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@arrl.net
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|