Hi Everyone. Casio CTK-80 plays notes I didn't ask for.


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When I press certain keys, the adjacent key, a half-step away, also sounds at the same time. However, it doesn't work the other way--when I press the second key, the first one does not sound.

The way the keyboard works is: there's a plastic strip holding rubbery membranes containing little dots of conductive plastic. The keys press the dots against little black coated traces on the PC board. These traces are like interleaved little fingers, set up so that if anything conductive hits anywhere on these black areas, one circuit is connected to another.

I thought the problem was mechanical: maybe the membranes were hardening from age, and when one was pushed down, the dot next to it also made contact with its respective area on the PC board. And I noticed that if I was very careful when pressing one of the messed up keys I could get just the note I wanted and not the one next to it. But it was like thousandths of an inch difference before the second one sounded. So I cut the plastic holding the membranes between the two keys, so that the motion of one could not affect the other. It made no difference whatsoever.

I also powered up the unit with the PC board out and tried pressing a screwdriver against the black areas. Two notes a half step apart sounded at every site, not just the ones with the problem.

So I suspect the answer involves whatever coding/multiplexing scheme is used: The cable from the keyboard to the chip has only something like 14 conductors but transmits information concerning 49 different keys and combinations thereof. Obviously, this is the first keyboard I've ever tried to do a complex repair on--or understand in detail. Can anybody help me out here? Thanks.
 
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