Moral:
When powering up any unknown or suspect piece of mains powered equipment,
from hairdryers through to mission control, get youself a standard mains
cord and in series with the hot or phase lead wire a socket for an ordinary
electric light bulb or lamp.
Power up with a low watts lamp in circuit.
If lamp glows bright then solve the problem before moving up to higher power
lamps and finally to the raw power.
Run this course with the amp.
BTW
Do this whenever fitting replacement electrolytic caps. They need a period
at low volts to form them.
Resistance checks of a transformer may help (check windings to frame) and if
the transformer is good will give you an unforgettable demonstration of the
ability of a collapsing magnetic field to induce voltage in an inductance.
But resistance checks are unlikely to reveal a common failure mode of
transformers, shorted turns.
In the case of the transformer you mention, it is likely that the fault is
in the secondary, but other components eg the rectifiers, may be at fault.
If the electros failed they will likely take the rectifiers with them,then
the transformer, but the fuse should blow first.
Capacitance tests alone will not identify faulty electros. Filter caps do
not live by capacitance alone.
Yes, you can rewind the transformer secondary by hand, not a big deal.
If you want to know more let me know after you have diagnosed the fault.
73
end
Barry Kirkwood PhD ZL1DD
Signal Hill Homestay
66 Cory Road
Palm Beach
Waiheke Island 1240
NEW ZEALAND
www.waiheke.co.nz/signal.htm
----- Original Message -----
From: Heath Albritton <halbritt@harm.org>
To: <amps@contesting.com>
Sent: Friday, April 05, 2002 10:46 AM
Subject: [Amps] SB-220 Transformer Problem
>
> 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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>
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