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IC-736 most common service | Radioaficion Ham Radio

IC-736 most common service


Most Common Service Questions for the Icom IC-736 & IC-738

©1997 ICOM America, Inc.

Unless otherwise noted, the following service questions apply to both models.

Symptom: Frequency jump in LSB after changing modes.
Probable Cause: LSB BFO is not being switched properly by logic; unit may need factory update.
Cure: With radio turned off, measure resistance of pin 17 to ground on IC24 on main unit. Should be 47k. If it measures above one meg-ohm, install a 47k, 1/8 resistor over the top of IC24 between pins 10 and 17.

Symptom: Radio is off frequency.
Probable Causes: Reference oscillator needs alignment.
Cure: Field: First reset CPU to clear all BFO presets, then tune in WWV and align L29 on PLL unit for zero beat in USB mode. Shop: Reset CPU. Align L29 for 60.0 MHz at P2. Check alignment of BFO.
Remarks: Before performing this alignment, make sure that update to prevent LSB frequency jump has already been installed in radio (see above cure).

Symptom: In IC-736, TX SSB audio sounds harsh. Average talk power seems low.
Probable Cause: Driver and PA idle current is set too low.
Cure: Realign idle current as per procedure on page 5-8 of the service manual with a change in specs: Adjust R8 for a 300mA increase and R44 for a 500mA increase. Total idle current draw should be about 1000mA(1A).

Symptom: In IC-736, unit will not turn on. Unit powers up if VCC cable is disconnected from PA.
Probable Cause: Shorted driver and PA FETs.
Cure: Replace driver and PA FETs. Inspect PA unit for overheated components and replace as necessary. Re-align idle current after repairs as per new spec. (see above)

Symptom: Automatic antenna tuner cannot find tune on some bands, typically 40 and/or 80 meters
Probable Cause: Burned capacitors on tuner unit.
Cure: Replace capacitors C2 & C6 (82pf & 100pf) on tuner unit with 500 volt (or higher) versions. Look for other burned capacitors in same area and replace as needed.

Symptom: Automatic antenna tuner hunts but cannot find tune on any band.
Probable Cause: Bad detector diodes on tuner CTRL unit.
Cure: Check these diodes on tuner CTRL unit: D1, D2, D3, D4, D5, D6, D11, and D12. Replace as needed.
Remarks: While tuner is disassembled, we recommend replacement of C2 & C6 on the tuner unit with 500-volt equivalents to prevent future failure. (see above)
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