Thanks for the advice. Firstly I tested this board again in a working polysix and the volume was still an issue (therefore it had to be on the elm 368 effects board). I checked one cv at a time. The effects speed voltage was crossing to IC21 pin7( patch level), altering the voltage. This board did have battery damage which I had previously cleaned. Not knowing what else to check I re-cleaned the battery damaged tracks. This gave a slight improvement. I then took out my etcher and etched inbetween all the damaged tracks and this fixed my problem. I initially was trying to remove battery damage, but I gather now that the battery leakage is also conductive. Is this correct?
I've tested the keyboard twice tonight and it is still working. The voltage is constant on IC21 pin7 and not affected by the speed dial. I'll test it again tomorrow night.
Once again thanks. Hope this helps anyone in a similar situation.