Discussion about the Korg PolySix synthesizer

I wonder if anyone would have any idea about the following problem...
It's a friend's Polysix we both recovered from battery leakage a year
ago, and left it untuned after we experienced some problem tuning it.
Now we got back into it and we realized that on the tuning circuit
KLM-366 new production):

VR2 - just doesn't make any change to the tuning... (the resistor 
itself is ok)
VR1 - When adjusting it the pitch will drop or raise the first second
but than will slowly get back to the initial pitch before changing
its setting !
The VR determing the D#4 to E4 gap seems to barely make any effect...

All other VRs (Mid tune, VR3 low tuner, VR10/11 on all vco's) seems
to work, but strangly VR15 makes change to the sound but not to pitch
(I understand it is also a kind of mid-tuner).

We replaced both 4051 in the tuning circuit and the two 072 and it
didn't make any change... Well, actually, after replacing IC39 the
low tuning started to work more or less ok (it is tuned more or less
at the lower octaves but not at all at the higher octaves...)

I wonder if anyone has any idea what could cause it ...

Thanks all.