Blown Neon transformers


   I completed my large transformer today, and two of the four neon 
transformers went bad after just a few minutes of operation.  I was 
using four 15kV neon transformers in parallel, one 60mA and three 30ma. 
 I checked the phasing before connecting them to the rest of the 
circuit, and it was correct.  I was tuning the primary and had just 
begun to get some very intense blue arcs when everything was reduced to 
a hum from the blown 60mA neon.  I am wondering if I have caused this, 
or if it is to be expected with old transformers.  I am also wondering 
if there is anything that I can do in addition to the normal RF 
filtering that will help prevent this type of failure in the future.  A 
brief description of my circuit follows.  Please excuse me if I have 
mentioned these details to you before.

   From the neons, my primary is protected by a typical protection 
circuit.  The bypass caps equal 500pF in each leg, and the chokes equal 
about 50uH.  My primary capacitor is a .025uF-at-20kV Condensor Products 
capacitor.  The spark gap consists of two cylinder gaps in series with a 
rotary gap.  The wheel of the rotary gap is 10" in diameter, has eight 
tungsten electrodes, and it is driven by a universal motor capable of up 
to 15krpm.  The primary coil is wound with 50' of 5/8" (OD) 
refrigeration tubing spaced 3/8" apart;  the inductance measured 49.5uH 
before I tightened the spacing up slightly (from 7/16" to 3/8").

  The secondary is wound on 10.5" OD polyethylene sewer pipe (the walls 
are 5/8" thick).  It has approximately 1150 turns of #21 magnet wire, 
which are a few too many, but I figured it would be easier to remove 
turns than to put them on.  The inductance measured 87mH and 40.19 
ohms.  The toroid consists of a 16" diameter aluminum plate with 4" 
aluminum tubing (24" overall).  The dedicated RF ground that I am using 
consists of a single copper clad ground rod.

   When I was tuning this coil, the only sparks that I got were some 
very intense thick blue arcs that went from one point on the toroid to 
either the beginning of the primary (about 3') or the beginning of the 
RF gorund.  The arcs were very fast, went straight down, and I could not 
tell precisely where they were striking.  The point that the arcs 
originated from was the blunt end of a brass screw that was pointing 
down (it is now very smooth and is pointing up).

   I would appreciate any comments or suggestions that you can find the 
time to make.  I would like to think that the transformers were weak, 
but I just don't really believe they were.  I don't want to replace them 
and have the same thing happen again.


          Jim Watson
          Pensacola, Florida