Hmmm, sneaky. Your KEY pot behaves like it is not connected somewhere. The microcontroller polls the eight knobs and reads their voltage, so many times per second. It maps the voltage onto a table to decide what value is desired. If it reads 0 volts, as when a pot is disconected, it will always display the first value in the table, in this case C. So since the pot seems completely unresponsive, we think there is a broken connection somewhere. Here’s how to check.
1. With the unit off, measure the resistance between the two outer pins of the pot. If it IS properly connected into the circuit, you should read 1.9kOhms. If the pot is NOT properly connected, you will read something higher.
2. If that’s the case, check all the solder connections associated with this pot, including its corresponding pin on the multiplexer.
3. Look also for solder bridges. This is an unintended connection causing a short. A bridge can consist of even a tiny whisp of solder touching two pads together that are not meant to touch. Tiny bridges can be popped off with a little gentle scraping. Larger bridges may require melting and removing the excess solder.