Home » Questions » Goods and Services [ Ask a new question ]

How dangerous is it to work on a CRT display?

How dangerous is it to work on a CRT display?

I've heard that taking apart a CRT can be dangerous, so before I go ahead on the eMac teardown I plan to create for iFixit, I'd like to know any hazards. Is taking apart a CRT really that dangerous?

Asked by: Guest | Views: 65
Total answers/comments: 6
bert [Entry]

"A CRT can hold several THOUSAND volts of electricity in its flyback circuitry. If you do not have the necessary experience and tools to properly discharge and make safe a CRT, you should not even take the plastic cover off. There is also the chance of physically damaging the fragile neck of the CRT, leading to a violent implosion as the vacuum is released. This could spray poisonous, phosphor-covered shards of glass all over the room.

Mac Geniuses must pass the CRT safety section of the certification exam with a 100% score every year - it's that serious of a safety issue.

Also, don't think a CRT which has been powered off for a long period of time will be safe - the CRT will slowly continue to build static electricity just due to the Earth's rotation and magnetic fields.

Peachpit Press has an excellent excerpt from its Training Series books discussing CRT safety here:


Servicing a CRT-based Macintosh should be left to the professionals due to the inherent dangers. I avoid them as much as possible, even though I am thoroughly trained, with decades of experience working on CRT-based computers, monitors, and test equipment."
bert [Entry]

Working at an authorized apple repair center a few years back, I was discharging an eMac CRT, as I was prying off the cap the alligator clip popped off the ground and made contact. I was blown black into my other bench, hand smoking... Worst pain i have ever felt, I think I even peed myself. I can still remember the feeling today. So just be very very very very careful when you discharge it, or if you don't feel comfortable, take it to a tv repair shop and let the pros take the risk... I no longer discharge the emacs, I leave it up to my other techs.
bert [Entry]

"I agree with the cautions above. The bigger the screen (distance between the cathode and anode) the higher the voltage, which can be 25 000 volts or higher. Voltage does not kill, but current. The higher the voltage, the larger the current that will flow through your body and affect it. So, take extreme care. I had years of experience, but was always on edge when working on the high voltage circuit of CRT's.

Implosion is not that likely: put the viewing side of the CRT on a stack of towels, you'll be OK. These CRT's have to be tested before they get to be put in front of anyone's eyes, and are designed for really low pressures (vacuum). Just don't apply any uneven pressure, and stay away of the neck because that's the weak part.

As with all devices, disconnect from power source as a first step.

There's a cable with a rubber plug or shield on top which is the culprit. It keeps it's high voltage because there's a whole array of diodes and capacitors to get the electrons going the right way. The diodes keep current going in one way, the capacitors work like batteries. This cascade achieves a high voltage using an alternating voltage, and it requires extra circuitry and a risk of failure to make it discharge when the set is powered off. There's little current needed to illuminate the screen, but as it takes some time for your CRT to actually light up, you can figure there's a lot of electrons just waiting to take a route through your body.

We used to disconnect the rubber plug on top (anode) by using two long screw drivers with good isolated handles. As extra, wear rubber gloves. One would touch the metal frame, one would be used to pry undert the plug, then cross these two metal rods to short out. Another method would be to take a wire with crocodile clips and connect it to the frame on the one side and the screw driver on the other end. Pliers can be used to press together the metal clips under the rubber shield, when at least one side can be pryed loose, it will come off. You should always try a second discharge as a final check."
bert [Entry]

"Grounding it to a wall outlet does nothing; there is a grayish-black coating on the outside bell of the CRT (called Aquadag) and you will notice some type of metal spring or finger touching it - THIS is what you want to ground the wire to.

But even more important the CRT is made of glass and it contains an extremely high vacuum. If it breaks it will implode and glass will go flying everywhere. I strongly recommend at least wearing safety glasses before handling it.

Update (11/28/2011)

I have been working with CRTs since I was about 10 years old (1970) and the points raised are all very valid. I illustrate the fact that it can be worked on safely - even by a kid - but if you read this and do not feel comfortable with this, please, don't touch it and leave it to qualified personel.

The second anode connection on the side of the bell is the high voltage connection. In ALL cases make sure it is not plugged in! After carefully removing the cover, you should be aware of exactly what you are trying to discharge.

A capacitor is nothing more than two conductors separated by an insulator, so yes, even a line cord or a network cable and a line cord also have capacitance (and inductance, but that is another story).

There are no high voltage capacitors as such - the aquadag coating of the CRT forms a capacitor. The inside of the CRT bell (silver) is one plate, the glass of the CRT is the insulator, or dielectric, and the thick black coating on the outside is the other plate and it is normally grounded, whether you have a TV, monitor, eMac or iMac, regardless of who made it,

There will be something making contact with the outer aquadag - it could be a coil spring, a flat leaf spring, a clamp or some type of metal comtacting it, this is the proper place to ground it. Attach a wite to this point and the other end to a flat blade screwdriver. Work the screwdriver under the lip of the rubber cup on the side, You'll hear a snap and a spark when ot arcs, but go further untierl you actually touch the metal connecton. If you need to disconnect this you can lift the edge of the cup and see how that is done. Usually you push it in slightly and push it to one side and und unhook it. Then touch the button on the side of the CRT with the screwdriver once again, just to make sure,

But that does not eliminate al the dangers! Two other things you need to be aware of. As already stated, the CRT is made of glass and it is fragile. It is under a very high vacuum and if broken it will implode and send broken glass everywhere. If you have to handle the CRT, and / or disconnect the deflection yoke and / or the gun connector, please treat them with care.

So now you have the CRT discharged and safely set aside. All is safe now, right? Not necessarily. Granted, the second anode is the high voltage, but there also is several hundred volts on the electron gun and deflection yoke, with it unplugged you probably won't have any voltage there, but there are capacitors in the power supply, and you could get an unexpected shock. Usually the circuitry will discharge by itself within a half hour, but that depends on the circuit design.

DO NOT try to discharge the CRT to earth ground! You are not completing the circuit and actually creating a more dangerous situation than you started with. The ONLY safe place to discharge it is by electrically connecting the inner and outer aquadag - because that is actually where the voltage is stored."
bert [Entry]

"Just to add to all this, a family member who did repair work for over 20 yrs, and a person that does not exaggerate this kind of thing witnessed a CRT implode and put a hole in a basement CMU wall. Someone hit the tip/neck.

This was for a TV im sure, since it was a long time ago. The implosion is of such force everything goes in opposite directions just like a bomb, just with negative pressure from vacuum and nasty gasses. Once of the few electronic devices that can really hurt or kill you in more then one way.

Boiler systems can do this as well, usually only causing damage to internal components. If your working on taking a boiler off line and don't vent it, you can create some really bad negative pressure. its enough pressure to suck a man threw a hatch doing maintenance without follow the SOP. (resulting in death). It was a big boiler.

Safety tech is good now, but isn't enough to completely eliminate human error, just like fixing one of these older models, a simple mistake can result in craziness, not worth it to me fixing such an older computer when I could pick up a working powerbook etc for approx. $100"
bert [Entry]

I agree that CRT Monitor Has 25000 wait it Enough to Kill you in Seconds. Don't Try it all .