Discussion about the Korg PolySix synthesizer

I have an old P6 that I'm restoring. I've manged to get every part 
working but the damn thing will not tune. I substituted the voice board 
and CPU board over and still the problem is the same. The power supply 
rails are rock solid. It has been suggested that the boards I have 
fitted (obviously second hand) could be suffering from an 
inherent/common problem and I'm beginning to agree this is possible. 
Basically if I tune C6 as described in the service manual all is well, 
I then adjust VR1 to get C2 into tune but when I do this C6 is now 
pitched at D#6. On checking C0 or C1 it reads B flat (C2 remains in 
tune and C3 is more or less in tune). I have set up several Poly 6's in 
the past and have never come across this problem. When I measure the CV 
with C0-C5 held, the voltages are not exactly 0.5v apart but I can't be 
sure if they are usually precisely 0.5, 1.0, 1.5v etc at the octaves 
are triggered. I've read on the forum about the opto but because the CV 
descrepancy I'm not too convinced at the moment. Any ideas,,please help

Thanks