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How to discharge a CRT safely.

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#1 marchie

  • 6502

Posted 06 April 2012 - 06:37 AM

Hey everyone. I wrote this a decade ago in the old forums and thought it might be appropriate to repost.

(Or at least as safely as possible)

By Marchie

Ok, Pay attention, it's not hard. I will also be outlining my PERSONAL mistake, that resulted in me discharging it across my chest.

~ Metal shank, flat screwdriver (not TOO big), rather long. MUST have heavy plastic or wood handle with NO metal screws or pocket holders.

~ A GROUNDED electric outlet (or a ground rod, pounded into the ground)

~ 4 feet (or as much to get from your outlet to your screwdriver) of 12 guage wire (minimum 12 guage... 10 would be fine to)

~ Alligator clip big enough to fit on one end of the wire

~ banana plug that fits in the ground plug of the aformentioned outlet.


1) strip the wire on each end.

2) attach the alligator clip to one end of the wire, and crimp (solder if you want)

3) Attach the banana plug to the other end of the wire, again, solder if you desire.

4) plug the plug end of the wire into the GROUNDED outlet.

5) clip the aligator clip to the screwdriver.


1) check your outlet. here's where you have to know aabout the electrical system wherever you are doing this. If you have a 3 prong outlet, it can be wired in one of 3 ways:

A) the ground plug is not connected to ANYTHING
B) the ground plug is wired to the negative (cold) wire of the outlet
c) the ground plug is wired to an actual ground.

If your plug is of types A or B STOP NOW. you'll only hurt yourself, hurt the computer, and possibly start a fire.

2) if you have are TRULY grounded, then hook your CRT Discharge tool into the ground plug.

3) Look at the back of the CRT itself. See the Suction cup? under that suction cup are a couple of bare wires/prongs. Touching these will discharge the electricity from the CRt, up the discharge tool, and to Ground.


5) CAREFULLY, slip the screwdriver tip under the edge of the suction cup.

6) slide the screw driver in, until you touch the prongs. You will see a little spark, and maybe hear a litle pop or click (depending on what CRT it is, and how much of a charge there is)

7) remove the tool, and put it down

8) you are done.

That's all you REALLY need to know to discharge a CRT, but there are some OTHER peices of information that are essential to keep you and your Mac safe. Here they are, in easily digestable chunks:

~Just because the CRT has been discharged, don't think that it, and the rest of the inside of your Compact Mac can't kill you, or be killed by you. It isn't so. ("Contrariwise, if it was so, it might be; and if it were so, it would be; but as it isn't, it ain't" -Tweedledee)

~Part of what makes a CRT work, is that inside, is a vacuum. A Rather powerful one too. If you bump it too hard (especially the "neck", or long back end) you can crack the glass, and the entire thing will implode, and spray lead, glass, phosphorus and mercury ALL over the place. Be careful moving around in there, especially when:
~swapping out drives; and
~unplugging that damn power cable to the mother board that just REFUSES to come off unless you haul on it.

If you must remove the little circuit board on the back of the CRT, be gentle, and wiggle back and forth while providing even pressure. Be patient.

The Analog board (the vertical board that has the power supply and video hardware on it) has capacitors on it that still hold a charge, that you CAN'T discharge. Touching them wrong could send some shocks into you, which probably wouldn;t kill you, but may not feel very good. Handle it gently, by the board itself.


1) I first used a FAR too small a gauge of wire, so my first attempt at discharging was incomplete.
2) I used a small pocket screwdriver with a clip, which meant I discharged into My hand...
3) and since My other hand was on the back of a metal chair, I discharged the CRT of a Mac Plus I'd had up and running less than 30 minutes before across my chest


#2 uniserver

  • Banned
  • LocationSouthfield, Mi

Posted 06 April 2012 - 03:29 PM

thanks for taking your time writing this post.

#3 JDW

  • 68000
  • LocationAichi-ken, Japan

Posted 09 April 2012 - 09:14 AM

Some years ago, our very own Dr. Tom Lee gave this simple yet excellent Mac CRT discharge advice:

Power-on your compact Mac. Crank up the brightness all the way. Yank the power cord. Wait two seconds. Now the CRT is discharged and you can pry off the suction cup with a screw driver.

I've tried it. It works well. No tools required.

#4 smrieck511

  • 6502

Posted 14 April 2012 - 03:07 AM

@JDW - Does that work even for the older CRT's and Flyback transformers (Pre-SE) that don't have that bleeder resistor ?

#5 JDW

  • 68000
  • LocationAichi-ken, Japan

Posted 14 April 2012 - 03:12 AM

Yes, of course. It works because it doesn't use the bleeder resistor at all.

#6 pbMacGeek17

  • 6502

Posted 19 May 2012 - 01:54 AM

I'm working on a MAC SE/30 and like your simple idea on discharging the CRT, but I'm a bit nervous trusting the technique since I don't understand how it discharges the CRT (and REALLY don't want to find out the hard way whether it's true). Did Dr. Lee explain why this works?

#7 JDW

  • 68000
  • LocationAichi-ken, Japan

Posted 19 May 2012 - 02:41 AM

It works because you have the juice flowing into the CRT maxed out (full brightness) and when you yank the power cord the CRT desperately is searching for more juice to keep itself alive. Because of that, it grabs all the available juice it can find in the electronics powering the CRT until they run completely dry. They run dry very quickly because the CRT is maxed out in brightness.

If you are dissatisfied with that layman's terms explanation, think of it in terms of a load on a capacitor. There are certain types of capacitors called "memory capacitors" that keep clocks or memory alive (for several days or weeks, until the capacitor runs out of juice) when the main power is cut. These caps can keep the circuit alive for a long time because the circuit attached to those caps sips very little power in the first place. Now compare that exaple with the capacitors in the power supply of a Mac Pro computer. How long do you think those caps can keep the computer running after you yank the power cord on it (however big, fat and physically large they may appear to be to your eyes)? Milliseconds at best. That's beause the computer requires so much more power thank those capacitors can supply. The computer drains all those PSU caps almost instantly when the main power is cut.

Simply put, the CRT when brightness is cranked up all the way draws more power than any residual power in the computer (after the power cord is yanked) can supply. Because the CRT uses up all that residual power in a few short milliseconds, there is nothing left in the video circuitry of the vintage compact Mac to shock you.

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