ICOM IC-781 TECHNICAL KNOWLEDGE BASE -------------------- Knowledge Base Article 5BGE535212 Issue My 781 has intermittent problems, including intermittent RX, TX, display, power up, and noisy RX/TX. Resolution Your regulator unit has problems. Heat and age is causing the regulator unit to produce unstable voltages to the radio. To fix: Recondition the regulator unit: Lift reg. unit PCB and rework all overheated and suspicious-looking solder joints. Repair cracked or damaged foil trace at L7 choke, if needed, and apply RTV compound between L7 choke and neighboring components to prevent it from coming loose again. Resolder joints at current-sensing resistor bar R22. Replace these commonly overheated components: L11 coil, C4, C28 capacitors, and R35 resistor. (For L11 coil, 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. 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 are in need of repair before any other repairs are commenced. -------------------- Knowledge Base Article 5BGE573121 Issue My 781 has 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. Resolution There are cracked solder joints at the crystal filters in IF unit. Resolder the filters. -------------------- Knowledge Base Article 5BGE592653 Issue My 781 has no TX output. PA is not drawing any current in TX mode. Resolution There is no 32 volts to PA unit from reg. unit. Resolder cracked solder joint at L7 choke on reg. unit and secure it with RTV to prevent it from shaking loose again. -------------------- Knowledge Base Article 5BGE5A6642 Issue My 781 shuts down when TX power is turned up. Resolution It has premature over-current shut-down. Resolder cracked joints at R22 current sensing resistor on reg. unit. -------------------- Knowledge Base Article 5BGE622A11 Issue My 781 has birdies, whistles, and/or hash noise in RX, especially on lower bands. Resolution The Reg. unit is running noisy. Resolder all suspicious looking solder joints on reg. unit. Replace overheated C24 capacitor and L11 coil in reg. unit. -------------------- Knowledge Base Article 5BH9421423 Issue In my 781, R20 resistor 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. Resolution This is a normal occurrence and does not mean there is a problem in the radio. -------------------- Knowledge Base Article 5BHB563839 Issue The scope on my 781 is inoperative, or the radio does something unusual (such as loses RX) when the scope is activated. The problem disappears when the scope unit is lifted or bottom shield is removed. Resolution Excessively long leads on bottom side of scope unit are shorting to shield. Trim the leads so they do not short against shield. Pay special attention to the leads around the outside edge of the board. These are outside the insulator inside the bottom shield. -------------------- Knowledge Base Article 5BHB5911A7 Issue There is no audio from headphone jack on my 781. Resolution Burned components on phones unit, most likely caused by RF feedback. Replace burned R1 and R2 resistors (10 ohms) with 1/2 watt versions. Check L8 and L11 coils. -------------------- Knowledge Base Article 5BHB6135A3 Issue My 781 has a noisy fan; rattles at some speeds. Resolution There are faulty bearings in the fan. Replace the fan. The fan cannot be lubricated or serviced. -------------------- Knowledge Base Article 5BHB6422A7 Issue My 781 has 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 connectors on RF unit. Resolution Bad solder at mixer FETs on either A or B channel. In the case of no RX, mixer FETs Q2 & Q3 may be bad. To fix: Resolder cracked joints in failed mixer unit mounted to RF unit. Replace faulty Q2 & Q3 FETs (both 2SK125), and burned L7 coil. The board runs hot around mixer section. Look for poor solder joints on this area on this FETs. -------------------- Knowledge Base Article 5BH9635A78 Issue My 781 has an AC hum or buzz in TX audio. Resolution Poorly shielded or improperly shielded cable is hooked up to accessory jack in back of radio. Replace cable and have accessory checked for faulty ground. -------------------- Knowledge Base Article 5BHA1126A3 Issue My 781 has intermittent CRT display problems such as loss of vertical hold, raster with picture, or complete blacking out. Resolution Possible causes: 1) Unsteady 13.8 volts or video signal to CRT unit. 2) Bad solder joints on CRT unit PCB. To fix: Verify steady and clean 13.8-volt supply at pin 1 of DC power connector on CRT unit. If this is OK, resolder cracked solder joints in CRT unit PCB. -------------------- Knowledge Base Article 5BHA134112 Issue The CRT on my 781 displays garbage or incorrect characters. Radio seems to work fine, otherwise. Resolution Bad CRTC unit on logic A unit. 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. -------------------- Knowledge Base Article 5BHA173982 Issue In my 781, the 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. Resolution Bad I/O transistors on logic unit. Replace bad Q3 and Q4 transistors (both 2SC2458GR) on logic A unit. -------------------- Knowledge Base Article 5BHB454826 Issue The antenna tuner for my 781 can't find resonance on some bands. Both motors turn when tuner hunts. Resolution Possible causes: 1) Presets far out of adjustment. 2) Burned variable capacitors C303 or C304. 3) Burned relays on AT tuner board. 4) Bad relay RL1 on AT controller board. To fix: 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 relay is bad on the controller unit, or there is a problem with the tuner SWR detection circuitry. Check for overheated RF switching components on the Connector 2 unit near the antenna connector. -------------------- Knowledge Base Article 5BHB481839 Issue The clock on my 781 does not hold time. Resolution Measure the lithium battery BT1 in circuit on logic B unit (top unit). If it measures less than 2.8 volts, replace it. -------------------- Knowledge Base Article 5BH9441834 Issue My 781 is dead, no lights, no display, no sound. Resolution Possible causes: 1) Open timer relay on PI unit. 2) Open R20 resistor (3 ohm, 2 watt) on PI unit. 3) Open start-up relay on PI unit. 4) Unit 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 resistor and main fuse to blow.) Correct each condition as needed. If R20 resistor 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. -------------------- Knowledge Base Article 5BH946693A Issue Power output from my 781 is too high (around 250 watts). Front panel RF power control and internal ALC adjustment have little effect. Resolution There are bad detector components on filter unit. Replace D8 & D9 diodes (both 1K60) on filter unit. Check L17 coil. Check filter unit for overheated components. -------------------- Knowledge Base Article 5BH9493681 Issue There is no TX power output from my 781. Inspection reveals that electrolytic capacitor C11 on PS unit has exploded. Resolution Biasing problem at driver transistors- base voltage is higher than .68 volts. 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 diodes (MV5 & 1S953). Realign driver and PA idle current. -------------------- Knowledge Base Article 5BH9512748 Issue There is no or low TX power output on one or more bands in my 781. PA-IC meter reading is high on faulty bands. Resolution Burned components on filter unit, usually toroids or relays. Replace faulty components and check the automatic tuner for a possible problem. Verify that the power output from the radio is not more than 160 watts. -------------------- Knowledge Base Article 5BH9531971 Issue No RX/TX on 20 kHz of each 500 kHz tuning step, especially when my 781 is cold or first powered up. Resolution PLL unlock. Adjust DDS-LPL in both PLL A and B units for proper lock voltage. -------------------- Knowledge Base Article 5BH9546776 Issue My 781 has a constant tone in the speaker when in CW mode. The radio goes into TX automatically in CW mode when VOX button is pressed. Resolution Shorted CW key line. Replaced shorted RF filter FI-103 on Connector 2 unit, and/or shorted F11, F12 and F13 filters on phones unit. Use shielded cable to CW key to prevent future occurrences of same problem. -------------------- Knowledge Base Article 5ATC6623AA Issue My IC-4KL will not turn on. No lights, no fan noise. A relay click can be heard when the power switch in the control head is pressed. Resolution Probable causes: 1) The circuit breaker is tripped. 2) A cable is not connected. 3) No +8 volts is being sent from exciter. 4) Power supply is blown. To fix: Check to make sure circuit breaker has not been tripped by turning it off, then on again. Check again to make sure all cables are connected. If they are, look for +8 volts DC at pin 1 of the DIN cable that plugs into the 4KL. (The IC-781 had an internal fuse on this line). If the +8 volts is present inside the 4KL, remove the cover to the reg. unit and look for obviously blown parts. If evidence of damage is present, the reg. unit should be sent to one of our factory service centers for repair.