CRT Discharge Tool

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Level 1

1 points

CRT Discharge Tool

I was wondering if anyone could enlighten me as to how I can obtain a CRT discharge tool. You can't get it through Apple anymore, and I've scoured the Internet trying to find them for sale to no avail.

I know as far as Apple is concerned these machines are obsolete, but I still have a number of them in my school district and I'd rather not throw them on the junk pile simply because they need a new hard drive, and I can't safely discharge the CRT in order to install a new one.

Thanks to anyone who can help.

iMac [Intel Core-Duo], Mac OS X (10.5.2)

Posted on Feb 28, 2008 6:39 AM

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Answer 1

Top-ranking reply

spudnuty

Level 5

7,128 points

Feb 28, 2008 6:29 PM in response to Daniel Baver

Hi all,
I've made 3 different ones using this site by Samuel M. Goldwasser. Sam being one of the most experience in this field:
http://www.repairfaq.org/sam/captest.htm#ctwtm
"The main filter capacitors in the low voltage power supply should have bleeder resistors to drain their charge relatively quickly - but resistors can fail. Don't depend on them. There is no discharge path for the high voltage stored on the capacitance of the CRT other than the CRT beam current and reverse leakage through the high voltage rectifiers - which is quite small. In the case of old TV sets using vacuum tube HV rectifiers, the leakage was essentially zero. They would hold their charge almost indefinitely."

My experience is that I have actually never seen an anode arc from a Mac. However I had also see none from TVs either until this one Sharp. Kablamo!
So my feeling is to always discharge the CRT if I'm anywhere in the area. There's always that one time - bleed resistor goes open - Anode insulation is faulty..actually it wasn't so much that the HV can kill you (which it can) but mostly you tear the crap out of your hands jerking it out of the chasis (old T.V. guy saw)
On the reasons to discharge it through resistors. (from Sam)
"Reasons to use a resistor and not a screwdriver to discharge capacitors:
1. It will not destroy screwdrivers and capacitor terminals.
2. It will not damage the capacitor (due to the current pulse).
( and also the flyback secondary )
3. It will reduce your spouse's stress level in not having to hear those scary
snaps and crackles."
This if from a typical Apple manual, after you discharge the CRT:
"2. Establish an ongoing ground by using a cable with alligator clips at both ends. Connect one end to the anode aperture, and connect the other end to the metal CRT frame"

Richard

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Answer 2

PT

Level 7

20,904 points

Feb 29, 2008 11:40 AM in response to Daniel Baver

For whatever it is worth, most modern CRT tubes now come with discharge resistors installed across the tube so when turned off, the tube is discharged. Likely the eMac has the same thing. Although if you think you will be near the CRT tube it is best to discharge it anyway just in case.
As to changing the hard drive, two other things to note. One, the hard drive is on the other side and so normally you would have no reason to go near the CRT anyway. And two, the contacts to the CRT tubes are under a rubber cap so you have to make a bit of an effort to get to under it and touch the contacts anyway. If you read the instructions on how to discharge the eMacs tube, you will note it tells you to slide the tool under the rubber cap until your touch the contact.
So in my opinion, which is of course worth about what you paid for this advice (nothing!), there is no danger changing a hard drive because 1) tube is likely self discharged with built in bleeder resistor, 2) Tube contact is far away from the area you are working on, & 3) the contact is normally nicely isolated and covered with a rubber cover.
I did my eMac a couple years back and besides a million little screws to keep track of, it wasn't a big deal.
Cheers,
Patrick

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Answer 3

Bill W

Level 5

4,755 points

Feb 28, 2008 11:04 AM in response to Daniel Baver

Many of us have installed new drives inside our eMacs and I'd be willing to bet most did not have a CRT discharge tool. There's differing opinions regarding exactly how long a CRT will hold a charge, but if you leave the eMac shut down and unplugged for a day or two, most of the charge should leak off. That's not to say you should tempt fate and touch the business end of the CRT, but replacing the drives in an eMac can be done without getting too close to the CRT connector. If you're careful avoiding the back end of the CRT, it can be done with very little hazard potential.

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Answer 4

spudnuty

Level 5

7,128 points

Mar 1, 2008 12:49 PM in response to Daniel Baver

Daniel,

I guess you could say I learned "by the book" using GSX and the Apple Take-Apart Guides. At the jobs I've had since, people do kind of look at me with a raised eyebrow when I tell them that the CRT needs to be discharged if it's going to be exposed.

Right and I've had people tell me "H--- it ain't loaded!"
I think you're just being cautious. Perhaps letting them hold the anode lead while you plug in the eMac and fire it up would teach them a stern lesson. ..
Wait they're clients so I guess you'll need them to be able to sign the check.

Perhaps I'm wrong about which part of the CRT holds the potentially-deadly charge.

It's actually the CRT itself which acts like a giant capacitor. It can actually charge itself up spontaneously. That's why Apple says and I quote myself quoting them again:

""Establish an ongoing ground by using a cable with alligator clips at both ends. Connect one end to the anode aperture, and connect the other end to the metal CRT frame""

That would be right where the flyback anode lead clips into the CRT. That actually reminds me of the lab capacitors in the physics lab that had shorting bars that had to always be in place when the capacitors weren't being used. There was a grad student killed by one of those.

Anyway Daniel, make one of Sam's discharge tools. It's basicly just a big resistor of appropriate resistance on an insulated stick. I used PlasStruct for my first one. Still have it.

Richard
Richard

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Answer 5

spudnuty

Level 5

7,128 points

Feb 28, 2008 7:00 PM in response to spudnuty

Addendum,

I put up a picture of my latest. I made it from an old EICO HV probe. It must have been made for a massive voltage. I couldn't measure the resistor that was in it. It was 3" long!
http://www.photolava.com/view/dypw.html
I feel like Dr Zarkov using it. I need those gloves like he had. Maybe lineman's gloves?
Daniel You could get a pair of lineman's gloves.
http://www.westernsafety.com/lmgloves.htm
I use the lower voltage ones for working in electrical boxes.

Richard

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Answer 6

Király

Level 6

10,035 points

Feb 28, 2008 10:12 AM in response to Daniel Baver

Two flat head screwdrivers connected with a thick insulated wire with alligator clips on each end will do the trick. ZAP!

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Answer 7

Daniel Baver Author

Level 1

1 points

Mar 1, 2008 10:21 AM in response to Daniel Baver

Thanks for everyone's advice...

Don't get me wrong.... I've changed many a eMac hard drive... I have my Apple certifications, and in my old district, we were a self-service provider, so I guess you could say I learned "by the book" using GSX and the Apple Take-Apart Guides. At the jobs I've had since, people do kind of look at me with a raised eyebrow when I tell them that the CRT needs to be discharged if it's going to be exposed. Perhaps I'm wrong about which part of the CRT holds the potentially-deadly charge.

I do find it kind of interesting (and frustrating) that there doesn't seem to be any of those CRT Discharge Tools left in existence... either that or the ones who have them will sooner die than part with them (irony?).

Once again, thanks for the advice... and if anyone does happen to come across one of those tools, and you're willing to part with it, feel free to contact me.

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Answer 8

Daniel Baver Author

Level 1

1 points

Feb 28, 2008 10:22 AM in response to Király

Indeed... though I am not so confident in my skills to attempt an ad hoc solution. Not when the results of doing it wrong could be a stopped heart... I don't like my job that much. 🙂

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