Quote:
Originally Posted by Rick62008
C68 had a bad joint and was in a pretty sorry state as you can see by the picture
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Yes, it is interesting how these capacitors fail, and they are supposed to be high quality and a safety rated part because if they lose capacity and the EHT climbs as a result, there is more opportunity for Xrays.
Of these types of capacitors I have disassembled for failure analysis, they have lost capacitance because the metal film has been eaten away and there is a variable amount of damage to the insulating layers. It appears that nearly all metalized film capacitors can do this. ( The worst cases are seen in X2 rated caps, It occurs when the peak currents are high. In those ones the paper can turn to carbon too and they start drawing current and burn up). Sometimes resistance develops at the points were the lead in wires and film are bonded.Ideally the capacitor would have a solid foil and this would be welded to the lead in wires, but many are not like this.
Film capacitors were thought to be a good idea as they are compact, but they have this vulnerability relating to the fact that the metal film can be too thin to support the peak currents in the external circuit, but it takes time for them to degrade.
On the topic raised by Ukcol, it is possible for a spark or plasma to puncture glass, even microscopically. I once saw an arc puncture one end of an Argon tube connected to a 15kV supply. Atmospheric gases slowly leaked in and its color in that area changed from blue to a yellow pink look. Over about 15min the whole tube had changed color, then it failed, it was quite a sight to watch.