servicing terminals/monitors

Mr Ian Primus ian_primus at yahoo.com
Mon Feb 23 08:57:16 CST 2009


--- On Mon, 2/23/09, John Floren <slawmaster at gmail.com> wrote:
 
> How
> long does it take an unplugged CRT to discharge? This
> hasn't been
> plugged in for over a month, so I figure it's probably
> safe right now,
> but if I test it and then want to check something else, how
> long do I
> need to leave it sit? Is there a safe way to discharge a
> CRT when
> you're working on it in the living room? 

If it's been sitting for a month, you're probably good. I've never seen one hold a charge that long.

In all honesty the hype and the fear of working on monitors is especially overrated. Yeah, there's thousands of volts in there. If you get bitten by it, it'll hurt like hell. But it's not going to kill you. And monochrome terminals use a lower high voltage than the average color television.

To safely discharge the tube, get a flat blade screwdriver, and a cliplead. Clip one end of the cliplead onto the metal shaft of the screwdriver, and the other end onto the CHASSIS ground of the monitor. Basically, you want to clip it to the metal mounting band around the picture tube. Hold the screwdriver by the plastic handle and carefully slide it under the suction cup until you hit the metal clip. If it's still got a charge, you'll hear a nice ZAP! If you do get a zap, pull the screwdriver out, then put it back in and do it again. This time, you should hear nothing - but probably a small click. The first time doesn't always get it all.

After that, you should be safe - at least from the picture tube. Other large caps in the terminal (and in the linear power supply of an ADM3A) might still be able to bite - discharge those by shorting them with a screwdriver.

There is nothing magical about a picture tube that makes it dangerous - it's acting as a capacitor. The inside conductive coating of the tube forms one plate, the glass forms the insulator, and the conductive coating on the outside (aquadag) forms the other plate. You're shorting this capacitor out with the screwdriver, discharging it's stored charge.

It's a good idea to perform the discharge ritual any time you go to disconnect that anode cap. Even if the thing has been sitting for a month, it doesn't hurt to go through the motions. Make it a habit, and you won't get bitten. And even when you do make it a habit, sooner or later, you'll forget, and pull the cap off a charged tube. It'll zap you, and it'll hurt. Your arm will feel tingly for a couple minutes. Then you'll move on with your life, and you probably won't forget again :)

-Ian



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