Weak NST

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Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Hi:

I found an abandoned high voltage supply where I work, that was
just a small NST, without any special (tar or similar) insulation
inside a box with two high voltage terminals at the top. Made by
the instrument builder Cenco. The NST has a 120 V primary winding
and a single 5000 V, 20 mA winding, each mounted at one side of a
core with two openings, with a shunt at the center. After cleaning
and disassembling the thing, I found that the NST works, but weakly.
The short-circuit current is really 20 mA, the DC resistance of
the secondary winding looks ok (~18 kOhms), but it produces at
most 1 mm weak sparks (5000 V should produce at least 2.5 mm sparks).
It can power my transformerless coils, but the output is insignificant
(I use normally a 5000V 30 mA NST with it, that produces far more
energetic sparks).
I imagine that this can be caused by internal sparking in the
secondary windings, a problem that appears only when the output
voltage is high. A carbon track crossing the paper insulation of
the windings is also a possibility. But the transformer doesn't heat
appreciably when left open, there is nothing wrong visibly,
and no funny smell.
Some idea about how to verify what is happening, or how to repair it?

Antonio Carlos M. de Queiroz