I had a bad Roland Juno-106. It came with a broken key on arrival, some dirt and a group of voices not sounding. So I took the entire thing apart, cleaned/washed the keys (got a replacement key from eBay), internal parts, membrane assembly, traced the circuit to solder back the connection for the voices, gave it a new fuse and wipe. Voila! Very happy playing the synth (but it was sold since).