[Hallicrafters] SX28 power transformer

[Todd, KA1KAQ]

On 7/28/05, Roy Morgan <roy.morgan at nist.gov> wrote:

> If "brought back to life n stages" means "bringing it up slowly on a
> vairac" that was your mistake.  That killed the transformer and did not
> bring it back to life.

Roy and I disagree a bit on a few things here. Roy thinks variacs are
evil and shouldn't be used because you will kill your radio eventually
if you use one. I disagree. I also disagree that you can't 'bring a
radio up slowly' with one because you can - it's just a lot more
involved than plugging it in and cranking the knob every few hours. It
involves doing things in stages, removing the rectifier tube(s),
monitoring the draw of the filter caps, and so on. Seldom is this
done, but if done correctly, it does work. It's also a way to isolate
bad components or specific areas/circuits as you go. Of course,
plugging it in and cranking the knob works well for finding the weak
points too, it's just a more destructive way. (o:

> >  It was receiving CW on 80 at 65 volts. Maybe I
> > > should have let it cook at low voltage for a few days.
> 
> No.  You should have dried out the transformer and reformed/replaced the
> filter caps and replaced leaky coupling caps first.

True. 

> > >   Or maybe the transformer was just ready to pack up.
> 
> Nope. What you did killed it.

Sadly, this is likely true also. There is a chance that the
transformer was ready to go regardless of what you did first, but
considering the age of these things and the processes used to
manufacture them (more primitive insulation materials, for example),
care needs to be taken with the initial restoration and power
application.
 
> Roll dice.  Get lucky.  Receive a replacement transformer from a Good Guy.
> Are you going to blow out the new transformer by doing the same thing to it?

I suspect (hope?) the advice offered here will be helpful to Jim and others.
 
> It's also not a big job to arrange a couple of lamps shining directly on
> the transformer to bring it's temperature up to dry-out level.  Ordinary
> desk lamps with 60 watt bulbs work fine.

True, but this tends to take longer and is more difficult to do
effectively since it requires building a real heat box with proper
insulation and enough heat to maintain the level of heat needed for
the period needed, without setting the box on fire. (o: Simply
sticking a 60w bulb near the transformer in a cardboard box for a
limited time is more likely to give you a false sense of security than
it is to really drive moisture out of a transformer. It didn't get
this way overnight, so it takes more than a cursory pass to drive the
moisture out.

> >... Check your voltages, change the necessary components
> 
> No. Replace the caps first.

This is another area where I disagree(with one caveat*) with Roy,
simply because the 'shotgun' approach can and often does) create more
problems than it solves. Wholesale replacement of components can
create problems in at least two ways: first, if you use this approach
be prepared to spend hours, maybe days troubleshooting the entire
radio when it doesn't work right (or at all) after you're finished. If
you feel you *must* repair a radio that isn't really broken by
assuming it is simply by its age, do so in stages and apply power
after each stage to check your work. That way if you create a problem
while doing so, it's easier to find before you go forward.

 Second, certain circuits are made with a given amount of 'play' (for
lack of a better term) in them by using components that allow this
(capacitors labeled + 40% - 10%, precisions resistors or lack
thereof). Sometimes people assume these components can or should be
swapped out for better values, when they actually allow the circuit to
perform correctly over time as components age, or as power levels
fluctuate, heating, and so on.

The one caveat*: circuits that have the potential to take out critical
components (difficult/expensive to find) like capacitor for the
mechanical filters of the R-390A. The audio transformer of the
SX-28(*), -42, and -62(*) could fall into this category depending on
your view of finding a replacement or rewinding. Same goes for the
power transformer.
 
> Sorry if I sound harsh. I just hate to see yet another fine old radio blown
> up because of bad restoration methods.

I agree with Roy here, and would also add "bad packing/shipping
methods" to his statement. As the saying goes, "They're not making
anymore of these", so each one lost makes one less available in the
future after we're gone.

~ Todd/'Boomer'  KA1KAQ