Disc Ceramic Caps failing, Doghouse HV...
 

I'm worried about the disc ceramics in the set I'm recapping. One was missing it's coating and 20% of the disc broken off. You can see it in the first photo. Curiously, it had no leakage at rated voltage and not surprisingly it was 20% low in capacitance.

The second photo shows a disc ceramic in the HV Doghouse. The coating on it is peeling. It was open on the edge all the way around. The missing piece is from me poking at it. This one didn't have a wax coating. Don't worry, the bumble bee is going.

It's not clear to me whats happening. The rest of the disc caps in the set are wax coated and the wax is blistering. Some of them appear darker in areas.

I'm worried that I may have to replace them all. Since a large portion of the 143 capacitors in this set are disc ceramics, that would take some time.

Does anyone have experience with this type of behaviour?

One disc ceramic had been replaced with a paper cap. And a tubular ceramic cap replaced with a bumble bee. Guess thats all they had on hand.

There is a one turn winding on the HV RF transformer for the 1B3-GT HV rectifier. There is a 3.6 ohm 1/2 watt ceramic resistor in series with it. That resistor is open, which surprises me. I would have thought that with it running at a little over 1/8 watt a 1/2 watt ceramic wire wound resistor would last forever. The 1B3-GT tests good.

John

I've never seen disc caps in a set of that vintage ever, I didn't think they started that till the mid 50's sometime.

Really? The Motorola TS-18 chassis has a few and I think it's of the same year.

I tried to find when they were in common use. The only thing I could find with Google was that discoid ceramic capacitors became available in the 30's. At least according to a wikipedia article.

I replaced a couple with silver mica since I didn't have the right value ceramic caps on hand. I assume that should be o.k. They're a bit physically large though. I thought about using film caps. They're actually smaller than the silver mica that I had. Maybe I should have.

I'm going to have to bathe the coffee can HV section with electronic cleaner. (I referred to it as a doghouse before, but I guess that wasn't quite the correct term.) The HV capacitor leaked oil and it appears that corona changed it into something really nasty. I tested the HV transformer to make sure the coating wouldn't dissolve. It seems safe.

John

I've seen very old ceramic caps like that, but I don't think that means that they are bad. They may have looked like that fresh from the factory.

DId I miss it? What make and model set are you working on?

I agree with the above comments. Ceramic caps can look terrible, and still be OK. Replace the ones which are physically broken, otherwise assume they are good until you have reason to suspect otherwise. In general, they are extremely reliable.

Hi John,

No, you didn't miss it. It's the Sylvania 1-108 chassis, model 1-076. It's a 1949 model, but was made in late 1949 or early 1950.

Thanks for the feedback everyone. All the paper caps that had bubbles were bad so I was concerned that the same might hold true for the ceramics. I've worked a lot with ceramic discs, but they were more modern ones that aren't coated with wax.

Having found one that was replaced, one that was bare, and one with the coating falling apart had me nervous about the quality. I'll leave the rest alone.

John

Paper caps can look perfect on the outside but still be bad. Water vapor invades the paper & wax and makes them leaky.

I would test all of the paper caps . . . or just go ahead and replace them all.

Phil Nelson

I've been replacing all of the paper caps whether they're good or not. I've been testing them out of curiosity and have only found a couple of good ones. Most have very high leakage. The ones that tested good also lacked the bubbled surface. Though some that looked nice were still bad.

The set came from Georgia so there was plenty of humidity around to absorb.

Replacing all paper caps is the one recommendation that I do follow. I've seen enough of the damage that they can cause. Besides, when testing them I always see the leakage current increase over the period of time I test them. That has to be a bad sign.

I'll probably start following the same advice for electrolytics after I get burned for ignoring it.

John

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.

i'd a guessed those as thermistors -

http://en.wikipedia.org/wiki/Thermistor


may be a good idea to measure the capacitance, before installation

They do look similar, but these are disc capacitors. There are no thermistors in this set. They are more common in later sets, especially in the automatic degaussing circuit of a color TV.

I was fortunate enough to receive a copy of the original factory service manual from bgadow before I even found this set.

John

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

I wonder if it is possible to place a leaky (not shorted) paper wax capacitor in a sealed container of molten wax and apply a high vacuum for a length of time to extract the imbeded moisture. Fred.

If the idea is to maintain the original look, I'd gut it and stuff it with a film capacitor.

You could get rid of the moisture this way, but I don't think I'd want to trust it. I do have the facilities to try this with little effort, so I will out of curiousity to see what the results are. I won't reuse the cap though.

John

Problem is that the moisture has reacted with the acid in the paper, and that has reacted with the metal foil inside the cap, and that causes the leakage. Basically, it's too late. Just replace the wax cap, and throw the wax cap out.