This made me laugh.

I have a Korg SP200 digital piano in for repair that had a block of 8 keys
not functioning. Took the offending keyboard PCB off to find the notes
would work if the scanning lines were manual shorted, but the conductive
pads refused to trigger them. (They have a resistance of around 60 ohms,
less if you press hard).

Couldn't work it out at all at first, the scan circuitry looked fine, the
pads just wouldn't trigger those notes.

On closer inspection, some of the '1' s of the scan datastream were low -
around 0.5 volts lower than the correct 3.3v.
It turned out that the scan circuity also scans the top panel switches and
LED's, and there was a partial short on one of the LED's, compromising that
one scan line that was also responsible for the non functioning block of 8
notes.


The machine is a single microcontroller, hence the use of the same port for
scanning both keyboard and switches/LED's, normally you would find 2
completely separate systems in use.



Gareth.