[Amps] SB-220 Shorted HV Transformer

[RDavis]

Hello amp-gurus.
I am in the process of restoring an SB-220 that was purchased from a SK's 
widow.
It had been modified to run on 11 meter only.  The tuned input was bypassed 
and the tank coil cut out.
 Yeah, a section actually cut out!
It also has a funky bias circuit, which I don't understand what it was 
supposed to do.

 When I first got it, I tried it on 10 meters to see if it was operational 
enough to consider rebuilding, and I got good power, ~ 1000 W on 10m.
So I acquired a tank circuit and bandswitch (the old one had the 40m tap 
torn off) and rewired it.

 Before settting out on so serious an enterprise as this, I thought it best 
to give the thing a bath.
 It was so full of dirt, dust and general crud. You know what one looks like 
when it has been sitting around in a garage for many years.

 I removed the tubes, front panel, and the electrolytics and put it in the 
dishwasher. No soap, just very hot water. I didn't know what the soap would 
do if it got inside anything.
It is not quite as clean as it might be if I had used soap, but it's in 
pretty good shape.

 Over the course of about a week, I went by the manual, step by step. 
Checking existing wiring, and undoing the mods to get it back to being an 
amateur amplifier again.
 Except for the bias circuit, I haven't done it yet. The grid RF chokes have 
30 ohm, 10W resistors in there place. The 0.82 ohm grid current resistor is 
1 ohm, and the parasitic chokes are a homemade looking affair.

 When ready to try it out, since it worked before, I went for broke and 
powered it up.
Meter light came on, fan spun up, HV went up to about 2800V, the tubes were 
bright.
Life was good.

 For about 10 seconds...
Then it started buzzing, smoke came out of it and one of the circuit 
breakers tripped. Subsequent examinations show negative grid current.
 This is all with only power applied. I had not hooked a transmitter or 
antenna to it yet.

 Fault was traced to a short in the HV transformer. One end of the winding 
is shorted (about 1 ohm) to the case.

 I have not heard that this is a common failure, so I am inclined to believe 
that I caused the failure.
 I have checked my wiring over and over, so I truly believe it is correct. 
And other than removing and replacing the caps, nothing was changed in the 
HV side.
I didn't change the electrolytics, but HV looked good.

The transformer is supposed to be potted, but do you think some water could 
have gotten inside, and if some moisture remained, perhaps caused an 
internal arc that resulted in a short?

 Is there anything else in the HV circuit that could have caused this, or 
was it just its time to go?

 I bought another transformer, but how can I make sure that I don't kill 
this one as quickly as the last one?

 Thanks in advance for you thoughts.

 73 es God Bless,
 Rick WD8JJA at arrl.net