[sdiy] Oberheim Two Voice help request

[Andrew Sanchez]

Hello,
 Long time lurker here. I'm repairing a friend's Two-Voice, and could
use some expertise from the list.

 The problem is that SEM 1 distorts if the resonance is set to a high
level while both oscillators are at or near max gain. It happens with
all filter types. What's making it difficult is that I can't see (or I
don't notice) the distortion with my scope.

 One of the initial problems this SEM had was Q43 was bad. This is the
transistor that feeds the current bias to the two CA3080s. It was
causing the cutofff frequency to be stuck at sub-audio frequencies. I
replaced the transistor, but I'm now wondering if there are other
things wrong in the associated circuitry.

 For reference, the schematic is here:
http://www.tcapp.com/pub/synths/Docs/2Voice_Schematics/SEM-1A%20Schematics.pdf

 So far, things that I've replaced are:

VCA - the CA3080, Q43, and the LM741.
VCF - A14 (LM741), C22, and to fix a different, previous issue A13
(CA3080), and Q42.

 All signals that I've checked so far with my scope (Tek 422) seem to
be the same as on SEM 2.

 I've also tried swapping SEM 1 and 2 inputs at the output module, and
the problem moved to the 2nd output channel.

In case it helps, I uploaded an MP3 of a comparison between the two
SEMs. First in the MP3 is SEM #1 with the distortion. I sweep the
filter and then detune the VCOs slightly. Around the 40 second mark, I
do the same with SEM 2.

http://synthasaurus.googlepages.com/stuff


 The amount of distortion varies with the front panel VCO level and
filter resonance settings. Nothing else seems to affect the amount of
distortion.

Thanks for any help,
 Andrew