Just repaired a Crumar Organizer with the percussion section not working. The percussion section is not – as the name might suggest – a built in drum machine, but an add-on to the standard organ sound. It is basically a separate mix of the harmonics run through a master VCA which is triggered on key press.
I could trigger the effect manually but it didn’t work when pressing a key. I found no schematics, but after tinkering a bit I found that there were two reasons for the percussion effect not working:
1) The common trigger bus trace was corroded. I simply bridged the corroded parts and it worked again.
2) Every once in a while the percussion effect stopped working. My first thought was another bad contact, but it turned out to be a defective spring, which caused one of the keys of the keyboard not returning properly and thus “blocking” the trigger bus.