Discussion about the Korg PolySix synthesizer

Hello-
I'm working on an older version voice board (with the 396 add-on board) 
that won't tune.
The six oscillators can be tuned to match at the low points & high 
points, but there's no way to tune them to play anything like a 
half-tone scale.

I've tried the usual (replace the optoisolator, replace the 4051, 
replace the dual transistor, replace the TL071 opamp), but nothing helps.

Any ideas where else I should be looking?

Thanks.

~G