The eMac was an all-in-one G4-based mac designed for the education market. It was the last mac to use a CRT display, and was sold at a low price to schools, and other institutions.

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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?

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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:

http://www.peachpit.com/articles/article...

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.

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You're absolutely right, but it is possible to do it on your own. You'd just have to build your own CRT discharge tool out of a coathanger, rubber handle, and a grounding wire. The 'leave one hand behind your back' suggestion is standard practice for dealing with high voltages.

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Does that apply to televisions as well ? A friend of my dad who knew a bit about them used to tinker with them and I'm 200% sure he hasn't even heard of any certification exam.

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Yes, some televisions (All Tubes) use highly charged CRT's as well.

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Ground was not ground and I got zapped.Old house and the only ground I am now sure of now is a water pipe. Also a plastic chassis was involved. How do I find a ground on a plastic chasis? Will high-voltage

lead grounded to water pipe work?

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Anky - my dad worked as a tv engineer and he sometimes got massive shocks of TV's - the thing is it's high voltage but low current, the charge is enough to bloody well hurt you (and possibly give you free flying lessons!), but unlikely to kill (unless you have other medical conditions). Similar to a tazer or an electric fence - unpleasant but not enough to kill. As for a tube implosion - that can be quite shocking to see - we had a big 29" widescreen TV that my mum knocked off a cabinet once - there was a loud bang, luckily most of the glass ended up in the plastic casing, fortunately the TV was off at the time! At present I'm faced with a similar problem of having to go into the CRT case - I've got an old Philips 21" monitor which still has an amazing picture (at 1900x1600 it actually beats the Panasonic full HD TV next to it), but the down switch is stuck on the contrast and I'm dreading going into the case to fix it.

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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.

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long screwdriver grounded by a 10k resistor worked for me well - just be AWARE

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A CRT, especially older ones, can act as a capacitor. That means that if you discharge the tube at the anode, it may still build up a charge over time and so you should always keep a ground attached to drain the charge. Older tubes tend to not discharge the entire charge initially and after a while the charge returns to dangerous levels. Working on one is better left to trained technicians.

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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.

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Actually, I think I'll just leave it to someone else to do a teardown, thanks for the answers!

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I partially agree with you. Yes, it would be foolish to try yourself. NO, you should not leave it for someone else, there are many good reasons documented here that tell us there should never, ever be a teardown done on a CRT.

It's just NOT A DIY DEVICE!

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No worries! I didn't mean to come across in such a pushy manner. Just semantics really, but I wanted to convey my thoughts that it really is "only for the pros".

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Yep, I understand.

I think I'll leave it to someone who knows what they're doing!

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Yes, a CRT can kill you with the electrical charge it carries.

And yes, older TV sets DO use CRTs, so you can kill yourself working on them as well.

So don't do it. Pay someone else (who has all the correct equipment and health insurance) to do it.

There's no shame in asking for help. After all, Superman has the Justice League to help him out when he needs it.

Let me say it this way: Any repair procedure that advises you to only use one hand and keep the other behind your back so you don't complete a circuit across your heart is something you really don't want to mess with, IMHO.

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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

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I worked on a lot of CRT screens back in the 80’s. That rubber cap on the back of the CRT is where the voltage from the fly back transformer comes in. WARNING: THIS VOLTAGE IS HIGH ENOUGH TO REACH OUT AND TOUCH YOU! Do not try to measure this voltage with a standard multi-meter; you have to have a special hi-voltage probe, which is long and insulated. That extremely hi voltage is needed to push the electron stream across the vacuum space in the CRT; I have seen in excess of 40KV on some systems. We used to use a long flat blade screwdriver with a ground cable attached. Push the screwdriver tip under the edge of the rubber cup, you may see a spark. Then it would be discharged. If you are disposing the CRT it should have the vacuum pressure released. To release the pressure there is a metal connector under the rubber cup. Once the CRT has been discharged, disconnect the connector under the rubber cup. You will see the little metal connector nub underneath. You can cut that with diagonal cutters and you will hear the vacuum hiss; that is air rushing in the CRT tube. After that you can safely break the back of the CRT tube if that is your plan.

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I agree totally with the answers on here,As a television engineer i worked on crt's in the early years and believe me! they can pack a punch even after discharge as they do built up static,i still have scars where they arced across to my hands and burned little pin holes in them! So i would advise you to leave it up to a professional.

Teddy.

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You must discharge the yolk but i agree if your new at it,DONT TRY IT

YOU can and Might KILL Yourself, if you must try to save your life you need thick strand wire thats heavely insolated ,ex. jumper cables (car jumper cables) and a true earth ground, best bet solid copper pipe directly pushed into your yard about 3 feet,and an electricians standard high voltage screw driver you can tell them by the rubber coating goes all the way to the tip well abot 1 inch, connect jumper cable clamp to the non insulated part of the screw driver and the other clamp on the copper rod you buried 3 feet in the ground,you cannot touch any part of the plain metal ,take the tip of the screw driver and slide it carfully under the suction cup on the tube aim towards the center

of the cup to were the anode is untill your hear a snap and a big blue spark.you could however realistically connect the cable clamp to the unpainted part of the monitor (CRT) frame and do the same thing,but to be safe go with the copper rod.

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

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W O W guys... I was about to try to disassemble an Apple CRT studio display and looking for an "how to" guide but was totally not aware of these risks... I wanted to make an aquarium from the casing but will think twice now... Thanks!

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There would be too many holes, all the fish would fall out.

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It has 5lbs of exposed lead inside of it.

You wear a "flack jacket" when you get an x-ray at the dentist for a reason. Also, many laws prevent businesses (or offer incentives) for not dumping broken CRT monitors because the lead leaks into the environment and causes alot of damage.

Lead is very harmful to the body, I wouldn't chance it if you'll going inside of it.

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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.

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Autoharp Bill, excellent. all you would need now is to add images to your answer so we do not have to try to visualize something like this "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."

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Autoharp Bill you are 100% right. And please do not take it the wrong way, but if you need pictures to explain these instructions then you need assistance or you may not live to regret it :)

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I've read all the posts about being careful when discharging CRTs. Have big hesitation when doing it myself... wow.

Found out though that some professional high voltage probes have built-in voltmeters, which would detect any charge on the anode. Kind of takes the guesswork out of the process. Much safer, IMO.

To test, per the YouTube video https://www.youtube.com/watch?v=UsL-rAUK... , after the probe is grounded, you can poke the probe under the rubber gasket while the TV is on. Once the TV is switched off and the probe is left in place under the rubber gasket, it will discharge completely through the same ground connection that was connected originally.

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