I am restoring a RCA TM21B studio monitor. The monitor was "upgraded" to a 21FJP22A CRT. The CRT has a terrible cataract, so I removed it and found that the safety faceplate just fell of when I remover the tape around the perimeter of the CRT, and the entire cataract film just pealed off. This is a good thing. But the safety faceplate appears very fuzzy when you look through it, even though both surfaces have been cleaned. When a small puddle of water is placed in the concave bowl of the faceplate, the view through the water and faceplate is clear again. I assume that this means the clear bonding agent they used between the faceplate and the CRT acted as a wetting agent for the back surface of the safety glass. I am also guessing that the safety faceplate is some kind of filter glass...? Anyone know about the safety glass on this CRT, and what bonding agent I might use to put it back on?

Thanks.

I've not run into this, but, I've been wondering about how they did it at the factory and how hard it'd be to replicate it. Heck, a service to do it wouldn't be that bad - send in your tube and get it back rebound.

I think the origional stuff was a sillicone of some sort. I'm sure there's a modern day one that'd work. It's holding it and filling it that's the tricky part, though neither seem insurmountable - use a jig and fill carefuilly. I'm imagineing something that holds the CRT horizontally, and then something to hold the glass in front of the CRT properly (suction cup device?). Seal the tube with tape around the edge and fill. I'm starting to think more and more that they wrapped the tape on the tube *before* filling it. Or, maybe they had a teflon band or something?

John,

I had a real bad cataract on an Admiral rectangular tube (knackered anyway). The safety glass was as you described. I inverted the tube in a container large enough to hold it safely and placed the cleaned glass right on it. At that close a distance to the screen, the fuzzy surface will appear clear. I just ran a large bead of clear silicone around the perimeter of the screen. The picture (as bad as it was) looked like there was never a cataract. I then sealed the edges with packing tape. I have before and after pics on my site.

John, the set pictured in my avatar is the one.

John,

A quick look at my TM-21D with the FJP (according to the manual) shows a bonded safety glass. Thick as a brick.

I can't help but wonder if your "wetting agent" was that good 'ol standby from magnifiers, that wonder chemical, mineral oil. Lay a pool in the glass and plop the tube in it and seal the edges.

Good luck. 50+ tubes and caps by the hundreds. Mine has a cold solder joint somewhere, so the cooling fan vibration gives it fits in the horizontal. It has not moved from my office upstairs in 10 years. Around 250 pounds, but does make a reasonable picture with no restoration yet.

It came from WFIL-TV (now WPVI-TV) here in Philadelphia and then went to Temple U. It left there and I traded a Majestic bakelite for it.

You post your pic and I will post mine.

Dave A

Yeah, the 21fj's were frosted. It was supposed to reduce glare. To me, it's like watching tv through a frosted bathroom window. If it didn't originally have the bonded glass, I would get rid of it. If you just take the glass off you have a 21fb. What sort of tube and safety glass was in it originally?

The original goo is called PVA. I hear it's tough (read impossible) to get all the bubbles out. Makes you wonder how the factory did it.

I've heard some rebuilders used to use a ring of double-stick tape to space the glass out (air in-between tube and glass) an then sealed the edge with tape like original.

I think most people use silicone like Big Dave did. Frenchy posted some pics a while back of this, too. As I recall he used some sticks to space the glass away from the tube while the first silicone dried.

John

Dave A,

I may be wrong about the wetting action, but the bonding material completely filled the gap between the cRT face and the curved safety glass. There was no mineral oil involved. The old cataract came off in one piece, maybe 1/8" thick, a little thicker at the edge. The center 1/3 was still clear, and is rubbery and flexable. I suspect if I just separate teh two with an air gap, it will look like crap. I may consider mounting the tube without the safety glass.

Blue Lateral,

The monitor originally had a 21CYP22A, and I would very much like to find one to put it back to it original configration. But in the meantime, fixing up my poor old 21FJP22 will have to do. PVA....polyvinal acetate?

Thanks guys for the info and suggestions, keep 'em coming. I will post some photos of the monitor once I get a cRT back in it and dust if off a bit.

---

John-

I think the PVA is polyvinyl acetate. I'm not really sure.

The frosted glass does look like crap IMHO (ducking and covering). I would only put it back in a set that came with it originally. Is the original safety glass in the monitor missing? Should it be flat or curved? If it's flat, I'd get something made. If it's curved, one from a 21fbp22 will work. They have a gasket, and four tabs on the crt mounting ring hold the glass in place. Now that the glass is off, you have a 21fbp22. AFAIK this is directly mechanically interchangeable with the 21cyp22a, only the 2nd anode connection is different.

John

I was surfing around in the Yahoo TekScopes group, as I have an old Tektronix 535 i'm fixing up, and I stumbled acorss a question about an old Tek 547 with an internal graticule. Apparently since the graticule was in the face glass of the tube, Tek bonded a piece of plexiglas to the face with some clear goo for implosion protection. The goo gets cloudy over time. Sound familiar?

Stan Griffiths, one of the resident experts on these older Tek scopes said (among other things):

-----snip------

Tek used to solve this problem by "repotting" a cloudy CRT at no charge to
the customer. When a scope would come into the Tek Service Center with a
cloudy faceplate, we would change out the black rubber pads to the newer
rusty-brown ones and send the cloudy CRT to Beaverton where the plexiglas
faceplate would be stripped off and replaced by a new one with clear
material. This is not easy to do in the field since it was done in a vacuum
chamber to keep air from being trapped between the glass and the plexiglas
and forming bubbles on the screen.

-----snip-----

So maybe a vacuum chamber is the secret.....

John

I have done 2 cataract repairs, both with the frosted glass. Like others I just used silcone on the perimeter with small spacers, nothing in between. I really don't see where it degrades the focus-as was mentioned, the distance between the face of the crt & the safety glass is so small. If you mounted it with too much space you would have a problem. I kinda like the frosted glass as it does reduce glare. But I think if this set was mine I would just leave the glass off & save it someplace in case you need to use the crt in another application someday.

Agree with everybody else on just sealing the perimeter with silicone, that's what I did, put in thin spacers to hold the glass above the tube, then run a bead around and inside the entire outer edge. Yes you will get a tiny amount of fuzziness due to the fact that now you have air between the outer glass and the tube instead of the old glue, but with the close spacing you should be using (same thickness of the old glue is what you want), it is unnoticeable really, even up close. Besides, even when new, that glue wasn't exactly the greatest material for a lens - and now you have eliminated it.

Read how I did it:

http://audiokarma.org/forums/showthread.php?t=6740

And Roundscreen's finished result is somewhere in a hread with a pic of the screen :)