Hello.

My university is currently re-hauling a lot of laboratories and departments, and dumping a lot of hardware in the process (most of it old, sometimes 486 or P2 old). While most of it is easily scavenged, i have come across a bunch of discarded computer CRT monitors.

I have heard that the tube itself is extremely dangerous to touch/disassemble, since it can shock people with something like 10kV. (Mind you i don't want the CRT's themselves, at least not for now.)

1) How can i find out it they are charged?
2) How do i safely handle them if charged?
3) Additional precautions?

Thanks and cheers;
Bruno

 

If they have been off for a few hours, the charge has probably reduced to a safe level.

To safely handle the CRT, you need to let air in. One way to do it is to use a long screwdriver and a small hammer. The rear of the CRT has plastic moulded on it to hold the connection pins. In the center of the end of the neck is a small "tit" where the vacuum pump pumped out the CRT. It has been flamed sealed.

You take the screwdriver and tap it through the plastic cap, almost in line with the neck. It will break the tit. You will hear the air rush in, and the CRT will be safe to take off the chassis. That is the one place you can break the glass safely.

 

I use pliers to crush the tiny "Tit" between the pins on the neck of the tube.
Wear saftey googles... accidents have been known to happen.
BTW, they can indeed stay charged for a long time... my first experiance with that was when changing an EHT rectifier tube back in the days of B&W, and embedding a screwdriver into the ceiling with the instant muscle spasm that followed.
Colour tubes can have a potential of up to 30kv, and the way I discharge them is with a cliplead attached to the earthing contacts strapped around the conductive (graphite coating) rear of the tube, the other end connected to the shaft of an insulated screwdriver.... and touch the screwdriver to the aquadag socket on the body of the tube. If it's charged up, you will see a nice wee spark leap out.

 

In the wise words of a certain master of ceremonies...

"STOP!
It's hammer time!"

Besides the ray tube, i believe there is nothing in the damned thinges that is worth the zap.

 

I've had a nasty shock off a charged-up CRT whilst replacing the front cabinet on a TV, and it isn't pleasent at all! I had my shock after I had already discharged it, so obviously I hadn't quite finished the job!

To discharge the CRT, find yourself a screwdriver with a very long shaft. Ground the shaft of the screwdriver via the earthing strap of the CRT and then, push the screwdriver underneath the plastic cap. You will hear a short "crack" as the CRT discharges.

If you prefer to see the spark instead, leave the ground off the shaft of the screwdriver, push it underneath the plastic cap and then, with a second screwdriver, touch the shafts together and touch the end of the second screwdriver to the body of the CRT.

You will see a nice arc as the CRT discharges. But be careful as this way is much more dangerous!!!

If you get a shock, suffice to say you won't want to go near the TV for the rest of the day.

Brian

 

Better still, buy or make a shorting probe. We used to call them "chicken sticks." You're not chicken if you use them, but you are fried like chicken if you don't. We also called them by another name, having to do with which profanity was uttered after being shocked.

http://tpub.com/content/neets/14188/css/14188_30.htm

Make sure the clip is soldered on, not just mechanically connected. Make sure the handle is made from a good insulator - plexiglass is a good choice. Make sure the shield is opposite the grounding wire. (Amazing that folk actually put them together backward, but it happens.) If making your own, insure that the grounding wire is big - at least an AWG6.