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I'm going to adjust the dynamic convergence and geometry on my CRT TV with magnets. However, my dad is very reluctant as he thinks the bell of the tube may have a high voltage current and that placing a magnet on the tube will give one a shock.

Is is safe to touch the bell of the tube and to tape magnets to it?

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    I'm voting to close this question as off-topic because this qustion is not related to Retrocomputing in any way, but a generic electronics question. electronics.stackexchange.com might be the better place to ask. – Raffzahn Jan 27 '18 at 10:10
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    We have a meta question about this here. – wizzwizz4 Jan 27 '18 at 10:49
  • @Raffzahn We've already discussed this in Meta. – Black and White Jan 27 '18 at 14:52
  • the CRT is vacuum electron ray gun where the ray is emited at the tube part and accelerated by electrostatic field (high voltage). Any touching to the CRT without proper HIGH VOLTAGE insulation is potentially dangerous (especially during operation). Adjusting magnets is the last resort (not by hand of coarse, not even by feromagnetic material like screwdriver). adding additional magnets (if strong enough) can bend the electron ray (out or towards the tube) transfering charge at high voltage so Yes your dead is right there is a risc of DC shock (which can be evil for blood). – Spektre Jan 27 '18 at 16:22
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    @wizzwizz4 THis question still has no relaion to computers whatsoever. It's exxactlyy the case even you described as not related. – Raffzahn Jan 28 '18 at 12:16
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It's weird that you know enough about the CRT to know how to adjust geometry with magnets, but not enough to know the way the voltage is stored. CRT repair is dangerous - you should understand how the CRT works - not just follow step by step instructions. This is true not just for safety but also because these are finicky analog devices that don't always respond the way the book says they should.

The short answer is that yes, the outside of the tube itself is perfectly safe. But that is not really enough to know, because you can still zap yourself.

There are two kinds of high voltage danger inside a CRT. First, the power supply, which can easily contain 200V or possibly up to 800V. Second, the tube itself, which can contain 25,000 (yes thousand) volts or even more in a large TV. The voltage in the tube is proportional to the size of the tube, about 1KV per diagonal inch, give or take. The power supply voltage can burn you and knock you down, the tube voltage can kill you. Most of the power supply voltage will discharge on its own given a few hours or days, though - especially if you unplug the CRT without turning it off, just like a PC power supply. You should still discharge any large capacitors safely before you handle them.

Voltage in the power supply is more likely to zap you because it's more exposed, but the danger from it is less. The tube voltage is by no means safe. If you look inside the case, you should see a large wire connecting the tube to a bulky component which is the flyback transformer. The tube end of the wire attaches to a "suction cup." It isn't really, it's actually an insulator, but it looks like one. This wire, the transformer, and sometimes some other components are all at high voltage.

It isn't always necessary to discharge the tube before working on the CRT. In fact, many adjustments can only be made with the case open and the display turned ON. Any activity inside the case when the tube is potentially charged should always be done using an insulated tool, wearing insulating gloves, no jewelry, no dangling hair or clothes, and with one hand in your pocket or behind your back. This will at least hopefully keep the current from stopping your heart if you manage to zap yourself.

To discharge the tube - which you should do before replacing any components - you connect a wire to a screwdriver, the other end of the wire to the chassis ground (NOT earth ground or electronic ground) and poke the screwdriver under the cap until it touches the terminal. Wait a few minutes, then go back and do it again. Remove the cap (it has clips under it which you can squeeze to release) and connect the ground wire to the terminal.

If you have any safety concern whatsoever, it's best to find someone who is willing to do this for you. There may not be any CRT repair shops still around, but there are still some people around who worked in them and remember how to do things. Safety is more important than color quality!

Edit to add: Most of what I wrote applies only to repairs or adjustments you might make with the CRT disconnected from power. If you decide for some reason to work on it with it turned on - which you probably will once you start fiddling with the picture quality - you need some additional safety precautions. The power supply voltage can kill you too if it's connected to mains electricity, and a few CRTs have a "hot chassis" - the whole metal frame is live! Needless to say this is very much not perfectly safe to touch. Because of these and similar issues you need an isolation transformer - with isolated ground - to work on a plugged in CRT. This will allow the CRT's voltages to float independently of the external environment, so you can connect things like oscilloscope probes and your fingers without trying to discharge the whole power supply through them.

  • Don't worry; I know how the high voltage is supplied in a CRT, but my dad was still cautious. Now I can prove him wrong! +1 – Black and White Jan 27 '18 at 14:51
  • Somewhat strange conclusion to an answer that stresses that, yes, there is high voltage on the tube and, no, the short answer is not enough to know. Poking around inside a CRT is ABSOLUTELY dangerous, and even in scenarios that seem safe there are so many non obvious circumstances that could occur that are hard to mitigate unless you're a professional. – Retrograde Jan 27 '18 at 20:01
  • The org. post suggests a rudimentary understanding of CRTs. Fine, expect if jumping to the conclusion based on a carefully qualified answer that it is safe to go ahead. Some of the less obvious risks are discharged caps recharging ("memory effect"), high voltage gradually rebuilding on the tube after it was discharged, air inside the set becoming ionized so that normal insulation properties degrade so that arcing to you or your tools could occur. You may choose to take the risk and may also come out of it unscathed. But understand the risk. Dunning-Kruger and high voltage is a poor mix. – Retrograde Feb 3 '18 at 10:39
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I can't tell you whether touching the bell of the tube specifically is dangerous, but I wouldn't do it if you're not sure.

Your best bet is to find an expert who knows exactly what they're doing and can show you how to deal with CRTs and high voltages safely.

If you decide to ignore my advice, at the very least make sure that you:

  • Turn off the power and unplug the CRT
  • Don't touch any capacitors
  • Work with someone nearby who can help you in case of an accident

Some people recommend discharging the CRT, but if you don't know how to do it correctly, you may be exposing yourself to additional risk.

Be safe!

  • Wow! Takes me back 50yrs. We used what was called a "cheater" power cord that allowed the TV set to be plugged in with the back off. – jwzumwalt Feb 3 '18 at 3:12
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A color picture tube can be made safe by (1) disconnecting power, and (2) shorting the anode to ground (with a probe under the suction-cup-like connection and wire(s) to the frame around the faceplate).

But, you cannot do dynamic adjustments, or static magnet adjustments, when it is safe. Those require a fully powered display. So, those adjustments are associated with safe-to-touch control knobs, or controls that can be reached with nonconducting tools from a comfortable position that doesn't require your body to be near the hot wiring.

If you can find a service manual, even for a slightly different model, it will help.

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