Discussion about the Korg PolySix synthesizer

I've done some tests today, but still haven't figured it out. I have traced the audio signal to pin 9 of ic15. At this point the speed potentiometer does not influence sound level. By IC 21 pin1 the sound level is effected. Weirdly pin7 of ic 21 voltage level changes when I move the speed potentiometer. I replaced ic21 but the results are the same. Not sure why vca mod would be changed by the effect speed potentiometer...this doesn't occur on the working board. Any thoughts welcome. Thanks