50 Degaussing (demagnetizing) a CRT:
Description
This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
50 Degaussing (demagnetizing) a CRT:
Degaussing may be required if there are color purity problems with the display. On rare occasions, there may be geometric distortion caused by magnetic fields as well without color problems. The CRT can get magnetized:
* if the monitor is moved or even just rotated.
* if there has been a lightening strike nearby. A friend of mine had a lightening strike near his house which produced all of the effects of the EMP from a nuclear bomb.
* If a permanent magnet was brought near the screen (e.g., kid's magnet or megawatt stereo speakers).
* If some piece of electrical or electronic equipment with unshielded magnetic fields is in the vicinity of the monitor.
Degaussing should be the first thing attempted whenever color purity problems are detected. As noted below, first try the internal degauss circuits of the monitor by power cycling a few times (on for a minute, off for 30 minutes, on for a minute, etc.) If this does not help or does not completely cure the problem, then you can try manually degaussing.
Commercial CRT Degaussers are available from parts distributors like MCM Electronics and consist of a hundred or so turns of magnet wire in a 6-12 inch coil. They include a line cord and momentary switch. You flip on the switch, and bring the coil to within several inches of the screen face. Then you slowly draw the center of the coil toward one edge of the screen and trace the perimeter of the screen face. Then return to the original position of the coil being flat against the center of the screen. Next, slowly decrease the field to zero by backing straight up across the room as you hold the coil. When you are farther than 5 feet away you can release the line switch.
The key word here is ** slow **. Go too fast and you will freeze the instantaneous intensity of the 50/60 Hz AC magnetic field variation into the ferrous components of the CRT and may make the problem worse.
It looks really cool to do this while the CRT is powered. The kids will love the color effects.
Bulk tape erasers, tape head degaussers, open frame transformers, and the "ass-end" of a weller soldering gun can be used as CRT demagnetizers but it just takes a little longer. (Be careful not to scratch the screen face with anything sharp.) It is imperative to have the CRT running when using these wimpier approaches, so that you can see where there are still impurities. Never release the power switch until you're 4 or 5 feet away from the screen or you'll have to start over.
I've never known of anything being damaged by excess manual degaussing though I would recommend keeping really powerful bulk tape erasers turned degaussers a couple of inches from the CRT.
If an AC degaussing coil or substitute is unavailable, I have even done degaussed with a permanent magnet but this is not recommended since it is more likely to make the problem worse than better. However, if the display is unusable as is, then using a small magnet can do no harm. (Don't use a 20 pound speaker or magnetron magnet as you may rip the shadowmask right out of the CRT - well at least distort it beyond repair. What I have in mind is something about as powerful as a refrigerator magnet.)
Keep degaussing fields away from magnetic media. It is a good idea to avoid degaussing in a room with floppies or back-up tapes. When removing media from a room remember to check desk drawers and manuals for stray floppies, too.
It is unlikely that you could actually affect magnetic media but better safe than sorry. Of the devices mentioned above, only a bulk eraser or strong permanent magnet are likely to have any effect - and then only when at extremely close range (direct contact with media container).
All color CRTs include a built-in degaussing coil wrapped around the perimeter of the CRT face. These are activated each time the CRT is powered up cold by a 3 terminal thermister device or other control circuitry. This is why it is often suggested that color purity problems may go away "in a few days". It isn't a matter of time; it's the number of cold power ups that causes it. It takes about 15 minutes of the power being off for each cool down cycle. These built-in coils with thermal control are never as effective as external coils. An exception is the type in the better workstation CRTs that include a manual degauss button.
Note that some manufacturers warn of excess use of these buttons due to their designs (read: inferior) where certain components like the coil or control circuits may overheat. It has nothing to do with excess degaussing - just excess use of their degauss circuitry.
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