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Thread: CRT cataract please read...

  1. #1

    Exclamation CRT cataract please read...

    There are many stories on the web about removing
    the front glass from a crt to fix the cataract problem.
    Very nice, it makes TV sets and other CRT applications usable again.

    But every time I see some advices I get the creeps here...
    Especially when someone want to use a heat gun!

    I know the CRT's from a computer monitor/terminal are quite small.
    But still, don't underestimate the force which is applied to the CRT!
    Remember, vacuum gives a force of 1KG for each square centimetre.

    That means that even a little CRT from a VR201 as a force applied to the
    front glass due the vacuum of approximately 500 Kilograms! (about 1100lb)

    Early CRTs up to somewhere in the sixties, did not have an implosion protection.
    These TV sets had a safety glass in front of the set. These were very strong
    and hard to damage. But the CRTs them self were dangerous.

    The danger of a CRT is the avalanche effect when a CRT breaks. Glass flies trough
    the center of the CRT sratters due the great force and flies everywhere...

    I have seen three ways of protection trough the years to prevent this avalanche effect.
    The earliest was glass fiber glued on the back surface of the CRT. Then quickly after that
    the glued front glass appeared. And after that a the metal rim was glued around the CRT
    which was sufficient to stop the avalanche effect.

    The rubber as we all know is the troublemaker here. When you apply heat with a heat gun
    you don't know that you are doing. There will be high forces in the glass itself because
    you don't heat up the CRT entirely. Please read what can happen here for example (Scary!) :

    http://videokarma.org/showthread.php?t=247887

    When you have removed glass and rubber successfully you have a crt with the same safety
    as an 5ties tube. So safety at all! I've seen people placing the safety glass back in front of the CRT.

    BUT: The piece of glass in front of a VR201 is NOT safety glass. It is just normal glass. It is only
    placed in front of the crt to prevent the rubber from damaging and it is an easy way for production.

    So even with the "savety glass" in front of your CRT, the level of protection is the same
    as that of an 5ties tube. When something smashed the front of the CRT the "safety glass"
    will fly around in the same way as the CRT itself does...

    The best way to restore such a CRT is to put new rubber at the front of the CRT.
    Unfortunately this is a process which I can't do at home. And the most hobbyists cant.

    The only thing I want to say with this story: Please be aware of that you are doing...

    Regards, Roland
    WTB: Case for Altair 8800 ...... Rolands Github projects

  2. #2
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    Gave the thread a read, I wonder if someone didn't get a little crazy with the heat gun. When I've removed CRT cataracts, or seen RetroHacker_ do it, the glass of the actual CRT was warm but not hot. I'd imagine the face would get that hot if you happened to leave it in direct sunlight on a hot day. I've always worn gloves, safety glasses, and long sleeve work clothing but probably a face shield would be a good addition. And of course any nick in the actual CRT face caused by the use of a metal scraping tool or something would seriously add to the danger factor!

  3. #3

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    I have removed the front glass from one VR201 and one HP2640 terminal. I was also very reluctant to use the heat method. Instead I used Butylacetate which I injected in between the front glass and the CRT. Then I let the butylacetate dissolve the PVA and after a week, filling up more butylacetate a few times, the front glass came lose.

    Butylacetate smell a bit. Fruity odour. Like very strong banana or so. Butylacetate is a used a as asolvent in nail polish. Although it smells quite a lot it is not particularly harmful in lower concentration. A good thing is also that it has quite high boiling temperature.

    I know that when Rhode Island computer museum guys restored a VR14 display they put a sheet of lexan plastic glass inbetween the frontglass and CRT. The idea is then that if the tube goes bang then the lexan which is hard to break takes the hit. They heated the lexan in the owen to make it adapt to the curvature of the front glass.

    Mike can probably tell you more.

  4. #4
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    BA like its relative, Isoamyl Acetate, is used as a food flavoring as well as being present in fruit. It's not dangerous.

  5. #5
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    That sounds a lot more convenient than the "soaking in water" method!

  6. #6
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    The post is a little confusing. He says he was unable to get the safety glass off even after preheating, then starts the very next paragraph saying he got it off using preheating methods on youtube.

    From the half dozen tubes I've done so far I found that you need to be sure what you are using to support the body cannot thermally transfer or scratch the glass. The other is that you have to be patient. Those tubes contain a lot of class and can be thermally shocked easily if you don't want to wait.
    That being said, I don't fully understand the people who leave tubes out in the sun or soaking in their bathtub for days Leaving loose CRT's around like that always invites trouble for some reason.
    = Excellent space heater

  7. #7
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    Hmmm, i used a guitar string to remove the plate glass and then a new craft knife blade to clean off the excess "glue" left behind. I re attached the plate glass with double sided scotch pads, two on each side about 1 inch long and then went around the edge with ordinary tape. Seems to be ok but you have me a little worried now.

  8. #8
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    We replaced the PVA with a sheet of Lexan formed to fit between the two pieces of glass and glued at the edges. It looks great and provides a lot more protection than just air. You can see the work in this blog starting 08/07/15.
    Member of the Rhode Island Computer Museum
    http://www.ricomputermuseum.org

  9. #9

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    That looks like a good protection for the users / visitors to me!
    WTB: Case for Altair 8800 ...... Rolands Github projects

  10. #10
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