On Fri, Apr 05, 2002 at 07:00:44AM -0800, Richard wrote:
> - Filament/bias transformer failure is more common because the
> transformer is not fused. So, with the stock biasing scheme, a shorted
> tube can destroy the fil. transformer in minutes. Changing the stock
> config from V-cutoff bias to R-cutoff bias will prevent a repeat
> performance. [see SB-220 circuit improvements article on my Web site]
Some of these improvements have already been made to this amplifier. You have
an excellent site, btw.
> Original 220 filter caps typically fail in c.20-years, so it's probably
> time to change them.
I concur. I don't feel bad for ordering the new caps, I just feel bad about
the fact that I assumed that they were bad to begin with. There is also the
heat issue with the bleeder resistors that the Harbach part solves.
> Good means <200uA leakage at 400V - plus c. 200uF per..
I'm not sure about the leakage, but they did give 200uF per, actually closer to
270uF per. In actuality, they probably aren't "good". I'm certainly not going
to re-use them.
> >At this point, my main question is what is the best
> >path to determining and resolving the problem with the plate transformer.
>
> Unsolder one HV secondary wire, apply 120/240V. If it trips the
> breaker, there's a shorted winding. If this eventually happens to mine,
> I would phone Peter W. Dahl and order a custom replacement with a tapped
> secondary in lieu of a tapped primary since it makes for a more efficient
> transformer design.
That was one of the "various things" that I tried which resulted in more smoke
and a tripped breaker. Did I mention the smoke? I'll plan on making a call to
Mr. Dahl and see about getting a replacement, though I'm not sure what the
circuit should look like with a tapped seconary vs. a tapped primary. Making
that change might be beyond my expertise.
> >Assuming that there is a short in
> >the transformer, is there anyway to get the smoke back in? Is it possible to
> >fix by hand?
>
> It's potted. The End/RIP
I'm sure it was a good transformer in it's time, it will be missed.
Thanks very much for all your advice.
Heath Albritton (KG6GTX)
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