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

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

Original poster: "Bill Vanyo by way of Terry Fritz <twftesla-at-uswest-dot-net>" <vanyo-at-echoes-dot-net>

I've depotted one 15/60 Franceformer - successfully or not, I don't yet
know.  It looks pretty clean, and I don't see any damage, but I haven't
tried it out yet.  I'm a little unclear about what to do next - I've
read they can be run dry, but I'm skeptical.  It seems like it would be
pretty easy for the secondaries to arc to the core.

It certainly isn't (or wasn't for me) as easy as most descriptions make
it sound.  I used a hotplate, and as you described, got tar dribbling
out of the bottom seams.  Then I used the hotplate on the two sides, and
scraped out the tar when it got  soft.  It took a while, and there were
layers of mica and junk to pull out.  Then I split the case at the seams
with a chisel - rather easy, as it was spot welded.  With the help of
heavy tin snips, I got the case off (destroying the case, unlike
descriptions I've read where you just repot it  with vaseline in the
original case).  I chipped away as much tar as I could with a
screwdriver, but always being careful to drive it at a tangent to the
surface, especially near the coils.  Some large chunks peeled away from
the core rather easily when hot.  

Then, I soaked it in mineral spirits (you said mineral oil - did you
mean spirits?  I wouldn't think oil would work).  This was the messy
part - every few days checking it, seeing what I could get off, still
using a screwdriver (carefully).  I used my bare hands quite a bit to
massage off loose tar - it doesn't damage the windings, but... I had my
hands in that stuff quite a bit, and I could definitely feel that it's
not a good thing - strange sensations on the back of the hand afterward
- finally got a good pair of neoprene gloves.  Eventually got to the
point where I could seperate the two halves of the core (the 15/60
Franceformer has two E shaped halves), and remove the windings.  Then I
soaked the windings in a fresh batch of mineral spirits, and put the E
shaped laminations and shunts back in the original slop.

Soaking took about three gallons of mineral spirits, and two weeks, with
repeated taking it out and removing what I could however I could -
mostly screwdriver, some toothbrush near the end.

So now I've got it all clean, but I haven't yet figured out just how I'm
going to reassemble it, as far as insulating the secondaries from the
core.

It would be nice to be able to buy the basic parts from the factory -
the core, shunts, primary and secondaries.  After all, part of the cost
of the NST goes toward all the stuff we go through such great pains to
undo.  The parts - core,  shunts, primary and secondaries, new and
untouched by tar - should be cheaper than the finished NST's.  Perhaps a
special order - NST's  with just that last step - filling it with tar -
left out.  Case and all, with whatever kind of insulation (mica,
etc...), just ready for us to fill with something other than tar
(vaseline, transformer oil).  The case might need some kind of sealant
at the seams - not sure what would work.  The only problem being they
might not be able to factory test them without potting (perhaps test at
a lower voltage?).

BTW, I don't have a coil yet - I'm moving along slowly but surely toward
that.

	-  Bill Vanyo

Tesla list wrote:
> 
> 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.