Zenith CCII 25GC45 Oil Cap overheating

[wb2mep]

This is one of the vertical chassis sets with the constant-voltage (ferroresonant) power transformer and oil-filled motor-run capacitor. A few years ago the set would trip its circuit breaker after being on for a while. After a couple of resets, it eventually would trip as soon as you turned it on. I replaced that oil-filled cap with one from an even older CCII I had parted out (23", bad CRT) and all was well.

The set was kept in the garage and didn't get much use after the DTV transition, but I would turn it on every now and then to wake up the CRT and reform the 'lytics.

Last year we moved the set to the back patio and it started the breaker-tripping after warming up again. It would run a couple hours before tripping the breaker, and would always come back after the set cooled down. Last week, I tried replacing that oil-filled capacitor again, as the symptoms were the same as the first time it failed, and I had a N.O.S. (1976 date code) replacement. I left the set running with the new cap, and went in the house for a few hours, and came back and found the set off with a tripped breaker again.

So I set my DMM to monitor AC line current. From a cold start, the set was drawing 1.07 amps. It held steady for about 2 hours, then started creeping upwards. That oil capacitor was heating up. It was too hot to touch when the line current reached 1.3 amps, and the breaker tripped at 3.1 amps.

Is it possible that a N.O.S. oil-filled capacitor could have the same failure mode as a well-used one? I didn't think they had shelf-life issues like electrolytics. This capacitor is connected directly across the secondary of the power transformer. Is there any kind of fault in the set itself that could cause that cap to heat up? Thi pic & sound are good until the breaker trips.

[JohnCT]

That's weird. I changed a handful back in the day, and maybe "handful" was actually two or three.. I don't see how anything on the chassis could cause that cap to run hot.

IIRC, that capacitor is across a bucking winding and is meant to tune the transformer. I'm wondering if isn't a winding inside the transformer that might be shorting after the transformer gets hot (which those things do, even with no load).

Another possibility might be too much AC input? The way those were explained to me was that those transformers saturated at about 90V, and everything above that was wasted as heat. Not very efficient but foolproof regulation: they would output full voltage at 90V and could not go any higher despite the AC being increased beyond 90VAC. Back in the 70s, the AC was probably between 110 and 118V, but today, we can get up to 125VAC in some areas. As I type this, I'm getting 122.6 out my outlet.

This might be enough to aggravate the system.

What I would try is a motor start cap of higher voltage and mount that. There's plenty of room in there for alternate mounting if you need it.

John

[zeno]

The failure modes I have seen were
1) loud hum pops breaker. Dead shorted.
2) Runs hot, pops breaker after running a while.
3) pix pulsates with scene change. Open / ing.
I probably did a dozen of them from when new up to
the early 90's after that most got junked ( for no good reason).
And that was working on thousands of them.
I have seen quite a few bad ones on the forums lately.
Bottom line is GE ( made most of them) never expected or wanted
them to last 40-50 years ! That is overkill......
Test is easy. Unhook one end & see what happens current wise.
The only other thing I remember doing this was on REMOTE sets the
ON/OFF triac. That was rare. Other breaker pops were horz / HV related.

73 Zeno
LFOD !

[wb2mep]

Thanks guys. That confirmed what I was thinking, not much else other than the cap and the transformer to go wrong and these caps are reaching their "use-by" dates. But when a new replacement part fails shortly after installation, it usually means you fixed the symptom, not the real fault.
I read that the value of that capacitor on these constant-voltage transformers has to be correct as it forms a 60 Hz tuned circuit with the inductance of the transformer.
I still had the cap I removed from the set. It's a 3.5 uF, 6%, 440 V, but measured 3.99 uF on my DMM. I heated it with my heat gun and it rose up to 4.25 uF. So I think what was happening is that the resonant frequency didn't match the 60 Hz drive frequency, caused higher current thru the cap and transformer, heating it up, going further off resonance resulting in thermal runaway.
I do have one more N.O.S. capacitor to try when I have time to get back in to this set, this one measures 3.3 uF, increasing when heated.

[Tim Tress]

The issue with those capacitors is electrical leakage. That was a common failure in the old days. A leaky capacitor will give incorrect readings on a digital tester; the best instrument for testing capacitors is one which will test for leakage at the rated voltage.

[JohnCT]

Originally Posted by wb2mep

It's a 3.5 uF, 6%, 440 V

If you're not concerned with originality, you can pick up something like this:

https://www.digikey.com/en/products/...50AA4J/6556394

John