If a picture tube loses its vacuum...

[julianburke]

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

[John Marinello]

What about monochrome?

[julianburke]

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.

[John Marinello]

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

[julianburke]

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.

[John Marinello]

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

[MRX37]

Originally Posted by julianburke

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?

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

[ChrisW6ATV]

Originally Posted by julianburke

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!

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.

[andy]

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[fujifrontier]

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?

[julianburke]

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.

[julianburke]

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.

[Wigwam Jones]

A CRT without vacuum doesn't suck.

[julianburke]

Originally Posted by fujifrontier

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?

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)

[kx250rider]

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