icom IC-745 Most Common Service Technical Support
© Icom America Inc.
Symptom: Frequency instability.
1) Unstable trimmer capacitor C5 on 2nd L.O. unit.
2) Dirty RIT/XIT pot and switches.
Replace plastic trimmer capacitor with a 30 pf ceramic trimmer and realign. Clean dirty RIT/XIT pots and switches with Blue Showers solvent.
Unstable C5 capacitor is very common in the IC-745. The plastic version of this capacitor should be replaced on all units that are in need of repair as preventive
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.
1) Bad trimmer capacitors on PLL unit, HPL VCO section. Replace plastic trimmer capacitors C4, C15, C24, and C33 with ceramic equivalents.
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.
1) Cracked solder connections on RAM unit is causing logic errors.
2) Program in RAM unit is corrupted.
3) Burned/overheated R20 resistor on logic unit is causing low voltage in logic unit.
Check for burned R20 resistor (10 ohm, 1/2 watt) on logic unit. If it looks OK, resolder cracked solder 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.
Symptom: Noisy and intermittent in RX. The preamp is also intermittent.
1) Bad preamp relay on RF unit.
2) Cracked solder on RF unit in preamp section.
Replace bad relay RL1 (FBR221DO12). Carefully examine the preamp section of the RF unit for bad joints and resolder as needed.
Symptom: Intermittent low FM TX output power.
It has poor contacts in relay RL1 on FM unit. Replace bad RL1 relay (221B-D009-M).
Symptom: No preamp operation. Sensitivity of radio drops when preamp is engaged.
1) Bad preamp relay on RF unit.
2) Burned components in preamp section on RF unit.
Replace bad RL1 relay (FBR221DO12). Replace bad/burned preamp components on RF unit, usually RL1 relay, Q11, Q12 transistors, R58 resistor, C86 capacitor, etc. Also check Q10 transistor for failure. If bad, replace it with a 2SC2878B.
Symptom: Low, erratic TX output in all modes. ALC meter reading is unsteady and may drop out altogether. Removing the top cover may cause problem to disappear for a while. Pressing on the wiring harness near the IF unit may cause the power/ALC meter reading to fluctuate. Problem may be temperature related.
1) Cracked solder joints on IF unit.
2) Dirty RF power/mic gain pot.
3) Poor connections at IF unit is causing low TX drive. This is sometimes caused by oxidation at the point where the wires are crimped to the connector pins.
4) Bad relay on FM unit (if installed).
Clean dirty RF power/mic gain pot. Resolder cracked and suspicious looking solder joints on IF unit. If these do not help, remove and resolder the wire crimps in the RF connector pins in J2, J5, J12, J9, J19 and J20 connectors on the IF unit. Be sure to not add too much solder to the connector pins or they will need to be replaced. Be sure all connectors are fully seated after repair. Replace RL1 relay on the FM unit.
Although resoldering the connector pins is a delicate and time consuming procedure, it will save you a great deal of troubleshooting time and frustration.
Symptom: TX power is marginal on some bands on my 745. The ALC meter reading is low but steady, not erratic. Cannot use ALC to adjust for more power. Troubleshooting reveals that no specific stage is malfunctioning.
1) TX IF needs to be aligned.
2) Modification to IF unit needs to be removed.
Set unit to 14.100 MHz, TX in RTTY mode, and adjust L8, L10 and L11 coils on IF unit for peak ALC meter reading. If this does not yield enough drive, remove the 220-ohm resistor on the bottom side of the IF unit directly beneath L8 coil and retune. Be sure to readjust power, ALC level and metering, if needed.
If the above steps have no effect, troubleshoot for a defective TX stage, or follow steps for curing erratic TX.
Symptom: VFO tuning is erratic, it sometimes skips when changing frequencies or bands. Displayed frequency does not increment smoothly when VFO knob is turned slowly.
It has a dirty rotary encoder. Remove white plastic cover from back of rotary encoder and spray clean the carbon track with Blue Shower solvent. Spin VFO knob vigorously while spraying.
Do not let solvent saturate the display unit or allow it to come into contact with the plastic display lens. Stuff a paper towel between the rotary encoder and display unit to catch spray run-off.
Symptom: TX power gradually drops off when warm. ALC meter indication remains at mid-scale. Adjusting the ALC does not help.
There is an APC misalignment. Realign ALC, Ic METER, and APC as per pages 9-1 and 9-2 in the service manual.
Symptom: AC hum or buzz in transmit and receive audio when an internal power supply is installed.
AC in being induced into the PLL from the AC wires and power supply case. 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.
In some radios, especially older IC-745s, it is not possible to completely eliminate all AC hum when using the internal power supply.
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. The TX frequency response is also poor.
There is a misadjustment of the BFO or PBT oscillator. Align the BFO and PBT oscillator as per page 9-4 of the service manual.
Symptom: Noise in RX, birdies.
The voltage regulators on the PLL unit are oscillating. Replace overheated C128, C129, C132 and C137 regulator filter capacitors on PLL unit.
Symptom: No receive through LB antenna jack. No BC band RX.
Open DS1 fuse on RF unit. Replace the fuse, part number 5080000010.
1) No RX/TX at all. Display normal.
2) No CW operation, or,
3) Some front panel controls do not operate reliably.
Burned or open R1 resistor on reg. unit. Replace R1 resistor (4.7 ohm, 1/2 watt).
Symptom: TX oscillations/spurious output.
1) Bad Q10 transistor on RF unit.
2) Open parasitic-suppression resistor on PA unit.
3) Bad electrolytic filter capacitors on PA unit.
4) Bad drivers or finals.
Disconnect J9 connector on the RF unit and see if the problem goes away. If it does, then Q10 transistor is most likely bad (replace it with a 2SC2878B). If disconnecting J9 connector has no effect, check the value of R22 resistor (4.7 ohms) on PA unit. This resistor may look OK but still be open. Replace C18 and/or C25 capacitors if they appear swollen or overheated. If none of the above are causing the problem and ALC meter deflection is low, then most likely the drivers or PA transistors are bad.
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 not common in the IC-745.