Vintage Synth Repair

Must be a common problem.  Here's a cut/paste from my previous post to
sdiy.  Hopefully someone can help. 

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