Bought a non working KAWAI R-50 drum machine - please help : )

I would ask for some advice.
If it’s powered up the screen shows black squares for the first 8 characters, and the LED lights up (then goes dark slowly).
No sound, no MIDI output, absolutely no reaction.
I’d check the analog part of the machine, voltages are OK, also checked the traces, solder points.

So I think the problem is with the digital part. It uses M50734SP-10 CPU.
Thanks for any advices, ideas!