Discussion about the Korg PolySix synthesizer

My Polysix has developed a fault with its bottom 9 keys (G downwards) - 
first they started intermittantly failing to trigger when pressed, 
then they started doing odd things like being silent when held down together...... and now they're all just dead.

I dismanted the keyboard assembly and cleaned the conductive studs and contacts on the circuit board - didn't help. I doubt very much they're the problem, as swapping different rubber stud-holders around doesn't change things.
I'm wondering if the multiway cable between the keyboard and voice board could be the culprit? Maybe just one of those yellow wires is dud?
I can't even get it out to swap it, the keyboard end is stuck in its socket. 

Any suggestions?

(Oh, and it all plays perfectly if trigger exterenally via MIDI - I have Johannes Hausensteiner's retrofit)

David