RE: Do I need to retune when adjusting a spark gap?

["David Dean" <deano-at-corridor-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Garry

> Original poster: "Garry F." <garryfre-at-pacbell-dot-net>

> I think I do, although so far, if I don't change anything else
> but the gap, I
> seem to find the best tap position is unchanged.
>
Changing the gap _should not_ affect tuning.

> Also, one of the NST's I recently seems to have a dead short in
> the primary.
> Sparks fly out of the wall plug when I plug it in. About half an inch long
> sparks, out of the wrong place but it does show promise for a
> mere 120 volt
> wall plug to throw sparks like that. Hmmm, maybe I've stumbled
> onto a wallplug
> to carbon converter circuit?
>

Sounds like it.

> Is there a chance that the primary windings are not a melted blob
> of metal and
> that by rewinding it, I might find the short and fix it?
>
In my experience, when the primary shorts, it trips the circuit breaker as
soon as you plug it in.
Perhaps one or both of the secondaries are shorted, thus causing the NST to
draw full load amps. That would cause energy to be stored in the
transformer, resulting in an increase in primary voltage when the primary
circuit is broken, thus the long sparks from the plug.


> The two I got were out of the same sign unit. The fellow said he
> had dropped
> the unit (Both NST's were inside!) onto a work table. I opened up
> the dead unit
> and the tar inside was had more cracks in it than humpty dumpty!

When a NST or anything potted in the same fashion is dropped it can crack
the tar, thus opening up the path for a carbon track. A whole lot of cracks
may well be the result of drying out of the tar, something that comes from
running at a high temperature for a long time. I found a pair of 15/30
Franceformers that way about a year and a half ago. They both worked OK when
I got them, even though half the tar was melted out into the cans they were
in when I found them. They had been on a roof supplying power to some signs.
The signs had been replaced with fluorescent lighted signs approximately
eight years earlier, and the NSTs had been supposedly killed in place. I
found they still were hooked up to power, 24/7, and the high voltage leads
were shorted, by burning the insulation off the high tension wires, to their
can. I thought they were dead, and took them home, rather that to the
dumpster, just to save the insulators off of them to use perhaps for some
future project. When I tested them, they both worked! One of them did not
last but a few seconds in TC duty, (unprotected, BTW) when I unpotted it I
found the tar dry and hard to remove. I had to turn the oven up to about 400
degrees for a while to get the tar to melt. (Phew!) The result was not
encouraging. The paper separating the HV windings was showing early signs of
carbonization. I would have to rewind the secondary coils to restore the
transformer. It still sits in a pile in the back. When its sister died,(due
to a misfire of a SRSG and no safety gap) after giving about one hour of
good service, I did not bother to try to unpot it, figuring that I would
find the same situation. It is still sitting in the pile in the back.
>
> I am just hoping the one I am using isn't humpty's twin except
> not shorted yet.
>
>
>
>