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?