23 Feb
2009
23 Feb
'09
2:57 p.m.
--- 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