More destroyed NST's How do I remove stubborn tar?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Garry Freemyer by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Garry-at-NDFC-dot-com>

Well, today I destroyed my fifth NST out of six tries and I am at whits end
on how to prevent this nasty problem from recurring over and over and I want
to ask people out there a few questions.

The problem - When I try to disolve the tar off of an NST I continually run
into the same problem where about 80% of the tar disolves redily but about
20% of it is like hardened concrete and these mounds of tar are so resistant
to solvents, so hard to remove that the toothbrush I was using to brush away
the tar was literally wearing through the cover of the windings exposing the
wires below which allowed the wires to be swept to and fro until I had a big
mass of snapped and broken wires that were being obscured by the tar
blackened solvent. I was just scrubbing at a stubborn wad of tar on the
windings and suddenly, I see a busted wire sticking out.

What I was wondering is has anyone else had reasonable success in depotting
or are we all failing misereably when trying to depot an NST? I have
recieved a number of emails from people who have told me that their attempts
at depotting were just as abysmal. I suspect a few are embarrassed to say
they failed and so they don't say anything, thus giving the false impression
that depotting is simple, easy and has a high degree of success.

As I recall, there are lots of sites describing depotting as being very easy
and simple to do successfully.

Claim: One book I have says just three days of soaking in mineral spirits,
kerosene or gasoline removes all traces of tar or that what is left is
easily removed by bit of solvent and a toothbrush. The book also says it
take only a gallon to clean an NST. I have heard this claim from several
sources.

My own experience is FAR from anything like this. I spent SIX WEEKS soaking
a transformer in mineral oil. I consumed 7 gallons of the stuff and the NST
still was so full of tar in the windings that immersing it in a fresh gallon
of solvent reminded me of the sight of an octopus spewing "Ink" all over the
place. The entire gallon was instantly black as night.

For those who have tried depotting, what is your experience? How many
gallons did you go through to clean an NST?

Many sources say one can melt out the core and most of the tar by putting it
into an oven at 200 degrees. Mine sat there for hours at 250 degrees and it
was still hard as a rock. Nothing even began to melt until 300 degrees and
that was only half. The rest was like concrete.

It is said to split the case at the corners but the ones I tried to split
were double walled at the corner with a PFC in one corner that got hit
because I couldn't see it.

One source said to turn the NST upside-down and lay a hot plate on the
bottom and let it fall out. This sounds great, but the allanson I tried this
on, simply spewed tar out the tiny cracks of the case and never did even
begin to loosen.

Maybe I should try gasoline. I am going to try resoldering these broken
wires by winding a copper wire around a thin needle to make a kind of sleeve
and slip the ends into the sleeve and solder it together. I doubt this will
work well tho as I can't see myself soldering this small of a connection
with my thumbs shaking like a blurred shaver head.

I haven't tried the freeze method. I guess that is next. I have my doubts
about this too tho but heck, I've destroyed all but one of my NSTs I have so
whats one more.

Any suggestions?

Anyone interested in depotting an NST for pay?

I remember Terry had a lead on a possible source of unpotted NST's from a
factory. I replied but I guess it fell through as I haven't heard anything
more.