip on PL unit. |
|
|
| Symptom: |
No RX/TX when cold or
warm. May have dead spots in tuning range of each band. |
| Probable
Causes: |
PLL unlock. |
| Cure: |
Align LPL lock voltage
on PL unit. |
|
|
| Symptom: |
Frequency unstable in
TX on 10 meters, especially after warm-up. |
| Probable
Causes: |
PL unit needs factory
update. |
| Cure: |
On PL unit, change
value of R163 from 3.3 K to 1.5 K, and spread coil L36. |
|
|
| Symptom: |
Mic gain is low when
using Heil microphone element. |
| Cure: |
Change value of R235
resistor on PL unit to 200 K. |
|
|
| 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
Cause: |
Misadjustment of the
BFO and/or PBT shift oscillator, or bad FL-30 filter on main unit. |
| Cure: |
Realign the BFO and PBT
shift oscillator. If this doesn't help, replace the FL-30 filter. |
| 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
Cause: |
1) Low power supply
voltage in TX. 2) 3rd party 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
a 3rd party 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. |
