OldSynths

Greetings,

I've been having fun restoring my Prophet 600.  It needed many things 
repaired/replaced and I pretty much have it finished except for one 
issue:  voice 3 is out of tune by a half step (bringing the rest of 
the synth out of tune with it when played) and the filter seems to 
run wide open most of the time.

I've replaced the power supply caps and two of the regulators to help 
clean up the power going to the rest of the circuitry.  The 3372s are 
a replacement set from Chip For Brains (who I highly recommend if you 
need any of his parts).  One of the original 3372s was shot.  I've 
tried swapping in and out 3372s from voice 3 and the result is the 
same.  I replaced the 4051s.  I also resoldered all of the CEM 
sockets.

Okay, so instead of willy-nilly replacing all of the caps for that 
voice, I'm looking at the schematic trying to determine what could be 
causing the problem. I've read that the original caps are a source of 
pain for this machine so my strategy was to go in and measure all of 
the caps in that voice to see what they're doing.

Anyone familiar with this machine or a CEM3340/3372 architecture and 
know if I should be looking elsewhere? I've ruled out the opamps 
because they seem to be functioning fine by virtue of the fact the 
other functions they control throughout the voices work fine. 

I suspect the 3372 and its associated circuitry (swapping the 3340s 
makes no difference) so I planned on focusing on that. It responds to 
all parameter changes except when the fiter sounds wide open and the 
filter knob has little to no effect.  So what looks like an opamp 
connected to pin 15 (filter CV) or that cap before the opamp are 
mytargets, but first I want to look at the 2.2 caps in that loop 
coming from pins 11,12,17 on the 3372.  

I don't see any caps in line on the +15 and -5 lines, and since the 
other voices work I've ruled out those lines as suspects for now. 

Any insights or comments are appreciated! Thank you!

Rich