[Hallicrafters] SX28 power transformer

[Todd, KA1KAQ]

On 7/29/05, Roy Morgan <roy.morgan at nist.gov> wrote:
> At 09:27 AM 7/29/2005, Todd, KA1KAQ wrote:
> 
> >Roy and I disagree a bit on a few things here.
> 
> Todd,
> 
> Well, not necessarily.
> 
> >  Roy thinks variacs are evil and shouldn't be used because you will kill
> > your radio eventually if you use one.
> 
> Not "will" but "can". And it all depends on how you use the variac.

Absolutely. My fault here. I took from previous posts that you were
very...anti-variac. It's certainly not an end-all for every problem
and does need to be used properly. Of course, you can turn a nut loose
with a screwdriver and they will destroy a nice radio, too. (o:

"Hmmm....lesseee what happens if I turn all of these shiny screws
unnerneath....should work better. And those little brass ones sticking
up out of the cans......yea...."

> >... 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.
> 
> I completely agree on this point.  My whole point is to get bad capacitors
> out of there before you put normal power to the radio.  (And get the
> dampness out of high voltage windings.)

Precisely. We tend to be a bit impatient with our new found treasures.
I confess - I have applied power to old radios before without checking
anything. The false sense of security it gives leads to those times
when P ON = MAX S(moke). I've been lucky enough to be greeted only
with a loud BZZZZ the times it went badly, no exploding caps. I'm much
more careful now.

> >...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.
> 
> This could very well be true. Maybe an initial resistance measurement
> between windings on the transformer would help you track how much moisture
> has been heated out of the thing.

It only takes one time to 'make a new man' out of you, as they say.
Granted, transformers can be rewound, but what a pain (not to mention
the expense). I go for baking a looooong time, and I use 175 degrees
only because my old electric oven is flaky and never gives the same
heat each time. Gary mentioned 225 degrees, I've heard as high as 250
works for others, so there ya go. Getting up to temperature is only
half the battle: driving out the moisture over some amount of time is
the rest.
 
> > > >... 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.
> 
> I was too terse: Replace the BAD caps first, and/or the ones most likely to
> cause damage (like audio output tube grid coupling caps.)
> 
> >  Wholesale replacement of components can
> >create problems
> 
> I agree. I don't recommend wholesale replacements of components (unless
> they are the failure-assured paper foil caps we hear so much about.)

See, I have a couple of thoughts here: first, a REALLY good visual
inspection using a magnifier will often show tiny cracks or other
damage, but nothing short of testing out of circuit will tell for
certain. I tend to go after these things after taking care of the
obvious and making sure there are no time bombs (crappy electrolytics,
direct shorts to ground or in tubes, etc) waiting to ruin my day. I'm
a lot more suspicious of iron than I used to be after being scared
that I had a short in the HV transformer in one of the big
transmitters. (Roy was actually at my place discussing this with me
during a visit). But if it looks suspect, take no chances. You can
always reinstall it later if it turns out not to be bad and you're
terribly fond of that particular component.

Second, (and I'm sure neither Roy nor I intend to indicate otherwise
by our remarks) you can go through all the steps, spend hours, days,
weeks checking things out, and the silly thing can still zorch when
you turn it on. While there are no guarantees, it's still best to
minimize the chances. Then, even if it does lose its smoke, you have a
better idea what is good and what isn't. Repair time should be
substantially less as a result (charred components stick out
like....charred components).

Now, lest anyone should think differently, I have met Roy and consider
him one of the finest, most decent fellows in radio and probably life
in general. He is a true gentleman and far more knowledgeable than me
(he works at NIST and used to fly whirlybirds: I'm a lowly mainframe
sysproggie and used to drive cars fast over a predetermined course).
True, he is older than me, but I'm catching up fast. If I read more in
or didn't read enough into his comments, I apologize as that wasn't my
intention. The main goal here is to save these wonderful old pieces of
gear without rendering them useless in the process; either by our
neglect or overly zealous attempts at saving them.

de Todd/'Boomer'  KA1KAQ (who is hoping to make Gaithersburg later
this summer and see Roy's radios along the way)