Mark, I have never personally done the actual repair. My work in the past was to design similar equipment. But, I just looked at the service manual. My guess (AND IT IS ONLY A GUESS) is that the switch assembly will need to be replaced. I don’t think that Technics provided individual switches for replacements. There is a switch assembly that contains switches for a complete octave. The part number is QMWG6009BAK OR QMWG6010BK. Not sure which one it is. One is for 13 keys and the other is for 12 keys. I would guess that the entire assembly for one octave will need to be replaced. But, I cannot over-emphasize that it is only a guess. If a circuit board assembly were the cause, I would expect that the symptom would involve more than one key.

Since this problem is not uncommon, it is possible that your technician has already done a similar repair. He would be your best source of info.