In conjunction with British weather/ garage storage. Well I can't believe I'm unlucky enough to have 2 such repairs. Earlier one Marshall 1962, year 2003 bluish-white flash from the HT area of the mains Tx secondaries. Small arc over just 1mm or so , moving the wires apart and cleaning and telling the owner to store the amp properly, cured that (or at least did not bounce back).
This time Marshall JTM Tremolo 50W , 1998. The giveaway is that the HT fuse failed at switch off to standby, mains sw on. Replacing fuse and running up on a variac amp would work fine up to 95 percent mains and switching back to standby but 100 percent and switch to standby and Tx saturation noise and dramatic long bluish-white arc from near an HT(ac) terminal. As fuse is in the ac line and the o/c voltage goes up on removing the load on the valve rectifier at sw off. Between the purple "o" s on this pic,
No obvious organic mold marks elsewhere on the amp or even abnormal rusting of the steel bits. Any opinions on the derivation of this problem. I assume marginal invisible mold growth that carbonise over amp use, and chafing? and 500V or so pk-pk then carbonising of the nylon or Tx lacquer. Hopefully recoverable by cutting away/cleaning nylon and silicone sleeving over the wire and exrending to rejoin the tag, to move it away from the edge. Would there be some Marshall treatment/coating that may promote the initiation of this problem? yes I know its due more to owners than manufacturer