Cataract repairs - are we missing the safety glass aspect?

[Kevin Kuehn]

Adding the bead of silicon around the perimeter of a decateracted CRT at least adds some level of impact and scratch protection. What you achieve is a nearly sealed cushion of air that resists any moving object from directly striking the CRT face. Effectively it spreads any impact over the entire face of the CRT. I wonder if the FB glass was not of different temper than a bonded FJ type?

[RetroHacker]

I've wondered about this many times myself - although, as it applies to smaller tubes like used in computer equipment. I collect old computers as well as televisions, and have done the cataract repair on CRTs used in old computer video terminals. It's the same process as on a roundie - just a much smaller tube. I've never had one implode, but I have often wondered about the ramifications of that safety glass being stuck on with tape or caulk. At least on a television, you're generally watching it from several feet away - but a computer terminal, you're right directly in front of it.

Also, I've wondered what the problems with heating and cooling cycles would be, on a tube that's had the safety glass sealed on with caulk. The air is sealed in that gap between the safety glass an the CRT face, what happens when it goes through a severe temperature change - say, for instance, in a car overnight in the winter, then brought inside? Has anyone ever had the safety glass crack in this situation?

-Ian

[Electronic M]

Silicone caulk stretches...Expansion should not be an issue.

[Eric H]

It seems it should be possible to re-attach the safety glass with the same material used originally, or it's modern equivalent.

The problem is it's probably not available to the general public, and if it is it's probably only sold in 55 gallon drums or larger.

[N2IXK]

Even if you could get the proper resin to do the rebonding, it likely needs to be done under vacuum to prevent entrapping air bubbles between the faceplate and the tube.

[snelson903]

i believe it was put on the front of the crt to prevent small pieces glass from flying outside the tv set only , i have a zenith roundie with oil between the crt face and the safety glass ,no pva material .

[DaveWM]

I think its important to distinguish between roundie color and rectangle. Since this is being discussed on the early I will assume roundie. I would highly recommend care when dealing with rectangle sets that had bonded faces. I have had one go off in the process of removal of a cat and it was something you will not forget, BIG chucks of broken glass flying everywhere covering 20ft or more. I would think that for those you should leave them be (if its just a slight green halo zenith) or maybe try banding, but not sure about how that would work. I suspect the rectangles were never designed to have NO protection at all.

[bgadow]

In the era when FBP & FJP were both being sold, the bonded tubes used frosted faceplates rather than clear (at least, every one I've seen did). I believe the PVA could help the optics in that case as just having the faceplate a hair too far away from the crt can result in a big loss of focus.

I've read an article in one of the TV repair trade magazines c.1959 talking about Sylvania introducing the first bonded tubes (b/w). I don't recall it mentioning safety, just how it freed up TV designers vs. separate glass. Personally, I do the edges either with double-sided tape or silicone & will continue to do so.

I suspect the original material had a very low viscosity when it was poured/pumped in. Maybe it was heated? Were it as thin as water or mineral oil perhaps air bubbles wouldn't be an issue. I'm not sure how they do it with modern auto glass.

Meanwhile, the vent windows on my '51 Ford have "cataracts" of their own. Rather than replace them I'm tempted to try taking them apart and then re-laminating them with silicone.

[matt.caputo]

This has been quite a discussion on this subject, so I'd figure I would chime in. It's always been my understanding the PVA bonded safety glass on the 21fj and others was used to protect the tube from external objects striking the face plate that could result in an implosion. Not the other way around. The 21fj without the safety glass is just as structurally sound as the 21fb, so it shouldn't matter how anyone chooses to re attach the safety glass. Think about it, there's no way these tubes had the safety glass bonded to them during the evacuation process ( I assume pumped down to the 7th scale) and exhaust process. The PVA bonding could not have handled that kind of heat for the time required. I also find it interesting that some of these guys claim the bonding or coupling agent is some form of optical oil on some of these tubes. That could very well be as that was the case on CRT based projection TVs between the tube face and magnifier lense.

[Tom9589]

There is definitely oil in front of some projection CRTs to keep the tubes cool. My son and I took a projection TV apart for the heck of it. I have even seen kits offered for sale which contain cleaning supplies and fresh oil as the oil gets cloudy after a few years of use.

[benman94]

I believe the real reason behind the PVA bonded safety glass might be to minimize reflections between interfaces.

Consider light emanating from a source outside of the set and falling on the screen:
In a set with a 15GP22, you have the interface between the air and the safety glass, an interface between the safety glass and the air behind the safety glass, and interface between the air behind the safety glass and the front of the tube, an interface between the front of the tube and the vacuum of the tube, and finally between the vacuum of the tube and the phosphor dot plate. There are going to be reflections at any interface where the light passes from a low index to a high index material. This is basic optics. In the case of light that makes it to the dot plate, there are now a bunch of low-to-high interfaces on the path back out of the set.

All this reflecting has the effect of washing out the picture and reducing the apparent contrast.

The later 15HP22, 19VP22, 21AXP22, 21CYP22, 21FBP22, etc, eliminated the interface between the vacuum of the tube and the dot plate by depositing the phosphor directly on the glass that makes up the front of the tube.

Now consider that the refractive index of PVA (1.48) is much closer to that of the various glasses (1.40 to 1.60 for most varieties) than it is to air. You would be replacing a air-to-glass interfaces with PVA-to-glass, and since the indices of refraction are much, much closer, you would minimize the reflections to some degree. Granted, there would still be reflections at the interface between the PVA-to-glass interfaces. Even glass-to-glass interfaces (with two seperate pieces in direct contact) must exhibit some reflecting as the structure of the glass itself is not continuous and there are gaps and voids between them, etc. Still, the severity of the resulting reflections is substantially lower.

The PVA bonded tubes should give less "glare" from ambient light than a 21FBP22 set. I can say that there is much less glare on my CTC-7 than the Westinghouse H840CK15, and I suspect that a CTC-16 with its bonded tube would exhibit less "glare" than say a CTC-7.

[old_tv_nut]

Originally Posted by benman94

I believe the real reason behind the PVA bonded safety glass might be to minimize reflections between interfaces...

Maybe not THE reason, but definitely a benefit. In medicine, the saying would be "There are no side effects, there are only effects."

[bandersen]

Here's the video showing the oil https://www.youtube.com/watch?v=VlWXNVJ5fN0

[old_tv_nut]

Originally Posted by bandersen

Here's the video showing the oil https://www.youtube.com/watch?v=VlWXNVJ5fN0

This video shows an unbonded tube. The 'oil' is exuded from the rubber edge gasket. While the rubber is still flexible on an old tube, the surface is not clean, but covered in this sticky substance, and may carry some black material from the rubber. I'm guessing that they were made with an excess of "plasticizer." In the case of my tube, there was some residue on the inside of the cover glass that never came completely off. So, there must be some volatile substances given off also. Not visible under normal operation, but still visible when the cover was off and reflecting a ceiling light. The gasket at first was covered in black goo, which I suppose was a combination of deteriorating "rubber" and plasticizer. It came off on my hands when I picked up the lens. After the first cleaning, I made the mistake of setting the lens gasket side down on the cabinet, where it immediately left a dark circle in the wood finish.