Vintage Synth Repair

Hi Nicolas,
I am not claiming to be a guru but I had a difficult Polysix here last 
weekend which couldn't read the Pot's correctly and did all manner of 
strange things as a consequence. Battery acid damage took out one of the 
multiplexers on KLM367, the CPU card.
Don't forget, most of the Sample and Holds for the Polysix are on the 
CPU board which is the one most likely to be damaged, and the keyboard 
filter tracking output voltage is on this (IC12 pin 7). Page 7 of the 
service manual is unusually clear for an old Korg manual. All the 
programmable Pot's outputs can be measured from the op-amp buffers on 
the CPU board (IC's 7 to 16, pins 1 and 7 on each) and they mostly swing 
from -5V to +5V, resonance is the exception swinging from 0V to 10V. It 
is probably worth checking that all these are correct before heading 
back to the voice card (KLM366).
Corrosion on IC23 is a little worrying, leaky batteries should not 
normally get that far, although the effects of leakage entirely depend 
on the physical attitude at which the keyboard was stored, as in was it 
on it's back, stored vertically either side down, upside down etc.
I had one here years ago where somebody replaced the bad battery with a 
cluster of Zinc Chloride batteries in a holder fixed to the right of the 
voice board, then stored it vertically pitch wheels to the ground and 
guess what, the NEW batteries leaked all over the Voice board.
I never could make this work properly, tuning was always bad, I had to 
buy a second-hand replacement board to make this machine work at quite 
an expense.
Do check all the Pot multiplexers, if all is good at least you know that 
setting up the basic test patch in manual mode is going to be effective, 
and will guide you more accurately towards the real problems.

Regards,
John.

www.retroactivesynth.com