Vintage Synth Repair

Hi,

Any been here and can give me a clue before I dive in a Juno 106?

Playing a key at a time, after the 5th key played, the 6th does not play. Starting to play again after the key that did not play again the same sequence happens: 1 to 5 ok 6th =no play. No matter what keys are played i.e. not in order, up or down etc.. each time the 6th does not play.
Another symptom that could/or not have to do with the above symptom is that a C note is heard in the background as soon as synth is switched on. I also noted (though I’m not sure yet) that this note in the background changes to the last 5th key note played after which the 6th does not play.

Any help please?
Btw.. the A1QH800170 chips are all original (coated/covered, in - epoxy?)

Thanks guys,
Noel