>its 3.3k resistor has literally burnt out too.
Wow!! It's too bad.
YM3421 is Dip IC but YM3421-F is QFP. (pin No, see the attached file).
It seems difficult to use alternative.
I asked Yamaha but YM3421 is out of stock.
But I/O board and Solenoid coil have stock.
The new parts price is 30000 Jpy and 900 Jpy.
My used stock price is 10000Jpy and 300 Jpy.
(shipping charge extra)
Please ask to your near Yamaha again.