NST REPAIR

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

This morning I tried repairing a 9 kV, 60 ma transformer which has had
one side shorted for many years.  It's the one I used to sit out in the
weather to power up my skunk zapper, with the bad side shorted with a
wire.  Must have run up about 4000 hours of operation in rain and clear
weather.  Anyhow, I've always wondered if a heat gun could be used to
melt out the tar, and decided to try it on that one, which used to be
one of my favorites because of the small size.  Bottom line was that
after about an hour of heating with the bad end almost vertical, and
scooping with a popstickel (sp?) stick I was able to clear the tar from
the area around the insulator on the shorted side, and the remaining tar
was hot enough that I could move the bolt to the outside quite freely. 
That end of the core was exposed where it butted up against the inside
piece of the insulator.  I was actually using two heat guns, a 750
watter in my left hand and a 450 watt one in the right.  Bottom line is
that this was a quick and relatively clean operation (all of the melted
tar just ran down and "froze" inside the case). If I hadn't dropped the
tarry stick in my lap there would have been no mess at all.I stopped the
work when the bearings in the higher power one (a beautifully built
little job  made in Germany) froze from prolonged ON in the heating
position.  It took me all afternoon to repair that, because of the way
it is built.  I had to take the whole thing apart to get at the rear
(fan) end of the motor where the bearing was frozen.  Worked penetrating
oil and then lubricating oil around the shaft and into the bearing and
it appears to be working fine now.  It was a horrid job because of the
mechanical design and the use of 9 screws of four different sizes and
three different thread pitches, with a couple of screws of the same
diameter having different pitch for no reason I can think of!!!!  Had
fits with the temptation to cross thread the screws to get done in a
hurry, but fortunately I looked at two screws at once and realized what
was happening.

	After that I measured the DC resistance of both sides with a digital
multimeter and got about 2.66k for the good side and about 2.95k for the
bad side, so I assumed I had done something bad to the winding.  Went
off and let the thing cool down for a couple of hours and went back and
measured the resistance again; it came out at about 2.65 on the "good"
side and 2.68k on the "bad" side. (Apparently the "bad end" was enough
warmer than the other one that the wire resistance actually had
increased by almost 20%.) I then measured the voltage output from both
sides and found it to be an identical 4300V for each.  The thing now
appears to be functional at both ends and the arc length at breakdown
appears to be the same for each.  I let it cook for about an hour and it
was still working at the end of that time.  I didn't want to let it run
longer because it was sitting on an open work bench amidst a lot of
clutter, and accidents can happen!

	The time working on the transformer was less than an hour and a half,
but it took over 4 hours to fix the hot air gun!

	I don't know whether the repair will last, but thought the procedure
might be of interest.

Ed