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Discharging a CRT Monitor

H

habibrobert

Member
Joined
Dec 18, 2012
Messages
39
Location
Florida
Hello,

Can anyone tell me how I can discharge a Macintosh Color Classic CRT monitor? I need to gain access to its hard drive and am a little nervous since I only used the computer yesterday. I have seen a lot of people using their own tools to discharge their monitors but I don't trust myself with make a tool that will have to carry thousands of volts through it! Did apple ever make any tools for this, where can I get one?

Thanks
 
You don't need a tool. Almost any wire will work. Be sure to connect it to a ground *FIRST* and then touch the other end to the HV terminal. BTW, 'thousands of volts' is not too relevant. It's the amps that matter.
 
I just use a long thin standard screwdriver with a clip lead... clip one end to the screwdriver shaft, the other end to an appropriate grounding point, then slide the screwdriver blade up under the rubber cap and wait for the snap (if there is one... the compact Macs don't seem to hold a charge for very long).

Edit: Oh, and if you're not confident in the insulating properties of your screwdriver handle, wear a gardening glove or something.
 
Old Thrashbarg said:
Oh, and if you're not confident in the insulating properties of your screwdriver handle, wear a gardening glove or something.
Click to expand...
Believe it or not... insulation is not too important here, as long as you are not grounded. If, however, you are grounded, it could become an issue. :)
 
I use an old test probe--clip the plug end (use an alligator clip) to the chassis, then stick the probe end under the anode cap. You'll know by the "snap" nose when the discharge occurs.

That's not the end of the story however. If you wait a few hours or days, you'll again find a (lesser) potential on the anode. This is due to a phenomenon called "dielectric absorption" and can take you by surprise. It won't kill you, but it can be an unpleasant surprise, so treat high-voltage gear with respect.
 
Chuck(G) said:
I use an old test probe--clip the plug end (use an alligator clip) to the chassis, then stick the probe end under the anode cap. You'll know by the "snap" nose when the discharge occurs.

That's not the end of the story however. If you wait a few hours or days, you'll again find a (lesser) potential on the anode. This is due to a phenomenon called "dielectric absorption" and can take you by surprise. It won't kill you, but it can be an unpleasant surprise, so treat high-voltage gear with respect.
Click to expand...

I can attest to this. Several years back I was working on an old arcade monitor and had discharged the tube, disconnected the anode and left for lunch and didn't do anything with it until the next day. Accidentally touched the anode connection on the tube and got quite a surprise! It was about like getting zapped briefly by an electric fence...after that I made sure to keep the anode grounded.
 
Be careful not to bump the neck of those little CRT's. Those necks are very easy to break. I know from experience. :(
 
I do antique TV repair as another hobby. When I first got my Predicta I was so excited about it that I forgot to discharge the CRT when I was taking the housing apart and that thing bit me bad! Good thing it hadn't been run in years or I probably would have broken it! Picture tubes are like a giant capacitor with a VERY low leakage therefor they can hold a charge for a very long time but not usually with enough ampers to kill you
 
Wow that is some scary stuff. Thanks everyone for sharing your experiences with me. I will definitely work with one hand behind my back when working with this guy. So then is there no "official" tool to use to discharge these guys? Nothing I can get off the internet that would be better built than anything I could make? I know its a very simple process, just make sure you have a grounded rod or screwdriver, just curious if there is anything out there.
 
My old man used to work as an avionics technician in the 70's for AWA, and as such played with radar screen CRTs and oscilloscopes etc as a matter of course, and all other kinds of CRT equipment. As such I got ntroduced to electronics and their principals at a pretty young age, and also the safety considerations that must me taken to account when working on electrical equipment, for the sake of preventing both equipment damage and more importantly personal injury.

Some of the earliest and best advice he gave me was that firstly, and foremost, treat every piece of circuitry as being live or as tho it could have stored current until you know for certain it isnt or doesnt... with that, the advice never to touch a potentially live component with the inside of a finger, as a shock will contract the muscles causing you possibly to grasp int involuntaryand prevent you letting go of it. As well as that, the second most important piece of advice, never EVER become an earth... Notice that birds on powerlines dont fry? its cos they arent completing a circuit by touching ground or a wire leading to ground. So basically, dont touch a positive and a negative uninsulated at the same time, which includes the chassis as it us generally part of the circuit, and always wear rubber soled shoes. He basically summed it up in one simple sentence, if ya work on CRT or a high voltage capacitor, until youre sure its fully disharged, one hand should be in your bum... lol
 
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