Polysix still not right after board replacement

Hi everyone

I am literally going insane on a Polysix repair. Thought I’d post here hoping to get advice by someone more experienced.

The synth has been fitted with a new KLM-367 repro board. I have salvaged the original 8045 MCU and a couple of other bits. The remaining are all brand new components.

I am having the exact same problems before the board change. Basically the filter cutoff, filter envelope amount, keyboard tracking and pw/pwm controls operate with a very slow response. As the pot are turned, the corresponding parameter slowly sweeps to the desired value.

Probing the outputs on the 4051 IC18 / IC19 while turning the knobs confirms that on the corresponding signals the voltage changes much slowly compared to the controls that are properly working.

I have tried disconnecting the other boards and just leaving the KLM-367 alone with the front panel boards and the same behaviour persists.
I have replaced the 4051 and the buffer opamps just out of desperation but obviously no luck.

I am pretty sure the problem must be on the KLM-367 and I am starting to really suspect perhaps the 8045 MCU might have been corrupted ? I don’t know if that would be even possible, I am making a guess just because the other parts are new.

I have completely ran out of ideas and it would be great if someone can give any suggestions.

Thanks in advance