[majoconz]

Let's not over-think this! It did work on the extension cable, you changed the extension and now the fault is apparent!

Is that a double-pole power switch on the back of the tone control. If only a single pole, which is more usual, why does it have the phase and the neutral connected to it? Unless, of course, the tag where the neutral is soldered is just being used as a tie point and another wire goes off to the transformer primary.

Incidentally, I don't know if it appears in the UK regs, but here you're not allowed the main earth-to-chassis connection to be soldered, it has to be a crimped terminal bolted to the chassis and it has to be near where the cable enters the appliance via a firm cable clamp.