When good picture tubes go bad

[RetroHacker]

This sort of goes along with my other thread about brightening weak tubes, but I just had a few thoughts and questions in my head, and I thought I'd start a seperate thread. Also not really sure what forum this should go in, there isn't a 'general television' category.

What exactly wears out in a picture tube, anyway? I mean, from what I have read and heard, it's basically just that the filament coating wears away, and no longer emits electrons like it should. But the rest of the gun assembly - does that really fail? Do those wear and get weak too? Is that what causes weak tubes to focus poorly, or is that just a side effect of the lessened emissions? I'm still a bit fuzzy on what goes on _inside_ a tube - I know the theory, and I know how the circuits around the tubes work for the most part, but I'm not exactly sure how that can translate into a beam of modulated electrons that can then be magnetically deflected by the coils on the outside of the tube. I mean, what emits these electrons? Is it the filament? Or is the filament just a catalyst?

And, if the major cause of a weak tube is an ineffective filament, wouldn't it make sense to make a tube with an extra filament in there? You'd only need one extra pin on the tube base, have one common pin connected to both filaments and then two seperate ones. When one filament gets weak, use the other one - a simple wire swap as opposed to changing the tube entirely. Now I'm sure there's a reason something like this wouldn't work, orherwise it would have been tried already. But it's a thought.

Also, when a tube is rebuilt, what all is involved in the rebuilding process? I know that they have to cut the neck on the tube, replace the filament and the gun assembly and base, weld that to the tube envelope, then re-evacuate the tube and seal it. But does the gun assembly really wear out? Does it need to be replaced? Or would it theoretically be possible to get by with replacing the filament? If so, it could _almost_ be done at home, in a homemade workshop. I mean, other than a vacuum pump, and something that can melt glass, what all would be needed? I know I read something about someone that rebuilt tubes himself. But I don't really remember.

Ok. I'm rambling. Any ideas? answers? comments? should I just shut up and quit being annoying?

-Ian

[compucat]

As I understand it, it goes something like this. When a picture tube wears out, the emissive material, that is the coating on the cathode element is diminished. It gets slowly consumed over the lifetime of the tube as a result of the act of boiling off electrons when heated by the filament. The filament is like a light bulb in that it either works or it doesn't. This reduced emission results primarily in a dim picture with poor focus. There are fewer electrons in the stream therefore brightness is reduced and focus is not as sharp because the other elements in the tube have little to work with.

The electron gun fails primarily from, open filament(s), weak cathode emission or shorted elements. Clolor tubes can also suffer from bent shadow masks which affect purity and convergence. I hope this helps.

[Chad Hauris]

I have very rarely seen an open filament...when I have, usually it's because the tube has been physically cracked leaking in air. Most bad crt's just have low emission.

In most tubes the filament is just a heating element that heats up the cathode sleeve, and this sleeve actually emits the electrons...there are a few small CRT's which have a direct filament cathode in which the video circuit is actually connected to the filament itself and there is no cathode sleeve, but these are rare.

When tubes are rebuilt they either just put on a new gun, or the phosphors/shadow mask in the screen are replaced too (this is where the screen has been burned or when the most thorough job is done).

I recall seeing a strange ad in a Life magazine from the late 60's by some big name supplier of military electronics supplier that told of them being able to repair a burnt out light bulb with some sort of high energy burst of microwaves or something......it cost somewhere around $60,000 each time to do this, but it was a technique they developed to repair esoteric and very expensive avionics tubes....especially ones in a semi-permanent installation.

Does this ring a bell with anyone? I may have garbled it somewhat, but I do distinctly recall such an ad along these lines.

Anthony

[bgadow]

Do you guys know how Sylvania started out? According to their website, they started out REBUILDING light bulbs! That was back around a hundred years ago. And to think, today we don't even bother to rebuild most picture tubes.

I would assume that for a shadow mask to be replaced they would have to cut the face off the tube? That part (and the respray of color phosphors) is unclear to me.

Some crts are much better than others; often there would just be no financial incentive for a company to build a more stout crt/gun. If it makes it through warranty, who cares? Most of us here have sets from 40, 50, close to 60 years ago that still operate fine with the original crt, while many bottles died after just a few years. Since very few of our old sets are today daily watchers, they don't have to have tons of life left in them.

I used to wonder why the gun & the screen couldn't be in seperate vacuum tubes, so that the gun tube could easily be replaced, but I finally figured enough theory out to understand it just don't work that way!

[rca2000]

It has been my own observation, that some of the BEST modern CRT's(1985-now,) are from rca--I seldom see a bad one, even from 15 or more years. They just go on and on. Now, in the mid-to-late 70's RCA had a pretty weak crt, used in the 25" xl-100 sets, that did not seem to hold up very well.

Then, of course , there is Zenith. from the late '60, mid-70' and their Chromacolor tubes, which seem to last FOREVER, to the early '90s, to about '99, tubes which are lucky to see 5 years, before shorting and killing the chassis.

[andy]

They can't replace the phosphor, or shadow mask in a color CRT. There's no way to get to those parts. If a color CRT has screen burn, or a damaged shadow mask, it's no rebuilable.

My favorite CRTs are definitley Zeniths from the 60's up to the early 80's. They became hit or miss until 1993 when they went to all miss. Those RCA/Thomson CRTs aren't the best looking, but they do seem to last a long time. A lot of them have poor focus, but otherwise look good. I don't remember ever seeing a weak one, but I have seen a lot with shorts, or open heaters.

Regarding rebuilding light bulbs....there was an early form of construction known as the "stopper" type in which the filament was contained in a sort of bottle stopper which was inserted in the base of the globe.....obviously this was a simple matter of removing and replacing, along with evacuation of course. Perhaps this was the flavor of bulb that Sylvania rebuilt. Way back when, labor was cheaper than parts so this may have made economic sense at the time.

Anthony

[bgadow]

Not sure if this relates to the topic, we shall see. After leaving Columbus the other day we headed south on US 23 toward KY & along the road, a few towns to the south, there was a big building with "Thomson" & "RCA" proudly displayed on the front. All kinds of what looked like ventilation equipment on the roof. The building did not look abandoned & seemed fairly modern. Could this be a crt plant? If not, does anyone know what they do build there (assuming it is still running?)

[Chad Hauris]

Originally Posted by andy

They can't replace the phosphor, or shadow mask in a color CRT. There's no way to get to those parts. If a color CRT has screen burn, or a damaged shadow mask, it's no rebuilable.

My favorite CRTs are definitley Zeniths from the 60's up to the early 80's. They became hit or miss until 1993 when they went to all miss. Those RCA/Thomson CRTs aren't the best looking, but they do seem to last a long time. A lot of them have poor focus, but otherwise look good. I don't remember ever seeing a weak one, but I have seen a lot with shorts, or open heaters.

What you are saying makes sense, but it seems like I remeber an ad for Sylvania rebuilt tubes in a trade magazine that said that they did replace the screen and it seemed like there were "new gun and screen" pictures tubes...could they have just cut the crt housing at the screen and then re-melted the glass to re-attach?

[andy]

I'm sure they did replace the screen on B/W CRTs, but I doubt they could do it for color. The screen of a color CRT is glued on with a white ceramic like substance which I've heard described as 'solder for glass'.