Disc Ceramic Caps failing, Doghouse HV...

[jeyurkon]

Originally Posted by John Folsom

Hmmmmm.... the only thing more unreliable than an old wax paper capacitor is an old electrolytic capacitor. No doubt one can get away with leaving old wax paper and electrolytic caps in a set and have it work..... for a while. But unless you enjoy taking the set apart to find the next failed original capacitor, I would change all the electrolytics and wax papers.

While we are in agreement that all paper capacitors should be replaced, I guess the agreement ends there. I've found electrolytics to be much more reliable than paper capacitors. The most common failure mode of the electrolytic is evaporation of the eletrolyte. This raises the ESR and in filter applications causes further heating and acceleration of the evaporation of the electrolyte.
http://powerelectronics.com/images/a...rolyticCap.pdf

I've reformed the electrolytics and measured their ESR along with the leakage. I've only left them in place if the ESR is exceptional. I have many 30-60 year old electrolytics in frequent use.

I've had many modern electrolytics fail in a few years and I've seen reports of specific brands that were prone to failure in very short order. I think much of this is due to how well they were sealed. The ones that have lasted and still have a low ESR probably have many years left. They haven't been over-worked and have shown themselves to have a low evaporation rate, perhaps because of a better seal mechanism that has lower diffusion.

Other articles:
http://www.evox-rifa.com/technote_pd...fe_factors.pdf
http://www.edn.com/article/CA6588368.html

Having a low ESR compared to the acceptable ESR for a particular Voltage Capacitance rating is key to its probable life left.

John