OBERHEIM SYNTH

See my comment below...

At 10:53 AM 3/24/2011, you wrote:
>Hi,
>
>I've got an intermittant problem on my 8-voice OB-Xa that I'm hoping 
>someone can help me troubleshoot.  All 8 cards are fully calibrated, btw.
>
>Sometimes when I turn the machine on, all the cards on my upper 
>board sound as if their filters are not opening all the way for the 
>patches I choose.  Doesn't matter if they are 2-pole or 4-pole 
>patches.  Kind of a muted sound and is more apparent on patches 
>where the filters really open up (making a "mwaaooooww" sound on a 
>bass patch, for instance).
>
>If I cycle the power on and off a few times, usually this fixes the 
>problem and all 8 cards sound the same.  Sometimes it takes longer 
>to clear up.
>
>Does this sound like the CEM3360 on this board going bad or does it 
>sound more like a cap somewhere on the control board or mother board 
>starting to go?
>
>Anyone experience this before or know the fix?
>

I would look at the TL084s in the S/H circuits (especially for the 
filter frequency CV) on the upper control board to see if any of them 
are going into oscillation, which sometimes happens, throwing the 
effective control voltage off.  The fact that this is intermittent is 
what makes me think this is the problem.  The fact that it's all the 
upper voices points to the upper control board.
I don't know offhand the exact reference for the parts which control 
the filter frequency, but if you have a schematic, you should be able 
to figure it out.  Or, just swap TL084s one at a time with a new one 
until the problem goes away (or doesn't).

Good luck,
Paul W.