Vintage Synth Repair

Hello!

Here's an odd one up for discussion. I have a Arp ProDGX. Everything 
works fine....except.....if I hit the top key (highest note) it 
briefly plays the note, then very quickly drops down to a much lower 
note (almost instantaneously). After this little event, every key 
stops working, other than a click for each note.

This happens on every patch.

Then, if I turn the power off, then back on, everything is fine again 
until the next time I hit that highest note.

Any suggestions on where to point my screwdriver and multimeter?


Thanks,

Jim Blair
Ape Island Sound
http://www.facebook.com/ApeIslandSound