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What are the main dangers of breaking down or tinkering with CRTs? From my electrical engineering background, I understand that typical CRT designs include very high capacitance that can be dangerous even when the device is unplugged. I also know that there are additional risks simply from working with a very large vacuum tube.

A friend was concerned that a young student might be getting in over her head by attempting a breakdown of electronics with a CRT, so he asked me about the risks, but I don’t have enough expertise in this particular area. Just how dangerous is that kind of project? Are there precautions you can take to mitigate the risks, that would be simple and safe enough for a child or teenager? Is this a project for experts only?

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    \$\begingroup\$ Insuffient research. Question should be closed. \$\endgroup\$ – Leon Heller Jun 12 '15 at 0:59
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    \$\begingroup\$ @LeonHeller Could you please be more specific? I have a good idea of the general nature of the danger from my EE background and initial research (as noted in the question). However, I don’t know just how serious the risks are, or what degree of expertise is required to keep them manageable. Asking on behalf of a friend who’s worried about some kids getting in over their heads, and I figured it was safer to ask here than risk giving him whatever random information I could find on the web. \$\endgroup\$ – Bradd Szonye Jun 12 '15 at 1:12
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    \$\begingroup\$ I can't speak for the dangers involved with any form of authority, so I'll leave this as a comment not answer. But speaking from experience of having disassembled a CRT computer monitor, there isn't really anything to be gained from doing so - inside there is just a circuit board and giant sealed vacuum tube. I took it apart to get a flyback transformer for generating a plasma arc. That was a pain in the neck, took about 2 hours trying to get the PCB out and then another hour trying to extract the transformer. Not an exciting or pleasant experience. Plus one small slip and you smash the tube. \$\endgroup\$ – Tom Carpenter Jun 12 '15 at 1:24
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    \$\begingroup\$ I'm all for taking things apart to see what's inside, growing up I did it all the time - ended up building my own PC (from bits from a computer store) when I was 10 or 11 as a result - more just to see if I'd learnt enough to make it work than anything else. But my suggestion if this is for the sake of curiosity to pick something which would be more interesting to take apart - e.g. an old laptop has more 'bits'. AM radios are quite interesting because of the ease of then saying, now you've seen what they are like, lets make one (google 'transistor radio', they are quite simple to make! :) ). \$\endgroup\$ – Tom Carpenter Jun 12 '15 at 1:50
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    \$\begingroup\$ For kids, more interesting and fun to mess with something like LittleBits or Snap Circuits. Plus, they are re-usable. \$\endgroup\$ – rdtsc Jun 12 '15 at 2:01
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There are some incredibly exaggerated claims about the supposed dangers of CRTs on the interwebs. Some people think they are

  • Charged for days or months with enough energy to kill instantly

  • Sitting like a bomb ready to implode and kill everyone in the
    room- spearing them with shards of glass from the implosion.

As someone who has gotten the odd jolt from an anode connector and deliberately destroyed a few CRTs (and one accidentally) they're not so bad. The HV circuit that powers a CRT is dangerous enough and you should make 100% sure that it's not powered and fully discharged. The mains voltage is dangerous too, and some TV sets had electrically hot chassis. Dielectric absorption (a kind of non-ideal behavior of the capacitor dielectric) can cause the output capacitor to appear to recharge itself a bit so avoid contact even after it is unpowered and discharged. Some cretins on the interwebs think this phenomenon is caused by wind and the earth's rotation or some such thing.

There's an anode connector on the side of the back of the CRT. You should discharge it to the black aquadag coating on the outside of the CRT when the connector is removed. It holds enough energy to give you a jolt and possibly trip or drop the CRT. If fully charged the CRT capacitance can hold approaching 1J of energy, which is of a similar order to what you'd feel grabbing an electric livestock fence- not an especially pleasant sensation. It will typically mostly leak off in a few hours or days, but discharge it to be sure.

enter image description here

Here is what Thompson says for a typical color CRT:

enter image description here

Danger from implosion is not huge, but caution indicates wearing safety glasses (!) -your eyes are irreplaceable- and gloves when the back of the tube is exposed. If the tube neck or filler pip is broken the air generally just whooshes in and that's it. If the tube is dropped from a height- it generally just breaks, no great drama. Here is what Thompson says about that- to the people who would handle them every working day in television assembly factories and service shops:

enter image description here

I suspect these stories got progressively exaggerated like 'Chinese whispers' as nobody wants to see a friend or a junior injured. If there is one substantiated instance of a person being killed by a CRT I have yet to see it. Really old color CRTs (the round screen type) might pose special dangers, and those sets emitted significant soft X-rays in operation until leaded front glass was universally used.

As an aside, television sets did start hundreds of fires in the early years. In the UK the rate was measured at more than 5 per 10,000 sets in 1948. Part of the cause was that the EHT was originally derived from the mains transformer. Using the horizontal flyback switching transformer greatly improved the safety. As recently as the mid 1990s televisions in the U.S. were reported to have caused about 20 deaths per year (0.5% of the total) and around 1300 fire 'incidents' per year (0.27% of the total).

Originally TV sets were designed so that when the back of the enclosure was removed to expose the set (including vacuum tubes which the user might take to the local drug store to test) the power connector came off. This interlock could be defeated by a so-called cheater cord. Don't do this.

enter image description here

TL;DR The CRT itself poses some additional considerations that need to be taken into account, but mostly worry about the mains and the high voltage supplies required by the CRT.

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I worked briefly on CRTs (not TV ones though).

Charged caps are one of the main dangers - a quick way to discharge is to turn them on again AFTER d/c the supply. Old ones had an on/off button which worked well enough. New ones have SMPS supplies which in standby mode can have the d/c bus charged - no help available for them.

Also keep in mind that the anode is at very high potentials upto 7000V, so ensure you are working on isolated platform.

Answers to your questions are:

  1. Dangerous - yes.
  2. Precautions - mentioned below.
  3. Experts only - yes.

Rules of thumb to keep in mind are (by no means an exhaustive list):

  1. This project is not for teens / children - and certainly not for someone who has zero experience with electronics.
  2. Always assume a cap is charged.
  3. Work with one hand behind your back so that there is no chance of a current passing from one hand to the other via the heart.
  4. Do not work alone - since if something happens someone is there to respond. If there is no choice ensure that someone knows that you are working alone.
  5. Make sure the platform is isolated - rubber boots, wooden chairs work excellent.
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When working with a CRT make sure you discharge the tube. I found out the hard way when I was a young man about 40 years ago that a CRT is also a large capacitor. It can hold a charge even when disconnected from the TV for well over a week. I vaguely remember discharging the CRT using a screwdriver with insulated handle and a 10 AWG cable attached to it. All I remember is a 4 inch spark to the screwdriver and a loud snap and then waking up on the other side of the room still gripping the handle of what was left of the screwdriver. The ground cable was no where to be found...I think it vaporized. My hand was paralyzed for about 3 days afterwards. I think the reason they refer to a flyback transformer as such is if you touch that sucker you fly back.

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