Any updates on RACS rebuilding early CRTs?

[ChrisW6ATV]

Originally Posted by John Folsom

Whatever material you use to attempt to seal a CRT must have three propoerties:
1. It must survive the 600F oven temperatures during processing
2. It must have a thermal coefficient of expansion which matches very closely the glass/metal of the CRT
3. It must be impervious ie vacuum tight over a very long period of time.

I am curious whether Vac-Seal has been considered, or ruled out already. If it is strictly because of the heat issue, could it not be applied after the CRT has cooled? Specifically, since the weld leaks may have existed since the CRT was manufactured and they did not cause failure for years if not decades the first time, why is there a need for them to be sealed before the rebuild?

[old_tv_nut]

Originally Posted by ChrisW6ATV

I am curious whether Vac-Seal has been considered, or ruled out already. If it is strictly because of the heat issue, could it not be applied after the CRT has cooled? Specifically, since the weld leaks may have existed since the CRT was manufactured and they did not cause failure for years if not decades the first time, why is there a need for them to be sealed before the rebuild?

I wondered the same thing, but I think it's iffy. The 21AXP22A that I paid Scotty to attempt went to enough air in 3 or 4 days to produce a bright blue glow from a Tesla coil test. Scotty applied vacseal to the neck/bell area just as a guess (he thought the most likely leak point was the metal pinch-off, which was replaced by the new all-glass gun). This is the tube that we did the helium test on at ETF this past spring, and determined that the leak was a "small" one at a tiny ding on the metal-metal weld. I'm now thinking that even small leaks might degrade the vacuum in the time needed to let the tube cool before the vac-seal could be applied and the getter could be fired, but your guess is as good as mine.

This does bring up the point that a tube could have lasted a long time and then fail quickly after rebuild - it appears that the stress of reheating can make a metal-metal leak much worse. I wonder how many more rejects RCA would have had if they went through a second heat cycle.

[ohohyodafarted]

Wayne,

My gut feeling is that VacSeal may possibly work. But prior to our experiments with the helium leak detector we always thought that the leaks were in the glass/metal jont. Now we realize the leaks are in the weld. So the next logical step is to see if weld leaks can be sealed with VAcSeal and if the product will hold up under the heat of the evacuation oven cycle. Unfortunately there is no oven to test the theory with since Hawkeye has closed it's doors.