Hello all.
I haven't had this problem, but having bought a used M13 that needed repair--and having seen the insides, I'd test those banks of black capacitors, one by one. The M13 I have had one cap terribly out of spec and another that intermittently leaked. I clipped the legs as close to the capacitor body as I could and soldered the new caps to the old legs, using clip-on heat sinks--I wanted to minimize heat transfer to the PCBs.
It's possible that a very small wire (like a brush wire from a pot) fell out--or a small wire or filing got in from somewhere else--and has established a bridge between traces. You might get lucky with canned air (make sure that whatever you use is guaranteed plastic-safe).
Tinker at your own risk.
