I have a prophet 600. Its membrane keypad stops working after the synth has been on for about half an hour. Anyone know a solution?

open the machine and locate where the membrane switch circuit panel has it's connections to the other boards and make sure their is no broken or coroded connections . SC likes to use cold solders ( so does everyone else ) but one reason why SC was bought by the Japanese is because their synths are UNSTABLE . But the PROPHET 600 does have historical value because it was the 1rst MIDI synth on the market .

It could also mean that your membrane keypad is simply going bad. Once that happens, your pretty much out of luck unless you can find another P600 for parts. You can still access the synth patches through Midi patch changes though...