Vintage Synth Repair

>he schemos in part are pretty horrible I have for this machine.  But I
>have two of them that exhibit this odd effect on certain settings
>where if you hit one not...or the other...you get say .1V peaks on
>signal.  Hit both notes together and you get 3X the peaks.   Were
>amplification linear this would not be possible.  Also there are some
>harsh distortions associated that ruin the overall impact of these
>otherwise gorgeous sounding machines.  I wondered if it was a design
>but that only shows up on certain parameter settings but I'm not
>familiar enough with them historically to know.  It almost sounds like
>a parphonic effect! But as I monitor the signal coming off the analog
>board it's quite normal.  It's the signal from the chorus board that
>is giving the problem.  There are of course a coupl 571 chips on there
>and I checked the caps around them and they seem ok esr wise.  Anybody
>got a clue before I go to the pain of trying to get diagrams that I
>can read the small print on?   (FIgured since I have two of these
>doing this..someone probably has seen the problem in the past or knows
>if it's a design flaw of some kind)  -Bob

Sounds like an op amp (or transistor) might be bad.  Check the power on the 
chorus board as well, just to make sure it's not the source of the problem.  
Tracing the signal can narrow the problem down considerably.

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