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Old 7th Jun 2021, 12:39 pm   #21
murphyv310
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Default Re: Cracked outer glass on twin panel CRTs

I agree John that heating is not good. Unequal expansion could easily cause an implosion.
I'm surprised these tubes have only a cracked faceplate. They've been stored in a shed with summer temperatures of 40c cold as low as -15c and often sudden changes.
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Old 7th Jun 2021, 12:41 pm   #22
Sideband
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Default Re: Cracked outer glass on twin panel CRTs

Quote:
Originally Posted by Heatercathodeshort View Post
I recently experienced a spontaneous implosion of a 9" MW22-7 tube that was fortunately in a packing box. It had been brought over by the owner for testing and went pop about one hour after we had placed it back in the car in it's box and that was bad enough!
I think some of these older tubes are becoming somewhat fragile. A slight scratch in the wrong place can strain the old glass (which I understand can become more brittle with age) resulting in implosion. I have always had a healthy respect for these potential 'bombs' even after 45 odd years of repairing/restoring. When I was at Philips, I heard one 26" CRT go for no apparent reason. The engineer (of long standing) had carefully removed it and laid it face-down on several layers of sponge while he fitted the new one. He then put the old tube in the new box with the requisite packing ready to go for re-gunning and while he was at lunch there was a (very) loud boom.....! Fortunately the box had contained the glass but we would never know why the tube decided to 'let go'!

I wouldn't even attempt to remove a cracked face plate. Something is likely to give and I wouldn't want to be anywhere near at the time!
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Old 7th Jun 2021, 12:49 pm   #23
PaulM
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Default Re: Cracked outer glass on twin panel CRTs

Heating tubes when in production and re-building was part of the process.
What's so strange about that? Ageing of the tube, perhaps, but it's 100% enclosed.
The guys on Youtube with hot air guns are the ones that terrify me! An oven raises the temperature fairly uniformly, the hot air gun is very localised and causes differential expansion - now that is a worry, especially as it's not enclosed.

The method works - I'll admit that it sounds crazy - but I'll post some pictures of the results when I have I have a moment. There have been no failures and no disasters but that's admittedly only with relatively small tubes.

Refitting is the easy bit with optical grade silicone.

It can be done.

Usual H&S warnings here . . .

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Old 7th Jun 2021, 2:13 pm   #24
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Default Re: Cracked outer glass on twin panel CRTs

As promised, some pics of the oven-ready CRTs.
OK, I'm an engineer not a photographer

First up is a 9" Conrac (possibly an RCA CRT) and second up a 14" Conrac.
Both with curved face-plates.

Third is a 7" flat face CRT for a broadcast camera viewfinder. Many are going like this and the final pic is of a really bad example showing a re-gunned Central Tubes unit. Perhaps they should have been replacing the faceplates after-all! To be fair, many others from several manufacturers are going the same way but not all - some makes are still perfect.

This is just to show that the technique works. Never had a failure, but one day there may be - it's about risk management. I'm also not saying that it should be tried on rare/valuable/large/fragile examples, but it is a valid technique and the tubes are contained within the oven. An industrial oven would, in any case, be needed for larger sizes.

Usual H&S disclaimers - own risk etc.

Best regards,

Paul M
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