Yes. Everything works until it doesn't.Anyone know why this problem didn't occur earlier and why?
Clean the return jack. The cutout contacts on that jack have become oxidized or dirty.
Opening the amp is straightforward. SHould be screws through the top of the cab into the chassis. REmove them. There may be several smaller screws across the rear of the chasis, if so remove them. The chassis should then slide out the rear. Be aware ther will be speaker wires and possibly reverb wires that must be disconnected before the chassis is completely free.
Sometimes the chassis might stock to the cabinet paint from when it was new, and sometimes the front hangs up on the edge of the grille.