A "touchy" subject

[smrieck511]

Hi, first of all, I have no intention of touching a charged CRT or analog board. I will take all precautions and discharge it properly.

That said, you often hear people say these things can kill you. Can in theory? Or are there actual documented cases of people who have been killed working on compact macs specifically?

just wanna make sure I understand what I'm dealing with. Thanks

 

[DetBenson]

my advice would be to wear rubber gloves, like the kind made for cleaning. Rubber is an insulator of electricity. Make sure you discharge everything and never actively work on the machine while its running.

 

[MrFahrenheit]

Hi, first of all, I have no intention of touching a charged CRT or analog board. I will take all precautions and discharge it properly.

That said, you often hear people say these things can kill you. Can in theory? Or are there actual documented cases of people who have been killed working on compact macs specifically?

just wanna make sure I understand what I'm dealing with. Thanks

It is my understanding that the volts themselves have been verified as being there in some cases. The number of volts have been documented to severely injure or kill a person.

So, the question you ask is kind of like this: If I don't wear my seatbelt, and drive 60mph for 8 hours a day, for a full year, will I die?

 

[Phipli]

Hi, first of all, I have no intention of touching a charged CRT or analog board. I will take all precautions and discharge it properly.

That said, you often hear people say these things can kill you. Can in theory? Or are there actual documented cases of people who have been killed working on compact macs specifically?

just wanna make sure I understand what I'm dealing with. Thanks

As dangerous electric things go, they're pretty dangerous. It doesn't take a whole lot to risk stopping your heart, which is why it is generally advisable to work on anything with high voltage with one hand and keep the other out of the way, it reduces the chance of the current tracking across your chest and through your heart... but, CRT voltages aren't "not much", they're substantial, and have nice big caps too.

For safety, there are bleed resistors on the caps to discharge them, but the risk is that these are broken.

I haven't answered your question regarding known cases, but it isn't entirely relevant - they are leathally dangerous, treat them as such.

 

[joshc]

Working on the machine while it's running is needed for some CRT adjustments. Wearing gloves while doing this is a good idea.

To some extent though, the dangers are exaggerated and there's no need to discharge if you are just doing a hard drive/logic board/floppy drive swap, as long as you don't touch the AB or flyback or yoke board.

It is normal practice to discharge the CRT if you are going to work on replacing the CRT or working on the analog board. High voltages are present, even after power off, and although there's a bleed resistor in the flyback of models after the Plus, this should not be trusted by default and you should still discharge before working on the CRT or analog board.

IIRC a compact Mac CRT can hold around 9,000-12,000 volts. Larger CRTs like those used in iMacs will involve even more voltage. They are definitely lethal if handled incorrectly.

 

[smrieck511]

Yeah, I am a little paranoid and will err on the side of caution after following the discharge procedure. Rubber soles, rubber gloves, one hand in a pocket, maybe even goggles.

 

[max1zzz]

I don't want to turn this into argument of opinions here but my understanding is that the HV output of a small black and white CRT would not normally be enough to kill or injure someone. Yes the voltage is high but the current is very low (a handful of ma from what I understand) and given this extremely limited current output it is very unlikely the HV output would be able to do you any significant damage

What is probably more of a risk is the mains section of the PSU which would be able to supply very high currents if you came into contact with it (I have actually read some reports that say the main danger of the HV output is not the HV it's self but that a shock form it might actually cause you to come in to contact with the mains side or the PSU or break the neck of the CRT as you try to pull your hand away)

It's also possible that a undischarged CRT may counterintuitively be more dangerous than a running one as it is essentially a big capacitor and might be able to actually supply a higher current when the HV supply can

With all that said I want to be very clear - I'm not suggesting you should be careless when working on CRT's. Discharge them if possible and the suggestion of wearing rubber gloves if you have to work on a running one is also a good idea. Coming into contact with the HV definitely won't do you any good so is best avoided even if it is unlikely to do you any real harm. All my "research" into this was also done in relation to small black and white CRT's which run at lower voltages (and currents?) then larger colour ones so may or may not be in any way correct for them

TLDR: My understanding is small CRT's are not really that dangerous but I'm a random guy on the internet so take that with a pinch of salt

 

[cheesestraws]

Yes the voltage is high but the current is very low (a handful of ma from what I understand) and given this extremely limited current output it is very unlikely the HV output would be able to do you any significant damage

A handful of mA will kill you very very effectively if it's propelled through your skin and across your heart. There's an article floating out there somewhere about how they're not that dangerous, but if I remember correctly, the maths was wrong in it. However ...

What is probably more of a risk is the mains section of the PSU which would be able to supply very high currents if you came into contact with it

In terms of risk, yes, people are more ... careless around PSUs. This is especially interesting given that one of the heatsinks on the 128/512/Plus AB is actually at mains potential.

So, you have at least two interestingly unpleasant ways to do yourself a major injury in there! They're such boxes of unmitigated fun!

 

[AndyO]

A significant variable is that while what @max1zzz says about the voltage and current is true, so generally speaking might not be expected to be lethal in the context of a B&W compact Mac, many people would prove more susceptible to a discharge shock than others, and it is not always clear to the individual where they may be on the spectrum of heart-related risk. Finding out the hard way is not recommended.

One other factor is that while the lack of modern-day first hand reports of death might suggest the risk is much lower than some would suggest, not only might that be more to do with the fact there are vastly fewer CRTs in use today than 20+ years ago, but that medical records and mortality reports are much more vigorously restricted too. The lack of reports to rely on isn't a reflection of the lack of risk.

 

[max1zzz]

but if I remember correctly, the maths was wrong in it

Ahh interesting - I know exactly the one you are talking about and was unaware that the maths in that had been disproved

especially interesting given that one of the heatsinks on the 128/512/Plus AB is actually at mains potential.

I have made that mistake before on another PSU and can confirm it is NOT fun! Don't go poking heatsinks in PSU's!

many people would prove more susceptible to a discharge shock than others, and it is not always clear to the individual where they may be on the spectrum of heart-related risk.

This is also very true, even if it may not be *that* dangerous to most people it not worth finding out.

 

[cheesestraws]

Ahh interesting - I know exactly the one you are talking about and was unaware that the maths in that had been disproved

I'm working from memory here, but I definitely remember that the article concentrated on the amount of energy that would be dissipated in the body, without taking into account the possibility of disruption to nervous control systems that really shouldn't be disrupted, and which are much more subtle.

I'm fairly sure the maths were also duff, but please do bear in mind I'm working from memory here, and how reliable that memory is is up for debate

 

[smrieck511]

Anybody here ever taken one of those shocks? Also, is the idea that the voltage and amperage can vary or is it an all or none thing?

 

[Macobyte]

Anybody here ever taken one of those shocks? Also, is the idea that the voltage and amperage can vary or is it an all or none thing?

I can’t speak for a CRT shock but when I was a kid and didn’t know what I was doing I took apart a disposable film camera and while messing with it I charged the flash capacitor and then touched the circuit. That was probably twenty-five years ago and I still remember the feeling of it running up my arm. I wouldn’t recommend experiencing what the numerous and much larger capacitors in a Mac can do.

 

[aeberbach]

I've experienced 240V mains a few times, it's never something you want to repeat. But this has been on a wooden floor, wearing shoes, one-handed. There's no calculating how bad it will be @smrieck511 - the consequences can depend on your personal conductivity and vulnerability, the path from the point of contact to ground, how an outlet is wired or how long ago the device was charged, so many things.

 

[ymk]

Rubber is an insulator of electricity.

Not at CRT voltages.

 

[joshc]

Another thing to note for @smrieck511 is to be careful if you need to detach the logicboard power cable - this is often when people accidentally hit the neck of the CRT with their hand as they pull hard on the logicboard cable. Once the CRT neck is broken, it's game over for that CRT and it would need replacing.

 

[rjkucia]

I think this might be a good summary:

If you know you have a heart condition etc., you should probably just avoid any risk of being exposed to high voltages and avoid this entirely. Otherwise, act as if it *could* kill or seriously injure you and take the recommended precautions:

• Discharge the CRT whenever you take the case off. The bleeder *should* work, but until you discharge it, assume there's high voltage
• Ideally only power the CRT on with the cover on
• If the cover must be off (for testing etc), don't touch anything
• If you must touch something (such as adjusting the CRT), use only non-conductive tools, don't touch anything metal
• Ask for advice if you don't feel confident or have questions

Additional maybe-obvious things:

• Keep children/pets away if you're working on a CRT
• While the voltage is definitely a concern, keep in mind we're also dealing with a giant tube with a ton of glass and a pressure differential. For example, you get shocked enough to startle you (but not enough to actually harm you), your reflex might cause you to damage something else, suddenly you've broken the vacuum and glass is flying everywhere. This might be harder to do than I'm imagining, though.

 

[Brett B.]

Interesting that nobody has mentioned discharging the CRT manually... a screwdriver with a piece of wire attached to it that is grounded is pushed under the rubber boot on the tube to touch the anode wire (equipment powered OFF, obviously.) Any charge remaining in the tube is gone at that point.

I don't worry too much about it. There are some dangers associated with working on electronics no matter what. I think just using common sense is the biggest thing.

Also, CRTs do NOT "explode", FYI, they contain a vacuum, not pressure. So even if you break the neck it's not going to send glass flying everywhere. Speaking from experience, we intentionally broke a lot of old monitors and TVs by dropping, throwing, etc. Even if you shoot one it doesn't explode, you just get a cool hiss and a pile of broken glass.

 

[3lectr1cPPC]

My question when it comes to CRTs is what part will actually zap you? Caps, fly back, anode I already know but what about the tube itself?

 

[rjkucia]

Interesting that nobody has mentioned discharging the CRT manually... a screwdriver with a piece of wire attached to it that is grounded is pushed under the rubber boot on the tube to touch the anode wire (equipment powered OFF, obviously.) Any charge remaining in the tube is gone at that point.

That's what I was referring to - although I realize now I probably should have expanded on exactly what to do for that for those who might not know, so thanks for doing that!

Also, CRTs do NOT "explode", FYI, they contain a vacuum, not pressure. So even if you break the neck it's not going to send glass flying everywhere. Speaking from experience, we intentionally broke a lot of old monitors and TVs by dropping, throwing, etc. Even if you shoot one it doesn't explode, you just get a cool hiss and a pile of broken glass.

Correct, they implode, which can still be dangerous. Compact Mac screens are certainly less dangerous than smashing a huge TV intentionally, but I think it's still fair to keep in mind that you can end up with broken glass if you aren't careful.