Vintage Synth Repair

Does anybody out there have any experience with the Arp Omni?  I have 
one I was working on for a while back to no avail.  Apparently the 
problem is in the actual tone generators - the filters work fine; 
when one presses the keys, you can hear the filter working its magic 
on the little "clicks" produced, and every key appears to function.  
There's just no tone to be filtered!  If anybody has any advice on 
how to repair the tone generating circuitry on the Omni, lemme know...

BaM