I'd try cleaning the control potentiometers/rotary encoders. Might not be the problem, but they should be fairly easy to clean. Remove the knobs and there should be small holes that you can spray some contact cleaner into. Give it a few small shots of contact cleaner and turn the knobs back and forth a few times. See if the controls are working. If so, give a tiny spray of WD-40 into the hole (optional) and put the knobs back on. If not, repeat cleaning process.
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