Need to Discharge Broken CRT?

[oldpcguy]

At extremely low current. As the article I linked to above explains, a stun gun is at least 10 times more powerful than the discharge from a compact Mac's CRT would be.

Most of the danger from electrical shock when working on consumer electronics is not from the shock itself, but rather the surprise causing you to bump into something or knock something over and cause injury that way.

None of which changes the fact the person I responded to was incorrect when they claimed there's only 12 volts present (ignoring the fact that the analog board has 120 volts wall voltage present).

With that said it doesn't take much current to stop ones heart. The reason stun guns don't (or shouldn't) kill people is the voltage doesn't pass through their heart. The same cannot be said when someone has two hands poking around in a compact Macintosh. The chances of having one hand touch the high voltage and and the other arm / touching ground isn't low. Do so and that current passes right across your chest risking killing you.

I agree, it is well known people have been injured or killed as you've described. However the threat of being killed by electric current is well known and present. Either way dead is dead. So anyone claiming there's low risk is, IMO, an idiot. I know that's not proper forum etiquette but such advice can lead to severe injury or death and I have no problem calling someone an idiot for offering it.

 

[Gary C]

Which is why people in the know tell you to keep one hand in your pocket when working on high voltage circuits. Serves 2 purposes... 1) to keep you from touching something and having the current path cross your chest... and 2) less things to move around when you're jumping back. :D

and you cant get your hand out of your pocket fast enough to save you from a fall

You should see the arc flash protection we have to wear these days when racking breakers in or out.

 

[twolazy]

Only circuit that would hold any decent amprage on that Mac would be the 5v and 12v line. 5v is mostly harmless. As I said before and will say again, unless you have some type of heart condition, it isn't going to kill you...

And why would the mac be grounded? Why would you work on the crt with it plugged in unless you are doing adjustments? Again he said he wanted to discharge the crt, not do adjustments with it on. So how is it going to ground?

 

[vwestlife]

With that said it doesn't take much current to stop ones heart. The reason stun guns don't (or shouldn't) kill people is the voltage doesn't pass through their heart. The same cannot be said when someone has two hands poking around in a compact Macintosh. The chances of having one hand touch the high voltage and and the other arm / touching ground isn't low. Do so and that current passes right across your chest risking killing you.

As already explained in this thread, the real danger of electric shock to someone randomly poking around in a compact Mac is from the capacitors on the analog board, not the CRT. And it's not like high-tension power lines where the conductor is exposed -- the HV lead of the CRT is a thickly insulated wire with a huge rubber gasket, and as that LowEndMac article explained, if you're not intending to remove the CRT or analog board, you're actually better off just leaving it alone than trying to discharge it with a screwdriver -- if you slip, you could damage the CRT and/or accidentally discharge it through you instead of through the grounding wire.

 

[twolazy]

Thank you VW. Most peeps seem to think modern crts are just like they were in the 40s and its not true. Most all have bleeder resistors. As long as you arent poking around with it plugged in you are fine. It really bugs me when people try to act like its going to kill you just removing the back panel. It makes the user not want to repair a machine, and if they do, they are super nervous and most likely will make stupid mistakes.

 

[oldpcguy]

Only circuit that would hold any decent amprage on that Mac would be the 5v and 12v line. 5v is mostly harmless. As I said before and will say again, unless you have some type of heart condition, it isn't going to kill you...

And why would the mac be grounded? Why would you work on the crt with it plugged in unless you are doing adjustments? Again he said he wanted to discharge the crt, not do adjustments with it on. So how is it going to ground?

As I said: It doesn't take a lot of amperage to kill you. Likewise the statement of yours that I responded to said nothing of amperage. It focused solely on voltage.

You appear to be ill equipped to comment on this subject. Given your advice could lead to injury or death I highly recommend you refrain from commenting on it. If you choose not to then I would recommend anyone reading it to ignore it. Those warnings on the systems are there for a reason whether you understand why or not.

 

[oldpcguy]

As already explained in this thread, the real danger of electric shock to someone randomly poking around in a compact Mac is from the capacitors on the analog board, not the CRT. And it's not like high-tension power lines where the conductor is exposed -- the HV lead of the CRT is a thickly insulated wire with a huge rubber gasket, and as that LowEndMac article explained, if you're not intending to remove the CRT or analog board, you're actually better off just leaving it alone than trying to discharge it with a screwdriver -- if you slip, you could damage the CRT and/or accidentally discharge it through you instead of through the grounding wire.

I know the construction of the high voltage line well. I've worked on many compact Macs. My interpretation of the OPs question is that he is referring to this particular voltage even if they were not accurate in asking the question.

In the end there are voltages present inside a compact Mac which could cause injury or even death. There are warnings on these systems for a reason. There's a reason Apple installed an insulating cover over the entire analog board. It's to protect people from possible injury or death. Anyone who pretends this risk does not exist should not be giving advice to others.

 

[twolazy]

Again where did I say there is no risk. I said twice unless you have some sort of heart problem it isnt going to kill you. This isnt rocket science. Ive been shocked by over 75000 volts and I am still here.

I also said only thing to worry about is the DC side, since the unit is not plugged in. It has bleeding resistors on the high voltage side. THERE IS NO VOLTAGE THERE. But hey fear monger all you want, and twist things to seem like you aren't wrong. Nothing is going to happen to OP unless he keeps the unit plugged in, period.

I guess next everything that says its going to give me cancer , means I have cancer now. You do understand how lawyers push that on everything. Even an electronic lighter has that same warning... Sheesh. Look out for that static electricity, its a killer since its 35000 volts!

 

[Chuck(G)]

If the CRT has been physically broken and has sat around, unpowered, for more than a day or so, it's unlikely that any appreciable charge will remain. In a normal environment with RH around 50%, charge bleeds off pretty quickly.

TL, don't fret--I got the same sort of response a few years back for using leaded solder. I'm still here.

 

[oldpcguy]

Again where did I say there is no risk. I said twice unless you have some sort of heart problem it isnt going to kill you. This isnt rocket science. Ive been shocked by over 75000 volts and I am still here.

I also said only thing to worry about is the DC side, since the unit is not plugged in. It has bleeding resistors on the high voltage side. THERE IS NO VOLTAGE THERE. But hey fear monger all you want, and twist things to seem like you aren't wrong. Nothing is going to happen to OP unless he keeps the unit plugged in, period.

I guess next everything that says its going to give me cancer , means I have cancer now. You do understand how lawyers push that on everything. Even an electronic lighter has that same warning... Sheesh. Look out for that static electricity, its a killer since its 35000 volts!

One doesn't need to have a heart problem in order to be killed. The risk exists in healthy people too. Many people have been shocked with high voltage. Those who survive likely didn't do so across their chest.

Yes, there is a bleed resistor...assuming it's still in place and functional.

 

[twolazy]

The Truth About CRTs and Shock Danger | Low End Mac

lowendmac.com

Read this article. It's says all you need to know...

 

[VERAULT]

I think you and I are old school and it never stopped us. I've been jolted by many things in my life, if you think that Mac is going to kill you OP, unless you have other issues, it isnt going to. I've had worse from car ignition systems. Sheesh.

I hate spark plug bite. Got bit a couple weeks back.. worse than anything I ever got from a computer or monitor.

 

[VERAULT]

With a "necked" CRT there is more danger from broken glass than anything else. And this is small potatoes compared to the people using a microwave oven transformer to do fractal wood burning because they've seen videos of it on TikTok and think it looks cool. 30+ deaths so far of people getting electrocuted while doing it.

Here's LowEndMac's advice about the dangers (real or imagined) of CRTs in vintage compact Macs:

The Truth About CRTs and Shock Danger | Low End Mac

lowendmac.com

Darwinism... maybe there is something right with the world afterall. Making it to old age is a reward.. not a guarantee.

 

[twolazy]

I hate spark plug bite. Got bit a couple weeks back.. worse than anything I ever got from a computer or monitor.

Amen on that. Was a 75kv Accel racing coil on a 429 cobra engine. Was adjusting timing and the new wires had a slice in the silicone. And sure as hell grounded through me since other arm was bracing the block. Talk about OW! lol

 

[oldpcguy]

The Truth About CRTs and Shock Danger | Low End Mac

lowendmac.com

Read this article. It's says all you need to know...

Sorry but I am not going to take an article on Low End Mac as an authoritative source of information on this subject. There is a risk of serious injury or even death when dealing with these Macs. This doesn't mean they cannot be serviced in a safe manner. What it means is one has to be careful and understand the risk. Anyone suggesting the risk is immaterial is an idiot.

 

[AndyO]

Sorry but I am not going to take an article on Low End Mac as an authoritative source of information on this subject. There is a risk of serious injury or even death when dealing with these Macs. This doesn't mean they cannot be serviced in a safe manner. What it means is one has to be careful and understand the risk. Anyone suggesting the risk is immaterial is an idiot.

While I don't have an argument on one side of this or another, since I don't have enough expertise on the subject to believe my opinion ought to matter to anyone, I would point out that however dubious the authority of Low End Mac, it is rather more accountable than an anonymous poster on a message board. Particularly since it seems improbable that they made up the attribution.

That said, I have heard enough of the horror-related advice to assume there is a danger, such that when I was about to remove the logic and analog boards from a Classic II for recapping, I asked an engineer who works on these things regularly for his advice, having had the Mac unplugged for over a month. What he said was that unplugged from the mains supply and unused for that long, there was no risk.

 

[Agent Orange]

Try this:

 

[oldpcguy]

While I don't have an argument on one side of this or another, since I don't have enough expertise on the subject to believe my opinion ought to matter to anyone, I would point out that however dubious the authority of Low End Mac, it is rather more accountable than an anonymous poster on a message board. Particularly since it seems improbable that they made up the attribution.

It doesn't matter if they're "more accountable" or not. The reality is the risk exists. Statements such as the following:

"...in fact, that it’s easy to believe that a CRT can vaporize humans and entire herds of cattle with a single discharge."​

do not inspire confidence that the advice given is authoritative. No one is claiming humans will be vaporized nor entire herds of cattle. What is being said is there is a risk of injury or even death so caution is warranted. I don't see recommending caution as being reckless unlikely those here who are advocating there's no risk. I wonder how many who are advocating this position would be willing to put one hand on ground and hold a screw driver in the opposite hand while pushing it underneath the suction cap of a fully charged 2nd anode? I bet none.

That said, I have heard enough of the horror-related advice to assume there is a danger, such that when I was about to remove the logic and analog boards from a Classic II for recapping, I asked an engineer who works on these things regularly for his advice, having had the Mac unplugged for over a month. What he said was that unplugged from the mains supply and unused for that long, there was no risk.

I would agree with this advice. The important thing is you, like the OP, were aware of a potential hazard and sought advice before working on the system. Thankfully you reached out to someone who provided sound advice and not one of the idiots here advocating there is no risk.

 

[AndyO]

It doesn't matter if they're "more accountable" or not.

Well, yes, it rather does in this litigious society. It tends to demonstrate at least some degree of confidence in the information being provided when the risk of it being wrong is potentially great.

I would agree with this advice. The important thing is you, like the OP, were aware of a potential hazard and sought advice before working on the system. Thankfully you reached out to someone who provided sound advice and not one of the idiots here advocating there is no risk.

With this, I totally agree.

 

[VERAULT]

You are all behaving like a bunch of little princesses. And DONT EVER TAKE MY WORDS OUT OF CONTEXT. I Never said there was no risk. I said They were making more out of the problem then there was.. Which quite honestly alot of you are still doing. I am someone who repairs CRT's all the time.. I am guess most of you waiting for your chance to jump in with a whole paragraph probably dont.. you just like to talk. ..And talk... And talk.... And you all seem to say very little.

WE DONT NEED TO HAVE BS DISCLAIMERS ON EVERY THREAD THAT STRING TOGETHER THE LETTERS C.R. And T. Unless the person is a complete idiot they will do thier research. IF they dont they will suffer the consequences. Yet for some reason we all need to suffer the consequences of these little princesses just wanting to spiel out the same drivel...

Lets not forget what this thread was.. a MAC SE with a broken CRT Neck.. Discharge it and move on.. This is not a big job.. its pretty routine and mundane. It been covered at nauseam online. IF you are too afraid to work on it Simply dont... My god come on end your little tea party.

If you dont have the mind to be safe dont punish those of us who do with Constant squawking.......