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NST death and resurrection
Hi All,
My 15/60 Allanson NST gave out on me a few days ago during
a run. The safety gap gave a loud pop and a flash and the coil
went silent. I disconnected the NST from the coil, hooked just
the static gap unit to it, and it would not even power a spark
around the 11 gap unit fully, but would do 3 gaps consistently. I
could tell there was some voltage present at the HV terminals,
but not the full 15kv.
The Allanson has a lid that is held on by 4 thin aluminum pop
rivets. I used my Dremel with the drill attachment to ream out
the rivets and had the top off quickly. I tipped the unit on it's
side, placed a 5 gal bucket underneath, and used a propane
torch to heat up the tar. After a few minutes, and using a
common wooden kitchen spatula, I was able to scoop out
enough gobs of tar where I could free up the HV secondary lead
wires on each side of the unit. I plugged the unit in and could see
quite clearly that one lead-wire was indeed shorting. I could see
where the lead wire went into the insulator that the short was
deep inside of the insulator, on the part of the ceramic that was
on the inside of the case. Where the wire entered the insulator,
I could see it arcing and melting the tar at the entrance to the
insulator.
I cut the wire at the point it went to the insulator, placed a test
lead on the remaining portion, and hooked it to the static gap.
It fired up all gaps. Since the short was at the point of the wire
where it was inside of the insulator, I busted the ceramic to bits
and saw that there was a inverted "U" shaped piece of metal that
surrounded the insulator on the inside of the case, but still buried
under tar. It appears that the short was from the lead-wire to this
piece of metal. It seems like the "U" shaped metal was part of the
insulator support system. I theorize that the insulator had a crack
on the inside where you could not see it, and the wire was causing
the short through the crack to the metal or it may also have been a
carbon track where the tar went alongside the wire as it went into
the insulator.
I removed the metal, and created a suitable insulator using two
pieces of PVC, one fiting inside of the other, re-soldered a longer
piece of wire onto the core lead-wire, and drew it through the PVC
to an end cap where I placed a 1/4" brass stud. I epoxied the entire
new insulator support system in place. I then put gobs of silicone
glue over both ends of the lead-wires, going to the core, and back
to the insulators. I allowed this to dry overnight, re-heated the tar
up and poured it back on to the exposed connections. After the tar
had cooled, I re-installed the NST back on the coil and works as
good as it ever did.
I was told later by a NST repair guy that this type of short is
pretty common in 4 out of 5 dead neons he sees come across
his shop. I can't say for definite sure if RFI kickback caused this,
as the unit was well used by the time I bought it from another coiler.
I did note some symptoms a few days prior - short pauses in the coil
firing so it appears that the short had been developing for a while.
I post this in hope that any coiler has this happen to them, try taking
the cover off and melt the tar out to where you can get to the lead
wires, plug the unit in and see if you can observe where the short is
located. Of course, this probably won't help if the lead wire has
been melted/burned at the core.
I priced a *new* Magnetek NST rated at 120ma and over $500,
so it pays to try and revive a dead NST, if possible.
Don
http://www.fwpd-dot-net/dona/tesla/teslacoil.htm