[Milsurplus] 1MC system and B+ problem

[Richard Brunner]

I suggest substituting a new filament transformer for the 866A
rectifiers only, then if it still blows there is something failing on
the B+ line.  If the 838's light before B+ is applied the filament
transformer is ok, except for a possible rectifier winding breakdown to
ground with B+. (which you can live with with the new 866A filament
transformer)

Richard, AA1P

On Sat, 2010-04-03 at 06:20 -0400, Rob Flory wrote:
> I had the thrill yesterday of sounding the general alarm on the WWII-vintage 1MC system aboard MASSACHUSETTS.  500W from 4 push-pull, parallel 838s.  "This is just a test, DANG DANG DANG....."
> 
> Rather annoying problem with a second PA unit that we were trying to get running again.  After isolating a leaky filter cap and everything downstream from the rectifier, we were still blowing fuses when we put the B+ on.  We could run the HV transformer alone, disconnected from the 866A rectifiers or even with either side of the transformer connected, but with both sides of the transformer connected to the rectifiers we got a lot of current as indicated by bright purple flash in the rectifiers before the HV primary fuse blew.
> 
> We suspect an arc in the rectifier filament transformer, which shows no sign of leakage from the rectifier winding to the case or any of the other windings under test with an ohmmmeter.  
> 
> The next step is to try replacing the filament transformer, which also runs the 838s, with a seperate winding for each 838 filament.  Weighing in at something like 50lb and located just above the deck, this is what we call fun.
> 
> Anyone have any other suggestions?
> 
> RF
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