Vintage Synth Repair

Hello, everyone! I have found an old yamaha CS01 synth in a pawn shop
and it worked fine so far until I borrowed to a friend and he had the
really stupid idea to hook a walkman into the breath control, hoping
to get some weird filter effects out of it and of course screwed it
all up! Long story short - it now has a flaw/feature Every time (and
only then) when I flip that switch below the mod/pich wheels to vcf
modulation and the mod wheel is set to 0, the filter(?) sounds gritty
and distorted which sounds kinda cool but I want to fix it now because
it's DEFECT and not a mod of mine! So where should I look? What should
I look for? I have a multimeter and nuff experience to replace some
caps and resistors and such. Just need someone to show me where to
start and what to look for.