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?
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.
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.
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.
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.
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.
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
I agree that CRT Monitor Has 25000 wait it Enough to Kill you in Seconds. Don't Try it all .
That is an answer that could get someone killed - yes, killed. I have been working with CRTs for over 50 years (we used to call them picture tubes). When a CRT implodes shards of glass can fly over 30 feet. I have seen it. Others in the room could be injured as well. The ultor (second anode) voltage is nothing to sneeze at either.
And yes, I have gotten shocks from 277 volts (one leg of 480 3-phase to ground) no fewer than three times. Your first could be your last. Understand that. Respect that. That will leave you sore for a week if it doesn’t kill you. Virtually every day I work with machines that need lethal voltages to do their jobs properly. I am not afraid of them but I am always respectful of the injury or death they are capable of.
But you needn’t be afraid of a CRT monitor - or an old school TV set either. Be respectful of the voltages it carries and the fact that any high voltage item - even if it is only a few milliamps - can injure you.
I changed my first picture tube (a 19” black and white) when I was 10 years old (1970) and my first color picture tube (25”) when I was 12 (1972).
Working with this safely requires one word - respect! The #1 safety device is the six inches or so between your ears. Never forget that. On an almost daily basis I work with equipment that won’t just give you a shock, but contains lethal power that can easily kill you!
There is one thing you should always remember when working with potentially harmful voltages - and remember people have been fatally electrocuted on as little as 24 volts. The important thing to remember is that your heart runs on a string of electrical impulses. If you get an electrical flow across your heart, such as one hand being on a voltage source and the other on ground, this voltage across your heart, which is much stronger and completely wrong for what your heart needs, can make your heart stop and start contracting in the wrong ways. Death or permanent and irreversible brain damage can occur in minutes.
Most professionals work in a way that they cannot get a hand-to-hand shock. Keeping one hand in your pocket is a good start.
As far as discharging a second anode, I have always used a flat blade screwdriver and a jumper wire clip, Nothing elaborate. But it is also important to know where to discharge it to. Usually the chassis, but possibly somewhere else. There should be a label saying where to discharge it.
In a glass CRT, there is a coating on the inside and the outside of the glass called aquadag. The coating on the inside is silvery, the coating on the outside is black. The black is grounded and the silvery coating is connected to the high voltage button on the inside. These are both conductive surfaces, the glass between them is a dielectric (insulator) and therefore it forms a capacitor, and this is what holds a charge.
If you are disposing of the CRT, it is best to let the vacuum escape. at the small end of the neck, there is an electrical connector. Unplug this, and in the middle of the pin circle there is a glass tube that appears melted, which is called an exhaust tip. Break this tip off with a pair of pliers and you will hear the air rushing in. Once that stops the crt is as safe as any other piece of glassware that size. But beware - if you didn’t discharge that second anode it can still hold quite a charge! That probably wont hurt you but it can make you drop the CRT.
In addition to the high voltage on the side, other elements can have several hundred volts.
There is no poison gas on the inside of a CRT, just a very high near perfect vacuum.
I don’t want to scare anybody, but it is best that you know what you’re doing and work carefully. I wish you all the best of luck!
What a bunch of kitten answers - 25-40k volts with low ams is nothing, go stick your thumb in a distributor whilst an engine’s running and you’ll get a feel for it. Wait till you get a mere 440 on high amps and see how that feels, not easy to let go of the offending item either and guaranteed hilarity for your co-workers! Or 90kv+ ESD’s from pipeline plastic coating/rollers in cold dry climates - essentially lightning, pick you clean off the floor and throw you 3 car lengths away with ease. Then you get up, brush yourself off and get back to work before the gaffer reaches you to see why productions stopped ;-)
Stuff a screwdriver in the anode with a wire/clip attached to a chassis ground (no need for external ground, resistors or any other bs); no spark/sound? discharged. Whip it out and nip the bud off the end to release the vacuum. Chances of it imploding are minimal, providing you are wearing standard PEE (glasses/screen/gloves) you will be fine even if it does.
As for the fish-tank that’s a good idea, just wear a respirator (or scarf and don’t breathe) when slitting it open. It’s not safe $@$* inside there, so don’t go inhaling the dust - it’s that simple. If you’re worried about handling it wear gloves. I did this for my retrofit Amstrad PCW to make the new screen look insconspicuous and was a 5 minute job from start to finish.
I have discharged several CRTs in my career. The only reason I have ever had to discharge a CRT was to dispose of it. The method I have always used is to get a long thick flat head (standard) screwdriver, connect my jumper cables to the metal shaft of the screwdriver (both black and red), then I attach the other ends of the jumper cables to a metal water pipe that goes directly into the ground. The pipe has to NOT have paint on it. It have it’s natural metal exposed to make proper connection for ground. Twist the jumper cables on the pipe to make a good connection and clamp them down if you feel the connection isn’t secure. Use a screwdriver with a hefty plastic handle and wear rubber gloves. Once you take the back off of the CRT, slip the jumper cabled connected screwdriver under the “suction cup” that is attached to the back of the CRT monitor and make connection with the flat screwdriver several times to the metal clamps that are clamped inside the CRT under the suction cup. Try not to touch ANY OTHER part of the chassis before you do this. This must be done first before you can safely remove the suction cup with a pair of pliers. Once the suction cup is removed, touch the grounded screwdriver to the metal contacts in the suction cup again just to be safe. Once the suction cup is removed, I then get my power drill with a very small bit and drill into the area where the suction cup was clamped. I apply very little pressure to avoid cracking / breaking the CRT. Once I have the hole drilled, I wait for the hiss to stop and I then know the CRT has been pressurized and can be handled safely.
if any of you have a better method, please let me know. I’ve been doing it this way for years.