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 Post subject: Zenith 4 lead capacitor.
PostPosted: Mar Wed 11, 2020 3:30 am 
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I have a Zenith 19FC45 TV that I got from an uncle quite a long time ago. I have been using it occasionally without quite thinking of it as an old TV. About a week ago I started using it a little after not having had it on for about a year or so. Well, after a couple of days one of the filter caps shorted out. So now I have it apart replacing caps.

But while working on it I noticed a sticker on the back that says "4 lead capacitor modified". And looking around I found a 4 lead capacitor. So I am wondering what that is all about. I am guessing that the capacitor has a lot of current running through it. And perhaps connecting two leads to each foil in different places allows the cap to handle the high current better. And I am also guessing that the original capacitor was not quite good enough and could fail.

But that is guessing. Does anyone know the facts?

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 Post subject: Re: Zenith 4 lead capacitor.
PostPosted: Mar Wed 11, 2020 3:50 am 
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Location: Detroit, MI USA
The 4 lead capacitor is in the horizontal output stage collector circuit and emitter circuit. It "tunes" the circuit to control the HV within a narrow spec, based upon the set's LV power supply coming from a regulated power transformer with a resonant cap. When everything works right, it's a decent design, but relies on the 4 lead cap always being functional as it was intended to be.

The two leads on each end of the 4 lead capacitor complete a circuit, so that if the cap fails, the circuit opens and kills the HV. At least that is how it was SUPPOSED to work. Like a fuse.

In reality, the original white 4 lead caps were garbage, and could open up the capacitor internally but leave continuity through the leads on each end, and at that point the HV could go up to 40KV or more.

This would result in arcing through the CRT's glass neck from the anode element of the gun to the ferrite pole pieces of the convergence coils. Next thing you would hear was a loud BOOM as the neck cracked and separated from the CRT. Frightened more than a few customers when that happened.

They quickly switched over to an orange 4-lead cap made by Sprague, which did not typically have the catastrophic failure of the original white part. A recall was issued by Zenith and supposedly most of them were replaced for free even on sets out of warranty. Of course, any resultant CRT failures were also replaced for free, and a new convergence assembly was provided as well. A very few sets would experience failure of the horizontal output transistor or damper diode (shorted) at the same time the 4 lead cap failed internally open.

I probably still have a few of the 4 lead cap replacement kits which were provided as part of the recall, we got them in large quantities from the local Zenith distributor. The part number was 800-860 and there are usually a bunch of them on eBay even 45 years later.

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 Post subject: Re: Zenith 4 lead capacitor.
PostPosted: Mar Wed 11, 2020 4:01 am 
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Thanks for the explanation. I was finally able to find the correct search terms and found this thread on the 4 lead capacitor. https://antiqueradios.com/forums/viewto ... 9&t=193605

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 Post subject: Re: Zenith 4 lead capacitor.
PostPosted: Mar Wed 11, 2020 8:40 am 
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Quite common in SS TVs because this cap is critical. You MUST use the right part.

Main reason for 4 leads was to provide some redundancy for the soldered joints, which can suffer from metal fatigue after many years.

Some designs used a series/parallel network of 4 or 6 caps for redundancy and to spread the load.

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 Post subject: Re: Zenith 4 lead capacitor.
PostPosted: Mar Wed 11, 2020 3:15 pm 
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I remember that fiasco very well. Zenith had an intensive rework program to replace the defective 22-7233 capacitors, but they couldn't catch all of them; we were still finding them in the early 1980s. The first SS Zenith color sets, from 1971-73, were the ones that had multiple capacitors. Those were the sets with the huge flat chassis; probably the best color sets ever made, by anyone.

The idea was that it the four lead capacitor opened, it would break the circuit, and prevent damage. In practice, things didn't work out that way.

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 Post subject: Re: Zenith 4 lead capacitor.
PostPosted: Mar Wed 11, 2020 3:29 pm 
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Tim Tress wrote:
I remember that fiasco very well. Zenith had an intensive rework program to replace the defective 22-7233 capacitors, but they couldn't catch all of them; we were still finding them in the early 1980s.

I had a set with the white safety caps that worked till ~2015 when the caps went and the HV shot up (I shut it off within ~2 seconds of the arcing starting so the CRT survived). I didn't much like the cabinet so instead of fixing it I passed it along to another collector.


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 Post subject: Re: Zenith 4 lead capacitor.
PostPosted: Mar Wed 11, 2020 3:36 pm 
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Yes, the early sets had the white capacitors, made by the same supplier. We had to be on the lookout for those as well, although the failure rate on them was much lower. The part number of that capacitor was 22-5001; like Dennis, I can still remember the numbers, 40 years later!

Zenith was still replacing CRTs from the capacitor failure, even when the sets were out of warranty. We had to send the serial number to the distributor, and have it checked against a master list. If the customer had never sent in their warranty card, the factory had no way to contact them for the recall.

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