Vintage Synth Repair

Does anyone have any info on the SX400. I've just acquired one which turns one with no sound output. It looks like the poly after touch
keyboard that's not triggering the synth. Well its been sat in a cellar here in the UK so its not in too good a condition. I'm looking to
either do a restoration or convert it to an expander unit if the keyboard circuitry is shot.

Thanks B

Doncaster