My friend has a Marshall JCM2000 2x12 combo that worked fine not along ago but suddenly stopped working. All the LEDs turn on and the channels can still change, but absolutely no signal passes through. Here is what we've checked so far: 1. The tubes seem to be ok - not sure if he checked to see if any of the tubes were dead but he made sure they were snug in the sockets. 2. It's not the guitar or the guitar cable - played it through his boss tuner and picked up a signal. I've told him to look inside the chassis and see if there are any visible issues but I don't know how much that helps. Can anyone help us please?