• Please review our updated Terms and Rules here

CRT cataract removal

falter

falter

Veteran Member
Joined
Jan 22, 2011
Messages
6,573
Location
Vancouver, BC
I picked up this LSI ADM-1. The screen is quite messed up. I assume this isn't just an anti glare cover but is actually glass protection.

I've read through the process of repair..it seems to be basically discharge CRT, heat old glue with heat gun, remove the plastic, remove glue from plastic, reapply glue and reapply screen?

Is it any more complicated than that or potentially dangerous?
 
Jerry Walker seems to be the only person to put out a good video out there covering cataract removal in a couple videos about ADM 3/A restoration https://www.youtube.com/watch?v=iTcZbBp5yGY

He talks about and shows the steps involved. My only fear is removing the from lens exposes the real CRT face which is thin and has high potential for implosion (wear a full face mask the entire process). You need to preheat the tube in an oven according to him.
 
I did this on a 1970's HP "wide format" terminal CRT maybe 10-12 years ago, with the CRT mounted sticking out of the top a bit in a custom plywood box, with a nichrome wire powered by the +12V line of an old AT power supply. Temperature was regulated by the length of the nichrome and I made wood and screw handles for the wire. I was extremely paranoid about implosion (I do NOT like to work with CRT's at all). I "cut" through the rotted material with the wire, removed the shield, and reinstalled it on the edges with optical-grade (fish tank) RTV. I would rather not have done this at all, but even then that tube was basically irreplaceable; the only replacement I could find of questionable availability was >$250. I wore very heavy clothing, welding gloves, and a full face shield.

There is probably a bit of hype about the danger of these tubes. By design, the "nipple" where the tube is vacuumed out is designed to be the weakest point of the tube, and I break these off (again, with full protection) before disposal. They are actually quite sturdy as long as you don't do anything "stupid" (i.e., lean or drop the tube on the neck).

Without that front plate, I have read that during WWII the onboard airplane radar screens had a nasty tendency to break, sending the (very hot) "gun" metal part of the tube forwards right through the front of the tube towards the viewer. Hence, the modern face plate.

My ADM-3A's tubes, which at the time were only about $60 each, did not have cataract problems, but had very bad screen burn. I've used them only very intermittently since. One of these days, I will make a hardware "screen saver" circuit for them.
 
Shango066 has videos on YouTube of doing cataract repair on old TVs, mostly roundie tube stuff though.
 
Many thanks! Yes, I am nervous about the potential to break the crt face. I'm also concerned about removing the plug or wires at the end of the crt neck.. I have always found it extremely easy to accidentally vent the crt, either by accidentally brushing up against the crt neck or one time when i tried removing those wires. I've watched videos of other people doing it and they always make it look like a simple pop off and pop on, but any time I've tried it it vents.

I would have thought in terms of implosion risk you wouldn't just be worried about glass shards but also the toxic stuff inside the tube no? I'm wondering if it wouldn't be worth the trouble to build a see through box similar to that used for sandblasting small parts, with only holes for your gloved hands and heat tool. That way if it blew up, at least it would be fairly contained.
 
You have never pulled a plug from the back of the CRT neck? Im surprised by that. ITs no big deal. I do it all day long. some are on tighter than others which may need a wiggle but some are barely on there. Youd be surprised. You wont break the CRT neck unless you have thich sausage fingers from working at the dock all day.

I dont think you need to go the length of building a box. Just wear a shield over your entire face. I have manhandled many a CRT; havent broken one yet.
 
The stuff inside the tube is largely--nothing (vacuum) :) There may be some traces of toxic elements, but not so much that it poses a danger. Keep in mind that these materials have been selected so as not to de-gas under vacuum. The danger posed by a small CRT imploding is very small--there's simply not enough energy in the volume. We used to take the 27" CRTs from TVs and place them in metal trash cans before letting the air in.

The most polluting part of a CRT is the "frit seal", the glass "glue" that joins the funnel to the faceplate. It often contains lead oxide, which can leach out into the environment when placed in a landfill.
 
As an alternative, people have used butyl acetate and a long needle syringe for injecting it deep into the PVA.
Nice that you picked it up before the ghouls noticed the keyswitches it uses
 
I recorded a cataract removal on an HP display tube a number of years ago using the halogen lamp and heat gun method. It's fairly lengthy but the entire process is covered from start to finish and to cover my bases for unavoidable flaming comments telling me I'm doing it wrong, I'm not wearing the proper safety equipment or there's some other way do do the same thing.
 
NeXT said:
I recorded a cataract removal on an HP display tube a number of years ago using the halogen lamp and heat gun method. It's fairly lengthy but the entire process is covered from start to finish and to cover my bases for unavoidable flaming comments telling me I'm doing it wrong, I'm not wearing the proper safety equipment or there's some other way do do the same thing.
Click to expand...

This has nothing to do with your thread, but your remark about 'flaming' kind of reminded me of the BBS day when someone would jump in your poop about using CAPS. :-D
 
VERAULT said:
You have never pulled a plug from the back of the CRT neck? Im surprised by that. ITs no big deal. I do it all day long. some are on tighter than others which may need a wiggle but some are barely on there. Youd be surprised. You wont break the CRT neck unless you have thich sausage fingers from working at the dock all day.

I dont think you need to go the length of building a box. Just wear a shield over your entire face. I have manhandled many a CRT; havent broken one yet.
Click to expand...

No. Not successfully. I've always been intimidated by CRTs. I have the world's worst luck with them. I've been zapped twice. And even taking extreme care not to break them, I've still managed to vent three of them. Tried to remove the plug (very gently).. whoosh. Was working behind the crt on my NEC APC, found a broken wire that may have caused it to be not working.. very, very lightly tapped the plug at the back.. whoosh. Vented.

I watch videos of guys removing the plugs all day long.. just *ripping* them off the back.. no problem. I breathe on my CRTs and they vent. I don't get it.
 
when you say Zapped do you mean AFTER doing a discharge? If thats the case I have had that happen about 7 or 8 times to myself.. Onetime through the front of the CRT to my stomach somehow! Risidual charge can build up after a discharge which can cause a small shock. Enough to alarm you (and piss me off) but not enough to hurt you. My remedy for that is leaving the discharge tool in for a couple minutes. I dont know why some CRT's are prone to do this build up and others are not but it happens.

Ill be honest I dont understand how the crt neck cap coming off can cause damage. The only think that occurs to me is slight corrosion on the pins causing heavy drag when pulling it off.

Its not quite the same but I was reading an article about electronics folks who use a grounding wrist strap vs folks who dont. The article was going into detail on how some people are much more prone to discharge static and others are not. IT went into detail on how it could be blood type dependent. Interesting stuff. Dont know if there is truth behind it. But never really used wrist straps and im Type A-Pos.. Who knows.

Anyway i wouldnt give up on CRT work. Practice on some equipment you dont care to much one. ITs not a big deal once you have some successes under your belt.
 
Yeah it's really just a confidence thing. I don't have someone experienced with me to sort of say, 'yeah, you're good'. You're dealing with something invisible and may give no indication that you got rid of it at all, until it's too late.

I got zapped actually when the units were plugged in and operating.. stupidly on one of them I used a metal screwdriver to adjust one of the metal trim pots and it happened to ground against the monitor's metal case. I didn't get thrown across the room or anything. It was more like getting a vaccine or IMS treatment.

I have so many projects lined up that I'd love to fix. I have a CM-2 that got bounced - externally is fine but the PCB got cracked. That's something I could fix. But instead i've let my fear leave it sitting for almost 3 years now.

What I would love to do is find a real CRT discharge tool, rather than messing about with a screwdriver. That'd really help my confidence a bit. But they seem to be hard to find.
 
Chuck(G) said:
A CRT that's been discharged can still retain a (weaker) charge enough to startle one. The effect is known as "Dielectric Absorption".
Click to expand...

I worked at a company that made very high power/duty cycle flash tube power supplies for photocopiers. Those capacitors had to be shorted at all times that they were not in a circuit. I worked with an engineer who briefly forgot about this and the results were not good at all.

"Getting a tiny shock that startles you" shouldn't be discounted. A lot of injuries/property damage from electrical shock come from the psychological reaction to being shocked, not the actual electrical shock.
 
falter said:
What I would love to do is find a real CRT discharge tool, rather than messing about with a screwdriver. That'd really help my confidence a bit. But they seem to be hard to find.
Click to expand...

You can build your own. That's what I did using the instructions from Larry Pina's excellent "Macintosh Repair & Upgrade Secrets" book. It's basically a clip that gets grounded to the CRT frame, an old test probe with a fine enough tip that it'll go under the suction cup but not so sharp that it'll scratch anything, and a resistor so that the charge isn't released all at once. I believe Mr. Pina recommended a 10 Megohm resistor, but I've seen other people say that that is too high and you'd still have a deadly charge even after touching the anode for a second. Perhaps that is the case for a big color tube as Pina's book was for repairing the original Macintosh which had a 9" black and white screen.

Here are his instructions:

LarryPina said:
The 10-megohm resistor allows you to drain the electrical charge slowly. Without it, you'll get a big spark which is potentially dangerous not just to you, but also to the CRT. An easy-to-make custom discharge tool is shown in place, in Figure 1-15. This tool is made from an insulated test probe, a green clip lead, and a 10-megohm 1 /4-watt resistor. To make your own tool, cut off one end of a green clip lead, unscrew the test probe handle, slip the handle over the wire, and cut the resistor leads to size. When you screw the handle back on, the resistor leads should be completely insulated by the test probe. There can't be any exposed metal. Readjust the leads as necessary, then solder everything together. For safety, cover the entire tool with heat shrink tubing. Never unscrew the handle while the tool is in use!
Click to expand...
 

Attachments

  • photo63751.png
    photo63751.png
    333.4 KB · Views: 1
Last edited:
You must log in or register to reply here.
Back
Top