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RE: Failed NST

>Original Poster: "Vivian Watts" <V.C.Watts-at-btinternet-dot-com> 
>
>Hi All,
>    Today was a bad day.  After several weeks of constructing a Synchronous
>Rotary Spark gap and trying it out on my coil it appears my NST has failed.
>I have the ability to alter the phase of the rotary in real time and was
>adjusting it while sparking when the sparks just stopped.  The rotary was
>still arcing but no sparks from the toroid.  I can only assume my safety gap
>was set to large an the NST has been damaged.
>
>The failure mode of the NST appears to be it now gives half the voltage
>expected.  That is 5000v instead of 10,000v.  This I have measured by
>applying about 24V in and get 500v out.  So this would be 240v in to give
>5000V.  The NST is centre tapped and each half gives half the expected
>voltage.  Short circuit current is as specification 50ma at 240v in.
>Resistance of each half is about the same at 2.7K.
>
>Never having seen the construction of such a transformer I can only assume
>some part of the high voltage winding has gone short but I am surprised it
>appears to have happened on both sides.
>
>Any thoughts on repairs before I hack into it!!!
>
>Regards
>
>Viv Watts UK.

Typically the cap in an NST powered coil is sized to be roughly
mains-resonant, and assuming one's static gap is set to a reasonable
value, the gap will fire multiple times per mains half-cycle.  If you
then use the same cap with a sync RSG, the gap, by definition, will fire
only once per half cycle.  This means that the charging voltage on the
cap will be significantly higher, quite possibly to the point of
damaging the NST or the cap itself.  Also, if the phase of the sync RSG
is not quite right, the peak charging voltage will climb even further.

The cause of the NST failure is probably carbon tracking within the
asphalt potting compound.  The chances of repairing it by melting off
the old asphalt  and repotting (I chose melted Vaseline petrolium jelly)
are good, but it's not easy or pleasant.

Regards, Gary Lau
Waltham, MA USA