My SP4-7 developed an issue last week: None of the controllers were working (volume, assignable know, pitch & mode wheels, sustain, expression) - but everything else were working fine. Did a reformat and OS reinstall, but the issues remained.

I tried reaching out to Kurzweil and they routed me to their India distributor, in Dehli, so not much help since I stay in another part of the country. Finally decided to dirty my hands.

Sourced the service manual which had the schematic and started tracing the circuit: The analog mux, controller voltage to balanced signals converter which is send thru the scan port to the engine board. (Found the engine board to be that of PC3LE!). Used a scope to trace the signals at the input at SH2-A (the 32 bit RISC microcontroller, a popular one for audio and instrumentation hardware devices). It leads to finding a corroded PCB track! Fixed it with a jumper and it fixed the issue!