I would suspect the voice board processor or its surrounding circuitry.
Two reasons:
1. It is responsible for the assigner buttons.
2. The "7th and 8th note are silent" problem.
If I remember correctly the voice assigner actually could serve different
numbers of voices and was also used in the Mono/Poly (with 4 voices). I
also recall seeing that the polysix key assigner could serve 8 voices max.
So it looks to me that the assigner somehow switched itself into "8 voice
mode" which could be a hint that the internal programm is faulty.
Two reasons:
1. It is responsible for the assigner buttons.
2. The "7th and 8th note are silent" problem.
If I remember correctly the voice assigner actually could serve different
numbers of voices and was also used in the Mono/Poly (with 4 voices). I
also recall seeing that the polysix key assigner could serve 8 voices max.
So it looks to me that the assigner somehow switched itself into "8 voice
mode" which could be a hint that the internal programm is faulty.