[Amps] SB-220 Transformer Problem

[Ron]

Hi Heath.
When you check the resistances , be sure to check from the primary to the core and
the same for the secondary as the insulation to the core that would normally be at
ground potential is sometimes a problem.
It is also possible to have a internal shorted turn on either winding and that
means a re-wind.  The shorted turn will not be detectable with an ohm meter but
will show up as a high current when the transformer is not attached to anything
except the primary that is energized.  Use a variac if you have one or a filament
transformer to apply low voltages to the primary to keep the destruction to a
minimum.
A few weeks ago , we were discussing the use of old microwave transformers for
service in amplifer use.
That might be possible if you find two that will fit.
---
Ron


Heath Albritton wrote:

> 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.
>
> In retrospect, I realized that I probably should've tested the old filter caps
> before I had ordered replacements.  After checking them with the cap meter,
> they all tested good.  At this point, my main question is what is the best path
> to determining and resolving the problem with the plate transformer.  Can
> anyone give me specifications on what the resistance should be on the primary
> and secondary?  I assume that my problem is a short somewhere and that I should
> be able to determine that with an ohm-meter.  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?  I suspect that even if it is possible to rewind it by hand that
> it would probably take more effort than it was worth.  I've seen original (used
> or NOS) transformers for sale for around $150 and I know that Peter Dahl makes
> one that runs about $255.  What would be the wisest course of action at this
> point.  Any advice would be appreciated.
>
> Thanks,
>
> Heath Albritton (KG6GTX)
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