Ive mapped out the circuit diagram, and can now start working out how to fix it.
My suspicion of the lack of 518kHz was with the transistors, but ive had them off the board today and tested, and they check out ok.
Now, I know the PIC oscillator is not working. The original crystal was corroded, so has been replaced by one close but not exactly right. I would have expected it to work but give perhaps slightly off frequencies. I suspect that at some time the antenna feed has been reverse polarised, and this has killed the chip. It might well have killed the MC1496 as well.
So, the plan now is to remove the MC1496 and the PIC 12C508 from the board, plus lift any associated components that might affect the signal path, and see if I can get it working just on 518kHz, by bypassing the conversion circuitry. Im also going to remove the transformers and test them off-board to ensure their windings are intact, as some corrosion/dirt can be seen under one of them. Testing them on board is pointless as the DC circuitry is routed through them/across their pins.
If I can get that working, then I can work on the conversion side another time.
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