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Re: Damaged / Destroyed NST?



Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Simon!

Sorry to hear you are still having problems :-( but it
doesn't sound much like a damaged NST . . . yet.

Date: 16 January 2001 02:30
Subject: Damaged / Destroyed NST?


>Original poster: "Simon Yorkston by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <quantumx-at-ozemail-dot-com.au>
>


<snip>

>When I powered up [w/o topload or tank caps] something
strange happened. On
>the safety gap, there was a continuous very "smooth" arc
[eg, not the loud
>usual "cracks"] jumping across the LARGER safety gap. WTF?
I can only assume
>that the other 'side' of the protection scheme is faulty-
good assumption?
>[[What's strange is that the resistors on both sides were
warm, so the power
>was getting through]]


Well, if there are no caps the arc will be as you describe -
like a Jacob's ladder without the ladder ;-)  Remind me -
how much cap are you using with your 15/30, and what kind?
I know the resistor chain is 8x 3k3 = 26k4, I remember doing
the calculations!  These ought to drop no more than 800V at
30mA draw, so there should be a good 14kV available for cap
charging.  What I don't remember is how big the primary cap
is.

If both resistor chains are warm that does suggest that
there is a conductive path i.e. a complete circuit between
them through which current can flow.  This is how it should
be.  However, (says he having made this mistake once!)
double check that the caps are in series with the primary
coil and that the gap connects directly across the output of
the filter resistors. I managed to connect things in
parallel and stood there scratching my head wondering how
come I had power in but nothing out . . . duh.  You only do
that once <blush>!  The conductive path ought to be an
ac-only one via the caps or in other words the caps should
prevent there being a dc path through the primary coil.

The other thing to check is the spacing of the main gaps,
around half a millimetre each, maybe 1/64 in imperial
measure. I'm a little surprised that the safety is firing
with only two of the main gaps tapped in - I wouldn't
encourage you to open the safety gap but I did wonder how
big the main gaps were.  Certainly when things are going
right you ought with 15kV to be able to run more than a
couple of main gaps without the safety firing.  With 10kV
I'm running eight 0,8mm gaps = 6,4mm total and it should
scale approximately, so I'd expect a total of 8-10mm or so
of gap when everything is right in your system.

>
>I retapped the Sparkgap to just 2 gaps, and powered up.
This moved the
>sparks into the sparkgap, with the safety arcing every 5
seconds or so.
>Strangely, the SSPG would suddenly stop firing, and the
spark would go the
>safety. I though it might be ionised air, but I have a fan
on top of the
>SSPG.
>
>Anyway, I seemed to fix it, and placed on the topload. The
SSPG made those
>louder 'smack' sounds that we seem to be used to, but I got
NOTHING from the
>topload, despite any taps on the primary. *sigh*
>


Well, I have to admit this *does* sound as if everything is
doing what it ought to.  Based on Ryan's unhappy experience,
just check that the former isn't arcing inside.  Sniff
around the caps for insulation burns/failures.  Give
everything a thorough inspection, and *please* don't run
full power until you have sparks at low power!  I'm assuming
you do have a variac to help with this - it'll help preserve
the resistors when the gap is shortened.

How's the earth connection at the base of the secondary?  My
500VA coil doesn't seem to care whether it has one or not,
but YMMV.  Oddly enough, my 15VA micro coil doesn't like
being deprived of a good earth!

Shad's posting really says everything else I would add.  You
can also try rigging a grounded wire a couple of inches from
the topload and pointing at it.  I use that when tuning my
coils as you can see if things are happening right at low
powers, lower than would ordinarily cause spontaneous
breakout, and at such low settings you are unlikely to fry
anything if way off tune (though of course the resistors may
start cooking as I mentioned in a previous posting).  Keep
runs as short as possible until you are sure things are as
they should be, that way you are least likely to fry
something.  I generally limit my coils to bursts of a couple
of seconds at reduced power until I see things happen "up
top".  It's just a matter of twisting the handle on the
variac up until you hear the main gap fire - take a look "up
top" - power down.

Keep us posted.
Dunckx