Ok so here I am with another update on this saga.
First up, do not EVER assume an IC is bad without checking all other components ! I recapped the power amp board and replaced the STK463 because I thought it was bad. The results were, well, not what I hoped for. The recap had made an obvious improvement because the DC was way down on the outputs BUT - the right channel was very quiet and distorted badly with increasing volume. After checking all the components of the power amp again, I decided to try dropping in the old STK463 again. As soon as I did that, both channels were back and going great !
Next up, I checked all functions of the EQ board, cassette deck and level meters and they all looked and sounded good. One thing that had been bugging me, however, was the odd sized capacitors on the power supply board. Reluctantly, I pulled the power supply out again and replaced the main power caps and filter caps. I also re-soldered the power switch was had cracking solder joints and was arcing quite badly !
I then moved on to the record player and tried to work out why the cartridge / door motor was running constantly. During this exploration, I switched on the unit only to be greeted by a big flash and a plume of smoke from somewhere near the main power switch. I turned out not to be as bad as I thought - what happened was the power switch was still not fully soldered down to the board and had arced again - this time burning a little bit of the PCB coating away, hence the smoke. I was relieved that I hadn't fried any components.
So right now, I have two things to do. One is to properly address the power switch issue and the other is to repair or replace the faulty 'player door open' microswitch. This switch is telling the player CPU that the door is open all the time (it never sends a low signal to the relevant CPU pin) hence why the motor is running all the time trying to close the door........
I'll be back with more updates soon.