Discussion about the Korg PolySix synthesizer

hello,

I have a big problem with my polysix.

everything is fine, but when I turn it on, after a minute or two the
filter makes some strange modulations..  sounds a bit like fm ... I
will post an audio file soon...

I already changed the voice pcb from another polysix, and found out
that there's a problem with the digital board (klm367).

I already changed all electrolytic caps from the klm367, but no
improvement. I replaced the battery some days ago, the original
battery was only leaked a bit, but the pcb was clean and all traces on
the pcb are looking good.

do you have an idea what could be a reason for this problem...??

thanks,
mat