If a picture tube loses its vacuum...
 

... is there anyway to restore/rebuild it? I'm guessing if there is, it would be grossly impractical and expensive and that replacement is a far more practical option, but what if it's a really rare picture tube? Could it somehow be restored?

Somewhere in the site I was reading up on that and I think if the phospher is ok, than it might be ok for a rebuild, but on the other hand, there is a chance that the internals could have come corroded and moisture could set in or something like that..

If its a metal cone CRT and it's lost its vacuum, I don't think it can be rebuilt. If the seam between the glass screen and the metal cone has been compromised, its not rebuildable at this point in time, though I believe Scott at Hawkeye Picture Tube Co. has been working on rebuilding 15gp22's that have gone to air. Its a work in progress...

Somebody asked me that question, cos apparently they have this prototype picture tube, there were only like three of them made or something like that. It's supposed to be very similar to a commercially manufactured tube, but the one that went into mass production has some small changes.

From what he told me, he stupidly knocked the tip of it off on accident and it lost its vacuum.

The only other stuff I know is that it's an all glass picture tube, was made sometime in the 80's. It's a color tube, and apparently it worked very well for being a prototype. No clue as to who made it.

Here's the deal: If the phosphor wasn't damaged (spots or specks, shadows, peeling, etc.,) you can have it re-evacuated and it might be fine. But chances are that if it's been to air for more than a short time, it may have gotten corrosion on the cathodes. That will mean it may be dim or have poor emission, or maybe it'll be fine. Depends on a lot of variables.

It isn't a big deal for a CRT rebuilder to evacuate and reseal it. All they do is to blow a new long glass tube onto the tip that broke, put it in the oven, evacuate it, and pinch off the new tip.

Charles

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If the tip was broken off the odds are the phosphor was damaged by the inrushing air and debris, unless it was a very small leak.

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Interesting. So it can be rebuilt. I assume it could have new phosphor and new cathodes put into it?

Heh, my friend's probably going to spend alot to get it rebuilt, because he really wants to get that tube working again. From what I know, it looks like an ordinary picture tube, but it has "PROTOTYPE" written on the back of it, along with a bunch of cryptic numbers. No brand name sticker, no date, etc.

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"When they rebuild CRTs, how do they let the air in and avoid damaging the screen?"

I am not sure, but I think I know from a dud gun color gun assembly that Scotty at Hawkeye sent to me.

The old color gun assembly was still attached to the bakelite socket and about 2 inches of the glass neck at the socket end of the neck. The glass neck assembly was severed from the rest of the neck in a very clean fashion. I examined the glass of the neck assembly closely to see how Scotty accomplished this feat. It appears Scotty scored a line with perhaps a dremmel tool where he wanted the neck to crack off. I assume he then tapped on the neck where it was scored and the glass tube made a clean break around the entire circumference of the glass neck. I am guessing prior to tapping on the neck to make it crack off, that Scotty might take some scotch tape and tape along the spot where the crack is going to take place. Taping along the point where the crack happens, would seal the air from inrushing to rapidly.

Like I said, all of this is just a guess from what I could deduce from the dead gun assembly Scotty sent to me.

Maybe someday someone will do a video of Scotty performing his craft so we can all see how this rebuilding is done. :D

sort of related.....
 

When I was a kid my first TV project was tearing apart our old Hoffman "Easy-Vision". My dad broke the tip off of the CRT so I wouldn't kill myself by breaking a "live" tube. I tried to re-evacuate it with the vacuum cleaner. :yikes:.
Good thing I couldn't get the hose to seal against the neck!

Looking back at all of the stuff I did as a kid, I'm lucky to be alive!

Rebuilders could possibly break the crt seal inside a vacuum chamber and slowly lower the vacuum(raise the air pressure) to avoid screen damange. Alternatively, the entire rebuilding process could be done inside a vacuum chamber.

I am not sure this is how it is done, but it is a possiblity.

Tube Rebuilding 101
 

Whoa! Whoa! Whoa! I often see misinformation in this group and this is one of them! Here in Knoxville there was a picture tube rebuilding plant of which I worked when I was in high school. I know every facit of tube rebuilding so let me set this straight of what's been said so far.

First, when a tube has gone to air you CANNOT "reseal" a tube and reuse the old gun mount as the cathode has returned to its' original form and will not work again. Once a tube has gone to air it now contains moisture which will attack the phospor if left there. Tube would also be so full of impurities from the tainted gun mount that it would not work anyway.

All of your theories of how they let the air in without damaging the tube are all wrong. After the tube is washed and cleaned and all of the old dag is removed with steel wool it is set into a rack gun side down. There are no dremel tools ever used in the process. (where would anyone get this idea? You scratch the glass on any surface of a live tube and you risk implosion and injury) Here you need a clean cut and a dremel tool cannot do it anyway. I have seen dumbasses scratch tubes before and the outcome is not good. When we inspected tubes for rebuilding, we look for scratches esp on the faceplate and they were REJECTED. At this point there are a pair of tongs with a nichrome wire that is squeezed around the base of the neck where the break is desired, wrapping the wire around the neck. About 12 volts at 10 amps, the wire gets red hot and about 8 seconds later it is removed and I would wet my finger lightly and touch the heated area and it would go "chink" with a perfect break completely around the neck where heated. We would then wrap a plastic bag and tape it to the neck which had a hose attached to it from a nitrogen tank. This kept moisture out of it. The vacuum seeps down slowly and no damage to the faceplate. When vacuum is finally down, you pull the gun assembly out completely intact. Now it's ready for the next step and this entails installing a new gun mount along with a glass lathe.

If a tube has been down to air for a period of time and the break was around the gun mount, it can be rebuilt but depending on the moisture content of the air inside the tube will most likely cause "staining" of the phosphor on the faceplate.

By the way-one more important thing: DON'T EVER *REPEAT* DON'T EVER REMOVE A PRESSURE BAND ON A LIVE TUBE!!! It will implode. I saw a dumbass TV repairman do this to make it fit into a set one time and he found out the hard way!


Summary:

*You don't just put a new cathode in an old gun mount. This is impossible unless you build gun mounts and it's not feasable anyway.
*When a tube is cracked via the hot tongs, you don't put any tape on the break.
*You cannot put a new stem on any gun mount (new or used) and evacuate the tube because the cross fires would damage the pins or wires coming out of the button of the gun mount. The pins or wires were annealed to that button and heating them will ruin that annealing process and cause a leak if not crack the button. New stems come with new gun mounts and you cannot weld on a new stem if broken. If broken, you throw the gun mount away.
*You cannot replace the phospors on a color tube. Again, this is a new manufacturing process and if you could do this, you wouldn't be playing with used tubes.

Any more questions for an expert?

Yea... How do you prevent a metal CRT from imploding while in the oven?

We rebuilt 21AXP22's in the same process by heating it up the same way in the ovens and had a 80% sucess rate

What about monochrome?

21AXP22's have to be put into the oven even when new which we did, and had a 80% success rate with no problems. They went through the same process as a 21FBP22. We never did any 15GP's (too old then) and they were not a good item in the '60's.

Monochrome = black & white. Those mentioned are tri-color tubes

B & W Tubes
 

B & W tubes are rebuilt the same way as color. All you are doing in the process is installing a new gun mount and evacuating the glass bulb.

Any experience in rebuilding metal B&W tubes? I was referring to the high failure rates with these while in the oven.

Sounds like you know your stuff. Thanks for the info.

Thank you for the fascinating details of CRT rebuilding. I used to pick up rebuilt CRTs for my company from a place on the South Side of Chicago in the early 1980's, and I wish now that I had kept up more with that business. I did see the whole shop area, and I remember the rebuilding machine was always dripping with water inside or outside it or something like that. The phone he had was still a hard-wired rotary phone from the late 1940's, the oldest I ever saw in service anywhere.

I should mention, though, that at least one type of 23" B&W CRT can have its band removed safely-in fact, you have to do so because new CRTs (National brand) in the early 1990's came without the band. We may have one or two still in the box here that I have not used yet. The band has a hex-head bolt and nut assembly that is loosened to remove the tube. I have always been a bit nervous when changing them, though, because I wondered if the band "held together" a CRT, and you just confirmed that.

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Not to highjack the thread, but what would happen if the button was broken, or it lost vaccum while a CRT was in operation? The heaters would burn up as soon as air enterd the tube, right?

Not much you can do except make sure the oven stays at a constant temperature (400 degrees) with no sudden temperature change. In the late 60's we never did many metal tubes because they were old then and Paul, the owner never wanted to mess with them because when a tube blows in the oven, you lose labor and materials. Plus you have to clean the oven out. All of our ovens had "scars" in them from when tubes went off.

There is a difference between a pressure band and a mounting band. The banded tubes use steel like a packing band and is crimped with a similiar seal. You can remove it AFTER the tube is let down to air which you HAVE to do when rebuilding it. You would normally use a hacksaw and can reuse the band on a smaller tube. The idea is that if a tube is struck from the front, the sides of the tube will have to expand in order to implode. With the band wrapped around it, the perimeter cannot expand thus making the tube safer.

A mounting band has a clamp that you can screw down and doesn't count here. Some tubes have the mounting ears made in with the pressure band for a specific application.

If you use a hacksaw to remove the band on a live tube, the band flies apart, dangerously and the tube suddenly "expands" enough that the bulb will rupture and implode. It doesn't bother a tube already let down to air because there is no tension on the glass bulb.

The glass bulb or envelope cannot be scratched anywhere because the scratch makes a weak point and you CANNOT put a scratched bulb in the oven as it has an 90% chance of imploding because of that scratch. (On the faceplate esp)

Somehow a few tubes get scratched on the face in the process (both new and rebuilt) that it was a good tube and putting a bonded face on it would hide the scratch with the resin between the two glass surfaces. Many others were called "seconds" if it wasn't a bad scratch as well as tubes that developed "pimples" in the phosphor. RCA made test jigs using a 19" tube (19EYP22) that had "TEST TUBE ONLY" sandblasted on the front upper corner of the tube so that you couldn't sell it as a replacement tube. They were that valuable back then.

A CRT without vacuum doesn't suck.

No. If a tube starts to lose its' vacuum during operation, it will immediately begin to arc all over the gun mount with blue sparks. This is how you can tell quickly in a solid state set if you have a broken tube or not and you can certainly hear it with a distinctive sound that sounds like a loud hissing in the tube itself. Strangely, the filament will not open up as it is very tough and goes to almost zero ohms as like a dead short. There is not enough current in the filament voltage to make it burn out. (later sets mostly)

I respect all these opinions, but I have personally had CRTs with a broken tip re-evacuated (and cooked in the oven) and they worked fine. I was advised that it might be a waste of money, but it did work. One was an 8" Arvin metal tube which had a metal tip that leaked, the other was a 21" B&W bakelite base tube. No phosphor problems, but the 21" did get a new gun. When they cook the tube, it forces the air, gases, and moisture out while they evacuate. The Arvin gun was still OK, although maybe a little less emission. In another case, I had a Sony 30" Trinitron CRT rebuilt after it had the neck cracked and lost vacuum. That one too got a new gun, since they were cheap and available. But again, no stains or phosphor problems.

I also had a 21AXP22 rebuilt after being with lost vacuum for a long time. It worked fine, but lost vacuum again.

As far as the pressure band, I have heard both ways on that. In theory, it shouldn't implode. The band is applied AFTER the tube is evacuated, and often my local rebuilder (Dunbar Sales in Los Angeles) would store the tubes without the band, so that they could slap it on with whatever mounts were needed for a particular request. The newer tubes all have the pressure band holding the mounts on, not like the older ones which had the pressure band plus another screw-tight band for the mounts. I'll stay on the fence on this issue of what happens when the band is cut off after the tube is evacuated, as I haven't done it (and DON'T PLAN TO).

Charles

Yes, I would call the band on these CRTs a mounting band, not a tension band. I wonder if the mounting band "protects" anything once it is installed?

If a picture tube loses vacuum in an empty cabinet, does anyone hear it hiss?

The biggest problem with rebuilding CRTs that have gone to air is that the phosphor degrades.

If a picture tube loses vacuum in an empty cabinet, does anyone hear it hiss?

The biggest problem with rebuilding CRTs that have gone to air is that the phosphor degrades.

A broken CRT is always a sad thing to see. Also sad...in the early 90s I stored a CTC-15 with a good Channel Master rebuild. Just a few years ago I decided it was time to get the set going. I put the crt tester on it first and thought it odd that the filaments were glowing very dimly. I upped the voltage slightly and still they were not to full brightness. I thought something was wrong with the tester so tried another tube, and it was fine. So, I hooked the tester up again and, one by one, I watched the filaments burn out. The crt had gone to air over the course of 10 years.

I have one of those RCA test jig tubes with "Test Tube" etched on the face. It's a non-bonded 21" round color. I sure wish the etching wasn't there. I have heard others say it could be buffed out, but I don't know if I'll ever try it. The man I got it from had the jig a short while when he happened across a great deal on a then late model set with a broken crt, so he swapped this tube in and used it as his main home tv. Eventually he saved up for a new tube and put this one back in the jig.

It depends on what happens to the tube. If it develops a slow leak at a glass-to-metal seal (as with 15GP22 and 21AXP22 often,) you'll never know it. It can take years and years. I had one 15GP22 that was starting to lose vacuum, as evidenced by bright blue glow in the neck. Later the filament got brighter and brighter, then after about 2 years, it started arcing when enough vacuum had been lost.

In cases where the tube gets busted, either by a high voltage regulation problem (many Zeniths in the 70s, and in many newer projection sets,) you hear "Krack- HISSSSSSSSSsssssssssssssssss". If you snap the neck off, you hear "KSHTHWUPF" (Try to pronounce that :banana: )

Charles

I have opened boxes with NOS receiving tubes that had gone to air without ever being used. Probably a manufacturing defect that caused a small crack in the glass.

It can be polished out as with scratches that are not deep. Julian

Ans. You prevent metal CRT from imploding through a very careful control of heat during the annealing process when it is in the oven.

There are different glasses and metals used in the manufacture of all CRTs. Each will have a different coefficient of expansion (rate at which physical dimensions change as heat is applied or removed). The trick is to match the coefficients between glass to metal or glass to glass so the different materials that are joined together have a similar coefficients of expansion. Otherwise, the materials joined expand at different rates and break apart from each other. The problem is compounded a bit with older CRTs. An old CRT has had time to "age" in a way that can affect chemical changes to glass to metal seal or glass to glass frit seals (15GP22 is a case in point) that may make it impossible to re-apply heat to those joints years down the road without causing them to fail. The secret to success is to find the minimum acceptable temperature and cycle time, or repetition of annealing cylces thereof, to safely anneal the join without inducing failure to other joints. A lot of experience and knowledge is necessary to determine the best cycling time and annealing temperature to achieve acceptable results so the tube won't fail. For the rebuilder it often means selecting a low temperature gradient and lengthened cycle time to minimize thermal shock.

Annealing used during the rebuilding of a CRT is not the same as the annealing process used during the original manufacture. That's why most CRT rebuilders either discourage this business or avoid it entirely. They simply may not know the correct annealing parameters. This is not really the fault of the rebuilder - even the manufacturer may not know without extensive resaearch. What's really needed is an exceptional understand of the material science and chemistry of the problem. Two very difficult skill sets to find in any practitioner with a sound understanding of vacuum technique.

The second reason to apply heat is to help the evacuation process. But this is a double edged sword. High heat destroys seals and may cause electrical feed throughs to fail. However, unlike the manufacturing process, the CRT rebuilder needs to take more time, often because the residual gas content of any old tube is hard to determine, unless RGA (residual gas analysis using a mass spectrometer is employed). This kind of anyalsis is often way beyond the ability of any rebuilder. Avoiding high "outgass" temperature is crucial to preventing implosion due to the mechanical failure of the envelope or failure of the seals. Typically in the world of restoration time is on our side. So, use the lowest heat possible and simply pump under vacumm for the longest possible time, and then seal the tube and flash the getter is often the best approach. Rebuilders who try to hurry the pumping process only short change the life of a rebuilt tube.

Rebuilding any tube is really a process of situational awareness involving knowledge of structurual and chemical engineering. Hidden pitfalls lurk everywhere. Breathing life back into any old tube can be a unique challenge every time.

I have another question now:

Did anybody ever try to make a CRT that instead of a vacuum, the tube was filled with an inert gas, like Nitrogen or Argon?

Actually, all conventional electron beam CRTs have some level of residual gas present in them, so in that sense they are filled, usually, with low levels of Hydrogen and Nitrogen gas, even small amounts of argon. But we're talking really low levels. Noble gases are usually always found several reasons, first, these gases are made from lighter elements in the periodic chart and can be often difficult for the getter to pump away. Second, they are typically stable and resisit breakdown, and thrid, slow leaks usually allow the lightest molecules to slip into the tube.

So called "gassy" tubes occur when very high residual gas levels are found inside the CRT. High residual gas pressures result in more significant collisions with electrons emitted from the electron gun of the tube. Gas molecules get in the way and collide with the electrons from the gun causing them to scatter, resulting in poor electron optics performance of the gun so fewer electrons focus onto the screen. Hence, we get a dim washed out picture. Sometimes you can peer through the neck of a CRT an see a purplish glow - usually indicated gassy conditions as the residual gas moelcules ionize in the present of high electric fields from the kinode.

But, I suppose what you really meant was, has someone deliberately made a CRT that requires a "gas" to operate. Well, the plasma display is one such device, invented by Robert Wilson in 1964. Think about a CRT without scanning electrons and where each pixel is represented by an single CRT. A plasma display is made up of hundereds of thousands of tiny cells filled with Xenon or Neon gas. It takes hundresd of thousands of these cells just to make up the picture. There are other examples too - limited only by your imagination. :D