[Amps] AL-1200 Intermittent HV Short

[Ed Briggs]

Three times in three weeks, when I turn the AL-1200 on, there is a flash and a bang from the RF compartment. This happens immediately at power up. Once it blew the diode that connects the b- to chassis (I replaced it with a 5408).  Once it blew the 15 amp main fuse. The other time, it did no harm.

Each time, the amp works fine after I shut it off and let my pulse slow down. 

I can't find the point of the arc.  I looked all over the place, inside the cover, at the chimney clips, etc etc. I can't find anything resembling pitting or carbon. And since I see a flash, I assume it can't be tube related.

The amp worked without incident for 3 month or so. Now this. What has changed. The "only thing" is that I changed the chimney from a 3-500 style chimney the correct 3cx1200 glass chimney.  No, I didn't bend the chimney clips. 

(Incidentally, I changed the chimney because it seemed to me that lots of air bypasses the cooling fins with the larger chimney.)

Ameritron says only thing to do is 'send it back', but since it is intermittent, I'm afraid it will come back 'no trouble found' or 'no trouble found, damaged by UPS on the way back."  
Has anyone seen this one before, or have any hints?     
I'm hoping to nail this before it kills something major like the operator, or even worse, the transformer.

Thanks for entertaining a vague, intermittent question.

Ed N1TS

"Life is hard, so why shouldn't amps be?"