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IC-781 most common service | Radioaficion Ham Radio

IC-781 most common service


Most Common Service Questions for the Icom IC-781

©1997 ICOM America, Inc.

Symptom: ANY intermittent problems, including intermittent RX, TX, display, power up, and noisy RX/TX.
Probable Cause: Regulator unit problems. Heat and age is causing the regulator unit to produce unstable voltages to the radio.
Cure: Recondition regulator unit: Lift reg. unit PCB and rework all overheated and suspicious-looking solder joints (you may find a lot of bad joint to solder on an old unit). Repair cracked or damaged foil trace at choke L7, if needed, and apply RTV compound between L7 and neighboring components to prevent it from coming loose again. Resolder joints at current-sensing resistor bar R22. Replace these commonly overheated components: L11, C4, C28, and R35. [For L11, a type LW-16 choke may be substituted for the LW-15 for increased reliability.] Examine the top side of the reg. unit carefully for more overheated components. Replace all (formerly) blue electrolytic capacitors that have turned brown with heat and age. Be thorough. Clean board once repairs are completed and examine your work. Apply fresh thermal compound to heatsink-mounted components.
Remarks: Because it runs hot, the regulator is a common source of trouble in the IC-781, especially in those units with many operating hours. We strongly recommend inspecting the regulator unit on all IC-781 that come in for repair before any other repairs are commenced.

Symptom: Intermittent low or no RX/TX output. Radio is vibration sensitive. Symptoms may disappear when radio is turned upside-down or when bottom cover is removed.
Probable Cause: Cracked joints at crystal filters in IF unit.
Cure: Resolder filters.

Symptom: No TX output. PA is not drawing any current in TX mode.
Probable Cause: No 32 volts to PA unit from reg. unit.
Cure: Resolder cracked joint at L7 on reg. unit and secure it with RTV to prevent it from shaking loose again.
Remarks: While reg. unit is lifted, it would be a good idea to service it as outlined above.

Symptom: Unit shuts down when TX power is turned up.
Probable Cause: Premature over-current shut-down.
Cure: Resolder cracked joints at R22 current sense resistor on reg. unit.
Remarks: While the reg. unit is lifted, it would be a good idea to service it as outlined above.

Symptom: Birdies, whistles, and/or hash noise in RX, especially on lower bands.
Probable Cause: Reg. unit is running noisy.
Cure: Resolder all suspicious-looking joints on reg. unit. Replace overheated C24 and L11 in reg. unit.
Remarks: While the reg. unit is lifted, it would be a good idea to service it as outlined above.

Symptom: R20 emits a small puff of smoke when radio is first powered-up, especially if radio has been turned off for a while. Radio seems to work properly.
Cure: Do nothing. This is a normal occurrence and does not mean there is a reliability problem in the radio.

Symptom: Radio is dead, no lights, no display, no sound.
Probable Causes: 1) Open timer relay on PI unit. 2) Open R20 (3 ohm, 2 watt) on PI unit. 3) Open start-up relay on PI unit. 4) Unit set for plugged into 110v when it is configured for 220v operation. 5) Bad timer IC on logic A unit. 6) Cracked solder joints in reg. unit. 7) Shorted rectifier D1 on PI unit. (this causes R20 and main fuse to blow)
Cure: Correct as needed.
Remarks: If R20 on PI unit is burned, there may be failure on the reg. unit. Also, mount replacement R20 away from the PCB to prevent it from ruining the PI unit should it ever burn again.

Symptom: Power output is too high (around 250 watts). Front panel RF power control and internal ALC adjustment have little effect.
Probable Cause: Bad detector components on filter unit.
Cure: Replace D8 & D9 (both 1K60) on filter unit. Check L17.
Remarks: Check filter unit for overheated components.

Symptom: No TX power output. Inspection reveals that electrolytic capacitor C11 on PS unit has exploded.
Probable Cause: Biasing problem at driver transistors- base voltage is higher than .68 volts.
Cure: Replace all obviously burned components on PA unit, including driver transistors. Make sure VCC to PA unit is around 33 volts. If OK, disconnect VCC drivers and measure base voltage. If over .68 volts, replace D1 & D3 (MV5 & 1S953). Realign driver and PA idle current.

Symptom: No or low TX power output on one or more bands. PA-IC meter reading is high on faulty bands.
Probable Cause: Burned components on filter unit, usually toroids or relays.
Cure: Replace faulty components and check the automatic tuner for a possible problem. Verify that the power output from the radio not more than 160 watts.

Symptom: No RX/TX on 20kHz of each 500kHz tuning step, especially when radio is cold or first powered up.
Probable Cause: PLL unlock.
Cure: Adjust DDS-LPL in both PLL A and B units for proper lock voltage.

Symptom: Constant tone in speaker when in CW mode. Unit goes into TX automatically in CW mode when VOX button is pressed.
Probable Cause: Shorted CW key line.
Cure: Replaced shorted RF filter FI-103 on Connector 2 unit, and/or shorted F11, F12 and F13 on phones unit.
Remarks: Advise customer to use shielded cable to CW key to prevent future occurrences of same problem.

Symptom: AC hum or buzz in TX audio.
Probable Cause: Poorly shielded or improperly shielded cable is hooked up to accessory jack in back of radio.
Cure: Advise customer to replace cable and have accessory checked for faulty ground.

Symptom: Intermittent CRT display problems such as loss of vertical hold, raster with picture, or complete blacking out.
Probable Causes: 1) Unsteady 13.8 volts or video signal to CRT unit. 2) Bad solder joints on CRT unit PCB.
Cure: Verify steady and clean 13.8-volt supply at pin 1 of DC power connector on CRT unit. If this is OK, resolder cracked joints in CRT unit PCB.

Symptom: CRT displays garbage or incorrect characters. Radio seems to work fine, otherwise.
Probable Cause: Bad CRTC unit on logic A unit.
Cure: Troubleshoot and replace defective components on CRTC unit. Most common problem is one of the RAM chips has gone bad: IC3, IC4, IC5 or IC6.

Symptom: Computer control does not operate in "transceive" mode.
Probable Cause: Unit is configured for DEFT (default) in CI-V set mode. [A bug in the 781's logic prevents the CI-V interface from working properly in the default mode.]
Cure: In the CI-V set menu, change setting from DEFT to USER mode.

Symptom: CI-V control-unit does not communicate with computer. Inspection reveals that no TTL signal is present at the REMOTE jack when the VFO knob is turned.
Probable Cause: Bad I/O transistors on logic unit.
Cure: Replace bad Q3 and Q4 (both 2SC2458GR) on logic A unit.

Symptom: Antenna tuner can't find resonance on some bands. Both motors turn when tuner hunts.
Probable Causes: 1) Presets far out of adjustment. 2) Burned variable capacitor C303 or C304. 3) Burned relays on AT tuner board. 4) Bad relay RL1 on AT controller board.
Cure: Connect radio to a dummy load and try adjusting presets. If a low SWR cannot be reached by manual tuning, then most likely there are burned relays or capacitors on the tuner unit. If unit is able to find a tune in the preset mode but de-tunes in the automatic mode, then either RL1 is bad on the controller unit, or there is a problem is the tuner SWR detection circuitry.
Remarks: Check for overheated RF switching components on the Connector 2 unit near the antenna connector.

Symptom: Antenna tuner doesn't tune on any band. Inspection reveals one of the motors is not turning.
Probable Causes: 1) Jammed motor gear assembly. 2) Tuning capacitor plate has warped from heat and is now jamming motor. 3) Blown motor driver transistors.
Cure: Be sure that the motor is not jammed into one end of its tuning range. The motor can be unstuck by applying external DC directly to the motor itself. [15 volts is usually enough. Disconnect J1 before applying voltage to prevent damage to the controller unit]. If the motor is not stuck, then the problem is most likely blown motor transistors. These overheat when they blow, so damage to the board will be obvious.
Remarks: If the motor drivers are blown, the IC6 (5-volts) regulator on the tuner unit may also be bad.

Symptom: Clock does not hold time.
Probable Cause: Dead or weak clock battery.
Cure: Measure lithium battery BT1 in circuit on logic B unit (top unit). If it measures less than 2.8 volts, replace it.

Symptom: Scope is inoperative, or radio does something unusual (such as loses RX) when scope is activated. Problem disappears when scope unit is lifted or bottom shield is removed.
Probable Cause: Excessively long leads on bottom side of scope unit are shorting to shield.
Cure: Trim leads so they do not short against shield.
Remarks: Pay special attention to the leads around the outside edge of the board. These are outside the insulator inside the bottom shield

Symptom: No audio from headphone jack.
Probable Cause: Burned components on phones unit, most likely caused by RF feedback.
Cure: Replace burned R1 and R2 (10 ohms) with 1/2 watt versions. Check L8 and L11.

Symptom: Noisy fan; rattles at some speeds.
Probable Cause: Faulty bearings in fan.
Cure: Replace fan. (Fan cannot be lubricated or serviced).

Symptom: Intermittent static noise in either A or B VFO channel, or no RX at all in either A or B VFO channel. Check reveal that both A and B PLL's are locked, and 1st L.O. injection signal is present and clean at J4 and J10 on RF unit.
Probable Causes: Bad solder at miser FETs on either A or B channel. In the case of no RX, mixer FETs Q2 & Q3 may be bad.
Cure: Resolder cracked joints in failed mixer unit mounted to RF unit. Replace faulty Q2 & Q3 FETs (both 2SK125), and burned L7.
Remarks: Board runs hot around mixer section. Look for poor joints on this area on this FETs.
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