Re: Blown Cap

TL>Unfortunately, one of mine suddenly lost about 4nF during
TL>a run with  a particularly nice secondary on Saturday
TL>night. The sudden detune caused the secondary to smoke at
TL>the base. On the surface it looks as  though one of the
TL>internal units has gone open circuit. I'll be
TL>double-checking this tonight but there is little doubt. I 
TL>placed it with my second unit and things were away again.
TL>I checked it with a known inductance (8mH) to determine
TL>what it was. Bizarre in a way. I wouldn't expect caps
TL>made for this kind of service to go open circuit (or
TL>short for that matter, although that is how I would 
expect o
TL>ne  to fail). 
TL>    This seems to be an isolated incident at this stage.
TL>I was using  it with a 12kV 60mA neon and the total gap
TL>length (two series gaps) was 1/4" max.


Malcolm and others

I would expect these caps to have been burnt in before sale: 
thus the effects of soakage would have been accounted for 
when quoting the capacitance. Have you stressed the caps 
with HVDC and then bled them through a few tens of megohms, 
before re-measuring the capacitors? Also you might like to 
measure the C with some DC bias present. 4 nF in 50 is 
getting on for 10 percent which is a feasible change for 
short term dielectric relaxation but I would expect you to 
be able to recover the missing C. You could also check by 
using the C in an LCR cct and tuning for resonance etc etc.

Richard Craven