Re: NST unpotting

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Ben,

The primary windings should have a very low resistance, like one ohm.

The secondaries will be in the 3 - 15K ohms range.

There should be no primary to secondary resistance (>10Meg ohm).

There should be a very high resistance from the primary to case (>10Meg ohm).

The center grounded secondaries should have, very roughly, 5k ohms to the case.

If I were going to test them, I would run them for one hour open load and
one hour shorted.  If that does not kill them, they are good.

To test the secondary voltages over such a wide range, I would simply see
if both sides arc to the case equally.  NSTs die one side at a time, so if
one side is bad it should be very obvious.  Unless an NST is bad, do not
worry with depotting them.  They can run perfectly fine as is.  NSTs
usually fail due to over voltage which LTR caps and protection filters can
easily protect against.  Simple safety gaps will protect against 90% of NST
failures.

You probably have a lot more questions and we have a lot more answers on
this subject :-))

Cheers,

	Terry
 

At 04:10 AM 11/22/2001 +0000, you wrote:
>  I recently acquired seven broken NST's for free. I have:
>3 4kV 30 ma Jefferson Magneteks, a 7.5kV 30 ma Magnetek, a 9kV 30ma 
>Franceformer, a 12kV 30 ma Jefferson (not a Magnetek), and a 15kV 30 ma 
>Franceformer.  All have center-tap-grounded secondaries, and all but the 
>9-30 are outdoor enclosure type.
>  They have high (6 to 10k Ohms) resistance across the secondary terminals, 
>low (~0 ohms) resistance across the primary, and no primary to secondary 
>continuity, although some have secondary to case continuity. Only the 15-30 
>appears to have life in it, drawing a 1" arc over a spark gap (straight from 
>120VAC, no caps, etc.)
>  Question: Are the resistances within acceptable range?  I am considering 
>de-potting them.  From the archives, I set each NST in a ~250 oven until the 
>black insulation gunk melts out, then I can add my own insulation, say, 
>submerge in a tub of mineral oil.  Would this work?  Anything different 
>about the Magneteks? I seem to recall cautions about depotting Magneteks 
>from previous threads.  What kind of ohms/continuity should indicate a good 
>NST?  What kind of sparks will I get directly off the secondaries?
>
>ben
>