Old Zenith safety caps.

If you watch how this type of cap goes open, it comes clear.

The two plates of the cap would very rarely if ever seperate, if a problem happens there they generally short out. When they open it is generally the connection to the plates. They are rolled up offset a bit so the leads can attach.

The four lead cap actually uses two connections to each plate, so if it opens up, there is no high voltage. The connection either to the emitter or collector of the HOT is broken. If not, the high voltage would rise quite a bit. It would cause Xrays and most likely blow something out, but if it didn't you might have a nasty situation.

The Zenith EC series used to use multiple caps for this, and sometimes one or more of them would open up causing an insidious excess of high voltage, because there was no shutdown circuit at all. Thus they needed the "4 lead capacitor modification". IIRC they came with a sticker for the back of the set, but newer ones might not.

What to do with them now is anyone's guess.

JURB

Thanks for the explanation. I never really understood the internals of those things until now. Now, yes what to do with them......Lenny.

They make good keychains.

If you have ever torn down a paper / mylar / poly capacitor to view its internal infrastructure, you would see the offsetting of the dielectric foil upon the dielectric separator and the clustering of the mass at both ends . That then needs to be collectively interfaced to the two wire connector leads. That is usually accomplished by a pressure compression of lots of surface contacting area or a enclosed crimping action and the other means being via a large molten solder "blob" which will properly meld with the tinnable copper wire lead and be dependent upon the other contact aspect to be the extensive area of foil that has solder interspersed within it.

Now getting to your special feature of the "safety capacitor". You will be finding it placed in the collector / to / flyback primary winding where it is resonating

along with the primary of that transformer, as that transformer action certainly is more efficient operating as handling a pseudo-sine wave versus a positive polarity sawtooth waveform.

With that resonance established between the two components of that tuned circuit , there will be

quite a degree of flywheel energy present and an actual lowered degree of required power required

as compared to a circuit with out that aspect being present.

Now lets look at that repetitive 15,734 ~ signal that is continually hitting that current loop in the hoz output stage, much in the order of an instantaneous power impact effect of a jackhammer.

If all is well in the hoz circuit, fine, no problems will be had.......but lets move back to the described capacitors internal structuring , if those high current pulse might find a weak / marginal contact area within that cap, expect it to arc over, vaporize metal and open a portion of a previously contacting area. That then leaves progressively less area to be carrying and sharing the current requirement.

A daisy chain or domino effect is then possible with the eventual loss of all contact area made to the foil and either arc over or loss of internal contact completely and the loss of connection and capacitance transfer.

Now IF that cap is effectively out of the circuit, by virtue of that opened internal connection.

Expect that previous effect of drawing less current while being in coincident or close to resonance state to the frequency that the hoz power circuitry was operating at, to no longer be valid.

The power consumption...i.e. current passing through that loop should increase, along with a shooting up of the high voltage level being produced at the fly /IHVT output by virtue of being hit by the sharp peak of a sawtooth waveform now.

That would be limit taxing on the HV components as well as possibly getting up into the X-ray level from picture tubes, or even up to the ultimate threshold of actually breaking the kine via a circular ring fracture about the yoke mounting area.

That is where the safety aspect of the mentioned capacitor comes into play.

In examining the circuitry, you will find that the caps dual leads are dependent upon completing specific interconnections.

That being in the closed loop from the B+ supply to the collector circuit of the Hoz output transistor. Within those capacitors, there are internal fusible links between adjunct leads, such that when operated beyond their current passage design center, an opening will occur internally, opening their circuit, such that the sweep circuit then becomes disabled.

Therefore being dubbed "safety capacitors", so that extreme condition never occurs, or merely rising upon that threshold for an instant.

Regards..... Edd

That is really an interesting discussion of how these things work. But it begs the question of why didn't they simply put some type of fuse or fusible link in the circuit? Lenny

A fuse won't work because current drain does not increase, in fact it goes down.

When what I call the damper cap opens up, the HV produced is softer, not as much current behind it, but at zero beam current rises quite a bit.

Interestingly something very similar happens if the core of the flyback cracks. I had one a long time ago, the damper caps were checked, the picture was narrow and the tech was scratching his head. He had the B+ turned down but couln't turn it up to normal because it would shut down. As usual in such a case, I wound up with it.

I took a screwdriver handle and pushed down hard on the flyback core and the picture widemed.

That's when it really pays to understand reactive scanning, although these modern pincushion circuits befuddle me. Even a CTC169, my logic says when the .056uF opens the picture should be too narrow. Then you got Sonys doing the opposite of what you think a given bad component should cause because of feedback. Then you have coils with magnets in them, geez ! See my thread about a new profession lol.

JURB

If circuits always failed in ways you'd expect them to fail, then it wouldn't be any fun any more, would it?

Have you ever seen the original three lead safety capacitor Zenith used? When they failed, a high DC current flowed in the yoke, which caused the neck of the CRT to crack. Plastic Capacitor Corp. made the defective three lead parts, and went out of business. I was working for a shop that sold Zenith, and went to the factory school on those sets. As soon as the problem surfaced, a mad rush was on to find every Zenith TV with the defective caps, and modify them to the new design, and apply the modification sticker to the back of the cabinet. They showed a 16 mm film of a CRT self destructing.