(OT):keyboard repair

[Dr. Jörg Schmitz]

Hi all !

Who knows how to repair the keyboard contacts of an old Korg Poly 61 ?
1/3 of the white and none of the black keys are working, I know this
from my (died) Korg PolySix. 
I suppose, that the PCB with these rubber-kind contacts must be cleaned,
but with what kind of
solution ? Alcohol ? Contact-Spray ? 

Any help ?

P.S.: I found no schematics of the Poly 61 in the net : what kind of
filter design did Korg use ?
There are no SSM filter chips, like in the Korg PolySix.