My Prophet 600 was fully working, except for the keyboard and its contacts. All buttons and knobs were OK, MIDI as well...but only a few of the keys worked, so I decided to take it apart and restore the keypads
First, unscrew all screws from the wooden sided and open the Prophet 600. Un screw the screws on the bottom to remove the keyboard assembly then unscrew the 2 long baords that keep the gum keypads in place. Gently remove all keypads (those keypads were used everywhere in the eighties, on Korgs, Rolands and so on.
I used a product called "Caikote" by Caig to revive all contacts on those keypads. It's basically a paste with silver inside, hence the reason for its cost.
First I cleaned all contacts with some alcohol, then appplied a small amount of Caikote on the black contacts.
I also removed all keys from the keyboard assembly. Apply a small pressure on the spring and then gently pull the key to remove it. Be careful not to lose the small springs used to keep the key in place and allow its movement. The keys are labeled on the back, "A", "B", "C" and so on...
I soaked all keys in water and dish soap for a while, then cleaned them with a small brush.
After cleaning all keys, reapply them with their springs, apply a small pressure on the key and push the key in place. Reattach all keybeds and screw all contacts back in place. Screw everything back after checking all keys are 100% working.