My friend just bought a used Marshall MG50DFX and the amp will not change channels. It won't change via the button on the amp or by footswitch. We took it apart to check for loose connections, but everything was fine. I have searched the internet for similar stories and found a fair amount of people with the same problem, but no solutions.

The only thing I found that was remotely close was here - but they get way to technical.

Anyone have any ideas on how to go about troubleshooting the problem or fixing this?

Thanks in advance!