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[Amps] SB-220 Transformer Problem

To: <amps@contesting.com>
Subject: [Amps] SB-220 Transformer Problem
From: 2@mail.vcnet.com (Richard)
Date: Fri, 5 Apr 2002 07:00:44 -0800
>
>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.  
end


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