Symptom:		Frequency unstable.
Probable Cause:		Unstable trimmer capacitor in BFO.
Cure:		Replace bad plastic trimmer cap C294 (30pf) on main unit with ceramic equivalent.

Symptom:		No RX, no TX. Dead spots in tuning range. Caused by PLL unlock.
Probable Causes:		Unstable trimmer capacitor is causing PLL unlock in DDS VCO.
Cure:		Replace bad plastic trimmer cap C205 (20pf) on PLL unit with ceramic equivalent.

Symptom:		No RX/TX, or distorted audio on one or more bands.
Probable Cause:		Unstable trimmer capacitor is causing PLL unlock in HPL VCO.
Cure:		Replace four plastic trimmer capacitors, C35, C46, C59 and C70 on PLL unit ceramic equivalents.
Remarks:		While the PLL unit is lifted, it would be a good idea to replace trimmer C205 (20pf) in the DDS loop.

IC-737 & IC-737A

Symptom:		RX/TX audio pitch change between USB and LSB, i.e. LSB has too many highs while USB has too many lows. RX audio may be "balanced" with the PBT control.
Probable Cause:		BFO or PBT oscillator is out of adjustment, or FL-30 filter is bad.
Cure:		Re-align BFO and PBT. If this does not correct the problem, replace the filter.

Symptom:		Unit will not switch between Antenna 1 and 2. Front panel switch and LED seem to work properly.
Probable Cause:		Stuck relay on ANT-SW unit.
Cure:		Replace bad RL1 (AE5343) on ANT-SW unit.
Remarks:		There is a factory update to prevent this failure in older IC-737s. Call Icom Technical Support for details.

Symptom:		Automatic antenna tuner hunts but will not tune on any bands, or sometimes finds a tune on some bands.
Probable Causes:		1) Bad detector diodes on tuner unit. 2) Bad tuner CPU.
Cure:		Check and replace faulty detector diodes D6, D7, D8, and D9 on tuner CTRL unit. Diodes are good, replace IC10 (uPD7533C) on tuner CTRL unit.

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