Macintoshguy1984
Well-known member
Whenever i open a macintosh up with a crt, I get this feeling that I might get shocked from the crt. Does anyone know the chances of getting shocked?
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Macintosh crt related deaths
- Thread starter Macintoshguy1984
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Reasons.
Well-known member
It's possible and would hurt like hell, but my understanding is that there isn't enough power in there to kill you absent a preexisting heart condition. An easy way to remove the danger is to leave the computer sitting unplugged for a few days before opening it. That'll bleed off the charge.
Elfen
Well-known member
It depends if you respect the machine and take your time or if you're stupid and try to do a rush job.
As for being shocked, in order for it to kill you, it has to travel through your hand, up your arm, around your shoulder, across your chest, THROUGH YOUR HEART, then out the other shoulder or chest or other arm or other hand. I Bolded that "THROUGH YOUR HEART" because in order for any electrical shock to kill you, it has to go through your heart, a minimum of 12V at 4 AMPs will do it. At 5V, it can be 6amps to kill you. At 10KV, I forget but 1/2 an amp will do you in.
But let me say this - for being stupid and rushing the job, I have been shocked by that Mac CRT a dozen or so times. Thing is, it did not go through my heart. It went through my hand, up my arm and out what ever my arm was touching, so I was lucky. BUT, IF FREAKING HURT LIKE HADES!!!! it feels like your fingers were blown off as the nerves yell at you for the next 1/2 hour "YOU IDIOT!!!" But like I said, the shock did not go through my heart. That would have done me in. And I'm still here to post this.
I give that analog cap a lot of respect but I'm still stupid.
If you want a taste of what it is like - sit in front of your mac, and turn it on. While it is doing the RAM Test and the tube lighting up, rub your fingers across the screen and collect the static off it. Then go touch something metal You will get a shock from that alone but it is a small shock. The shock from the analog cap is 1000X more painful!
As for being shocked, in order for it to kill you, it has to travel through your hand, up your arm, around your shoulder, across your chest, THROUGH YOUR HEART, then out the other shoulder or chest or other arm or other hand. I Bolded that "THROUGH YOUR HEART" because in order for any electrical shock to kill you, it has to go through your heart, a minimum of 12V at 4 AMPs will do it. At 5V, it can be 6amps to kill you. At 10KV, I forget but 1/2 an amp will do you in.
But let me say this - for being stupid and rushing the job, I have been shocked by that Mac CRT a dozen or so times. Thing is, it did not go through my heart. It went through my hand, up my arm and out what ever my arm was touching, so I was lucky. BUT, IF FREAKING HURT LIKE HADES!!!! it feels like your fingers were blown off as the nerves yell at you for the next 1/2 hour "YOU IDIOT!!!" But like I said, the shock did not go through my heart. That would have done me in. And I'm still here to post this.
I give that analog cap a lot of respect but I'm still stupid.
If you want a taste of what it is like - sit in front of your mac, and turn it on. While it is doing the RAM Test and the tube lighting up, rub your fingers across the screen and collect the static off it. Then go touch something metal You will get a shock from that alone but it is a small shock. The shock from the analog cap is 1000X more painful!
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flashedbios2012
Well-known member
I wear rubber gloves.
Macintoshguy1984
Well-known member
I took apart my macintosh plus I few hours ago and when i was putting back apart and I think the analog board was loose and I hear a snap, but nothing broke
Macintoshguy1984
Well-known member
I know this sounds dumb, but which part is the analog cap ( Whenever I open a macintosh up I don't touch any part of the crt)
See that little thing that looks like a suction cup stuck to the side of the tube? That, and the wire that runs to the black transformer on the analog board, are the most dangerous (or highest voltage, at least) parts.
An old book I had about amateur TV repair years ago recommended that any time you work on an open TV chassis (and that would include an open compact Mac) when you don't know if it's discharged and ESPECIALLY if you're working on it with the power on (ie, making adjustments) you keep one hand firmly in your pocket, thus hopefully preventing a shock from crossing directly through your chest. (As might happen if your off-hand was touching a more attractive ground than your feet.) I've always followed that when working on open TVs but I haven't ever taken a really nasty shock so can't vouch for it, uhm, making it not kill you?
An old book I had about amateur TV repair years ago recommended that any time you work on an open TV chassis (and that would include an open compact Mac) when you don't know if it's discharged and ESPECIALLY if you're working on it with the power on (ie, making adjustments) you keep one hand firmly in your pocket, thus hopefully preventing a shock from crossing directly through your chest. (As might happen if your off-hand was touching a more attractive ground than your feet.) I've always followed that when working on open TVs but I haven't ever taken a really nasty shock so can't vouch for it, uhm, making it not kill you?
Macintoshguy1984
Well-known member
If i let it sit unplugged for 2 days if i were to accidentally touch the analog cap would it hurt a lot?
novusgordo
Well-known member
Aren't you actually supposed to take a discharge tool (or a screwdriver with an electrical lead attached to it) and discharge the CRT before you start working on it or the analogue board?
Scott Baret
Well-known member
Most of the newer computers (SE, Classic) have self-discharging CRTs, as do many of the Apple monitors (I know the 12" RGB does). The older ones can either be discharged or you can let them sit for a good while.
Discharging the CRT is actually a somewhat risky procedure in of itself. I only have done it once and it was honestly sort of nerve-wrecking!
On the compacts, if you're really, really concerned, get a pair of insulated pliers and remove the anode cap with those. I do this quite often.
Discharging the CRT is actually a somewhat risky procedure in of itself. I only have done it once and it was honestly sort of nerve-wrecking!
On the compacts, if you're really, really concerned, get a pair of insulated pliers and remove the anode cap with those. I do this quite often.
I discharged a Commodore PET's monitor's CRT once using an ad-hoc tool consisting of a screwdriver grounded by heavy-gauge wire to a cold water pipe, and even though the shock didn't go *through* me the hand holding the screwdriver felt a distinct slightly-painful *tingle* when I made contact. I can totally imagine that the unfiltered experience must hurt like *ell. One correction, though:
Technically a shock *doesn't* have to "pass through your heart", a sufficiently large shock that transits *anywhere* in the central nervous system can cause irregular hearbeat and/or breathing disruption. Also, technically speaking, it's *mostly* the Amps that kill you, not the volts, and as little as 100ma at any voltage *properly applied* could kill you. What makes higher voltages generally more dangerous is they're more likely to overcome the resistance your body presents between source and ground. (Your body isn't a *particularly* good conductor so, for instance, there's a pretty good chance you'll get away totally unscathed by brushing dry fingers across both poles of a 12v car battery despite the potential for a positively massive jolt of amps therein, while the voltages you'll find in a flyback transformer are sufficiently high that it's possible that a spark could hop to you *across dry air* under the right/wrong circumstances.) If you've ever licked the top of a 9v transistor radio battery you'll know that it's *possible* to get a nice painful shock from even piddly low voltages if you overcome that whole resistance thing.
FWIW, it's a common myth (er, oversimplification, really) that electricity takes the path of least resistance - it actually takes ALL paths, but how much flows thru any given path depends on the resistance of that path as compared to the rest of the paths.
If, for example, you have a positive source connected to your right thumb, and a negative source connected to your right pinky finger, there will be some flow thru your heart - but it will be vastly smaller than the flow from your finger to your thumb, because the resistance of the path that goes thru your heart is so much greater. Usually, that difference is enough to make getting a shock that primarily goes thru a path other than across your heart a painful but non-fatal occurrence - and CRT's generally fall into this category.
Large high-voltage power supplies however, are a different story; getting shocked by an industrial power connection can kill you even if the primary path is just thru some other part of your body. Also, for high current discharges, inductive effects can come into play - lightning strikes, for example, tend to do this (most of the power of a lightning strike flows over the surface of the object being hit, but that much current makes quite the inductive whollop to anything conductive nearby). :O
Other factors can also come into play - your skin resistance, for example, is much higher when dry than when wet (and it's even worse when sweaty, with all the electrolytes in the sweat making it a good conductor); but the resistance of your insides is much lower than your skin in general.
Wearing gloves in general is a good idea when working with high voltage; but make sure that they can breathe so that you don't get sweaty in them.
If, for example, you have a positive source connected to your right thumb, and a negative source connected to your right pinky finger, there will be some flow thru your heart - but it will be vastly smaller than the flow from your finger to your thumb, because the resistance of the path that goes thru your heart is so much greater. Usually, that difference is enough to make getting a shock that primarily goes thru a path other than across your heart a painful but non-fatal occurrence - and CRT's generally fall into this category.
Large high-voltage power supplies however, are a different story; getting shocked by an industrial power connection can kill you even if the primary path is just thru some other part of your body. Also, for high current discharges, inductive effects can come into play - lightning strikes, for example, tend to do this (most of the power of a lightning strike flows over the surface of the object being hit, but that much current makes quite the inductive whollop to anything conductive nearby). :O
Other factors can also come into play - your skin resistance, for example, is much higher when dry than when wet (and it's even worse when sweaty, with all the electrolytes in the sweat making it a good conductor); but the resistance of your insides is much lower than your skin in general.
Wearing gloves in general is a good idea when working with high voltage; but make sure that they can breathe so that you don't get sweaty in them.
olePigeon
Well-known member
Leaving it unplugged for at least a day should be fine. The CRT should dissipate itself, but they're old, and it may not do that, so leaving it unplugged for a day should take care of it.
If you're really worried, you can make a discharge tool (it's hard to find a commercially made one these days.) You basically take a screwdriver and run a ground wire from the shaft to the ground on the chassis (gator clips are useful here.) You stick the screwdriver under the cup into the CRT, and you should hear an electrical pop. It'll discharge to the ground.
Make sure you're not touching the chassis when you do that.
If you're really worried, you can make a discharge tool (it's hard to find a commercially made one these days.) You basically take a screwdriver and run a ground wire from the shaft to the ground on the chassis (gator clips are useful here.) You stick the screwdriver under the cup into the CRT, and you should hear an electrical pop. It'll discharge to the ground.
Make sure you're not touching the chassis when you do that.
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tomlee59
Well-known member
There's all kinds of misinformation and downright nonsense in some of the posts here.
Current through the heart is indeed an important consideration. But comments like "It takes X volts at Y amperes to kill you" make no sense at all. Current and voltage are related by Ohm's law -- your body is a resistor, so if you apply X volts, the current will be automatically determined by your resistance. You simply cannot specify both current and voltage independently.
The charge stored in a 9" crt is far too small to do you in. Yeah, it can hurt like hell, but fearing it as a life-threatening event is overdoing it. And unless you specifically need to disconnect the crt, there is no need to discharge it prior to working on the mac. There are no exposed terminals with high voltage on them, so just leave it be.
If you do need to discharge the crt, clip one lead onto the metal band around the crt first, and then connect the other lead to a screwdriver (or something similar) and slide it gently under the anode cap (the suction-cup like thing on the bell of the crt; see the Classic Mac repair guide for pics). That will take care of it.
Connecting one end to a cold water pipe is not guaranteed to work, and is unnecessary in any case. All you want to do is to short out the capacitor, and that means providing a conductive path between its two terminals. No need to take the long way around! And the cold water pipe wouldn't work if the Mac isn't plugged into the wall (and you shouldn't be working on it with the thing plugged in). Don't be confused by the word "ground" or "earth". Even airplanes have "earthed" connections, but they don't achieve this by dragging a 5-mile long wire behind them.
Current through the heart is indeed an important consideration. But comments like "It takes X volts at Y amperes to kill you" make no sense at all. Current and voltage are related by Ohm's law -- your body is a resistor, so if you apply X volts, the current will be automatically determined by your resistance. You simply cannot specify both current and voltage independently.
The charge stored in a 9" crt is far too small to do you in. Yeah, it can hurt like hell, but fearing it as a life-threatening event is overdoing it. And unless you specifically need to disconnect the crt, there is no need to discharge it prior to working on the mac. There are no exposed terminals with high voltage on them, so just leave it be.
If you do need to discharge the crt, clip one lead onto the metal band around the crt first, and then connect the other lead to a screwdriver (or something similar) and slide it gently under the anode cap (the suction-cup like thing on the bell of the crt; see the Classic Mac repair guide for pics). That will take care of it.
Connecting one end to a cold water pipe is not guaranteed to work, and is unnecessary in any case. All you want to do is to short out the capacitor, and that means providing a conductive path between its two terminals. No need to take the long way around! And the cold water pipe wouldn't work if the Mac isn't plugged into the wall (and you shouldn't be working on it with the thing plugged in). Don't be confused by the word "ground" or "earth". Even airplanes have "earthed" connections, but they don't achieve this by dragging a 5-mile long wire behind them.
That would be my bad, but in my defense the site I googled up for instructions on how to do it gave me the bad information.
I will note that thinking about it further the advice about keeping one hand behind your back when servicing a TV was for the most part aimed at working on the set *when it was on*, and in fact the danger you were mostly working to avoid in that case was line voltage, not the HV circuitry. (Many television sets, even fairly modern ones, lack isolation transformers and therefore could expose you to risk of AC shock if you touch the "chassis ground" while the set is plugged into an incorrectly wired outlet.) I *have* in fact gotten a 120v shock off an old TV while trying to adjust a recessed control on the back without even taking the lid off; that's why back in the day TV repairmen would have sets of plastic "screwdrivers" for manipulating those controls.
Realistically, sure, the danger to you when working on a Mac, particularly if you're just opening to work on the "computer" parts of it, are indeed minimal. *Running* it with the top off is a somewhat different story.
Or, if you do, you are also implicitly specifying the resistance of the circuit. R=V/I - "Ya canna' violate the laws of physics, cap'n!" Lt. Cmdr Montgomery Scott.
I think it depends on who "you" is, and how unlucky you are. Sometimes people have heart conditions that they don't know about.
On the other hand, I just got back from traveling at a closing speed of over 120 miles per hour in close proximity to other vehicles being operated by a bunch of yahoos... any error during the course of which could well have been fatal (and indeed, something in the neighborhood of thirty-two thousand people die each year in the US alone from this activity). I'm probably going to do it again tomorrow, too... :O
What is this insane activity I speak of? Why, just going out to get lunch... and doing so in a car.
