Yes. TH3L20 has also gone but I have managed to find a replacement. The diode is fine. Only one output on YM3421 seems to have been affected (its 3.3k resistor has literally burnt out too)
You seem to know about these things. so I'll ask a question if you don't mind:
The PWM generator YM3421 just doesn't seem to be available anywhere but a chip called a YM3421-F is available. According to a service manual for a newer Disklavier This chip appears to have all the same functions as the old one but just different pin outs. Do you know if this will work if I simply re wire the pin numbers?