Casio PX-110 - Dead Zone in mcpz-kyb2a, replacing rubber contacts does not help

On the three boards under the keys, everything appears to be fine (no burned out diodes, no scoring or scratching) but a stretch of a handful of keys that are over the middle board produce no sound. The cabling appears intact. Swapping out keys and rubber contacts with known good ones from other locations on the instrument has no effect. I’m wondering where to start troubleshooting. If it helps I can get some pictures with the non-functional area later today.

Anyone have any ideas of anything I should be looking for? The area that doesn’t work is contiguous, but it isn’t all of the keys on the middle board, just a subset of them.

I would test the flat ribbon cable from the middle board that connects to the main board - make sure there is continuity from a solder point on the key contact board to a solder point on the main board (test at the solder points to eliminate the issue of a good cable making a bad connection). If every wire through that cable shows continuity, then there may be a problem with the main board.

Exactly which keys are not working … then we can narrow down the area of the problem.