I have had a Marshall 1960A Lead 4×12 cabinet in with problems.

Only two out of the four drivers were working on mono 4Ω mode and one of the four was making bad noises. This cab relies on the switch contacts on the two jack sockets to connect the two pairs of drivers in series (16Ω) or parallel (4Ω) depending on which socket is used. The switch contacts on the original sockets are nickel silver or tin plated and can’t stand much current without burning out. This is what happened in this case. The arcing had heated the contact enough to melt the plastic body.

To fix the fault I replaced both switch jacks with gold plated Neutrik / Rean NYS2152G replacements. They fit perfectly and should last a bit longer than the originals. I also sprayed the centre Mono / Stereo switch with contact cleaner. There was also a loose spade connector on one driver which I pinched up with pliers to solve the bad noises.