>
>I've received an SB-220 from a friend in non-working condition with the
>advice
>that the filter caps were bad and that I should get the replacements from
>Harbach, which is exactly what I did. After installing the new caps from
>Harbach and powering up the amp on 240VAC the amp did nothing more than throw
>the circuit breaker. After a couple of attempts while trying to troubleshoot
>various things the smoke escaped from the transformer. As I understand it,
>this sort of behavior is non-optimal and probably indicates that there is a
>problem with the plate transformer.
>
HV transformer failure is rare in the SB-220, but it happens
occasionally, especially with hours of berserko speech-processing and
screeching into the mic.
- 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]
>In retrospect, I realized that I probably should've tested the old filter
>caps before I had ordered replacements.
Original 220 filter caps typically fail in c.20-years, so it's probably
time to change them.
The 4-digit mfg date code should appear on the cases. The original
30k-ohms of the bleeder/equalizer resistors need to be at least tripled
to reduce the heat on the uncooled caps. Each 10ºC temp rise reduces
electrolytic life by half.
>After checking them with the cap meter,
>they all tested good.
Good means <200uA leakage at 400V - plus c. 200uF per..
>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.
>Can anyone give me specifications on what the resistance should be on the
primary
c. 1-ohm each.
>and secondary?
c. 12-ohms
> I assume that my problem is a short somewhere and that I
>should be able to determine that with an ohm-meter.
Not likely since the short is usually from winding layer to layer - hence
the DC-R decrease is less than tiny.
>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
>...
cheers, Heath
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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