RE: Warning about Magnetek(sp?) Jefferson NST's and a depotting question.

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Original poster: "David Dean by way of Terry Fritz <twftesla-at-uswest-dot-net>" <deano-at-corridor-dot-net>

Hi Garry,

I have unpotted several of these NSTs. They do not seem to be of very good
quality construction.
SNIP
>
> During the course of depotting I noted a few significant things. ....
>
> The tar was severely cracked and dried out compared to the other trannys
> that were in the same sign and were of the same age.
>
SNIP
> ---
>
> Also, what is the correct temp for melting tar out of a tranny in an oven
> without having the tar smoke a lot? I seem to remember 200
> degrees, but the
> tar wasn't even soft with this temperature after 45 minutes.
>
>
If the tar is in fairly good shape, 150 degrees F is plenty. When you get
the dried out cracked type tar it takes a lot more heat to melt it. I have
had to go up to 400 degrees and still had poor results. A lot of tar was
left on the core, etc, but it crumbles away easily while it is still warm.
Of course at 400 degrees the paper insulation between the layers on the
secondary windings will want to carbonize making the secondary windings
useless. I believe the failure mode leading to the dried out cracked tar and
possibly even the gaps in the tar, or uncoated spaces that you observed, is
that the transformer is run into a short circuit for a long time as well as
being in a location like up on a roof or somewhere that is exposed to the
heat of the sun. The heat from the sun combined with the heat from the
shorted transformer causes the tar to dry out by evaporating all of the
solvents in it. In the early stages, the tar is melted and leaks out of the
case leaving voids. I had three Magnetek/Jefferson transformers in that
condition. I also had two Franceformers from the same location that showed
all the same signs.

I would suggest if you have a transformer with dried out cracked tar that
you try soaking it in a solvent like kerosene or diesel. I have not tried
this, but I think that it should work OK. I tried a small sample of the tar
from one of the Franceformers that had not been heated in the oven (it came
from the container the NST was in on the roof, had already been melted out
of the transformer when I found it) and it dissolved quite rapidly. It may
be that the "heat work" is already done and all that is necessary is to soak
a little and the tar will come off easily.

Just a thought,
later
deano