Unpotting techniques...

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

Original poster: "Mike Novak by way of Terry Fritz <twftesla-at-uswest-dot-net>" <acmnovak-at-email.msn-dot-com>

Hello Everyone,
I thought I'd take a moment and share with you some of the little secrets
that I found make unpotting MUCH easier.

1.)Modified Freeze and Chip Method:
The original "Freeze and Chip" method was NOT as easy as everyone claimed.
It's dangerous for the windings and extremely messy as well. Here's what I
did; First, chizel off the case using a good sharp chizel and an angle
grinder to weaken the corners so they break easiily. Unclip the insulators
and disconnect their wires before removal. Then place the transformer
outdoors to aquire a good freeze (or in a freezer if you live in a southern
climate). Next, Using a chizel (or sharpened screwdriver in my case:) chip
off the thin layer of tar on the top to reveal the core. This IS as easy as
it sounds because the transformer's silicon steel doesn't bond with the tar
too well. Now that you've revealed the top of the core, locate the two pairs
of hot wires and the GND wire. Hook the input and the GND to a grounded
plug, and hook both HV leads to the core or GND connection. Once plugged in,
use a plumber's propane soldering torch to heat the core. If you use a fine
point flame, and stay to the center of the core, you can unplug it and turn
off the torch when the tar nearest the core melts to a real thin liquid (and
I emphasize THIN liquid, if you just soften it, it won't work as well). With
everything unplugged, use that sharp chizel and start removing chunks of
tar. It should come off real easily, and you should be able to pry off LARGE
sections at a time. If it didn't cooperate, just plug it in and apply heat
for another 10 mins of so, then try again. Now that you're done getting all
the tar goblins off your beautiful NST, we're ready to throw her in
solvent!!!

2.)Solvent Secrets:...
I've had a bit of troubles with solvents in the past. For instance, I always
have tar left in every crack which must be tediously removed by hand, and
when I'm done, there's a big old pail of  solvent which must be disposed of
:( If you're using solvent to get rid of the entire block of tar, you'll
have a REAL hard time getting rid of it. The best way to minimize the amount
of dirty solvent is to use the smallest possible container. What I suggest
is a 1-2 gallon rubbermaid container with a really tight fitting lid (i do
all my unpotting indoors during the winter so the lid is a MUST). Completely
disassemble the core before soaking, as It will save you time later. After
you've removed the bulk of the tar from the core and coils, you should be
able to put it in the sovent right away, just as long as the transformer is
cool enough to touch, although a warm transformer helps the tar dissolve,
you don't want it *too* hot because of the low flashpoint of pretty much any
solvent. If you have access to a heated garage, then here's one thing you
can do to speed up the process: Cut holes in the top of the container to be
used, place a couple peices of some kind of metal (must be at least 1"
thick) in the bottom of the container followed by a coarse screen. In the
space created beneath the screen, place a "bubble stone" (the kind found in
aquariums to add oxygent to the water) and run the tubing through the lid to
an elevated position in respect to the container so the solvent won't
"crawl" up the tubing. This bubbling action will act as a automatic stirrer
(if you can't afford a refugee from a third world country to do it for you
:).
Now that you've got a relatively clean transformer, you *could* immerse it
in oil. However, the oil will inevitably turn a lovely mud brown colour due
to the tar hiding in the cracks in the laminations. Of course, in a acrylic
enclosure, that gets quite unsightly. So, we're going to take the solvent
process  one step further. Get yourself a new gallon of solvent and transfer
the contents of the used container to a milkjug using a big funnel. Now,
take apart the core from the coils. Separate the shunts and the laminations
into piles to be cleaned. Aquire a bit of coarse sand (or maybe even
aquarium gravel might work). Start stacking alternating layers of a single
lamination, and some sand. You'll only be able to do half of the laminations
at a time if you've got the "optimal" sized container. If you've made the
bubbling assembly (or the heatless boiler as i like to call it:) you can
just turn it on and wait. However, if you didn't see the necesity of doing
so, you will now because you'll have to sit and stirr it for an hour or so
before the laminations are thoroughly cleaned. Repeat the process using the
other stack of laminations and the shunts as well. Once they've all been
cleaned, wipe the excess solvent off with a terrycloth rag ( paper towel is
just a total pain here).
Re-assemble the transformer the way you found it (minus about 6 leaves per
packet of shunts) and you've got a spotless transformer that's ready to be
mounted in any kind of enclosure you desire. I suggest HV oil, but Vaseline
is the next best thing.

Hope that helped you brave souls who dare to venture into the dark black
sticky depths of your NSTs...
Good Luck!!!!


Safe coiling !
-Mike Novak