CRT monitors, while not as widely used today, were once used on nearly every computer system. The bulky cathode ray tube screen was used for computer and video monitors, but has since been replaced by newer technology. The CRT basically operates by shinning electron beams on strips of phosphor dots. A shadow mask controls the beams striking dots, creating a mix of the three primary colors, which in turn forms the image on the screen. A strong magnetic field will disrupt the beams traveling through the shadow mask, creating odd colors and distortions. If this occurs the screen will need to be degaussed.
Locate the degauss button on the monitor or TV screen. Newer CRT monitors and TVs have a degaussing coil built in. The coil is a powerful electromagnet wrapped around the edge of the screen that is used to "reset" the local magnetic field to normal. Some TVs or monitors have a button or option in the menu that allows the screen to be degaussed manually. The icon for the degaussing coil is often in the shape of a horseshoe magnet. Check your documentation for the exact location and appearance of the screen's degaussing coil.
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Turn the screen off and let the unit cool down if there is no manual degaussing option. Units with a degaussing coil will always degauss the screen when the unit is turned on. This works most efficiently when the unit is starting from cold. After the unit has cooled, which may take from 15 minutes to over an hour, depending on the device, turn it back on. You may hear a loud click, pop or thunk and the image will wobble or shake. This process may need to be repeated a few times to clear the problem.
Use a hand-held degaussing coil if the unit is not equipped with a degaussing coil, or if the coil is not powerful enough. To use the coil, turn it on and hold it a few inches from the center of the screen. Drag the center of the coil to the edge of the screen, then use it to trace a full path around the border. After completing a full circuit, return the coil back to its original position and slowly pull it directly back away from the screen. Wait until you are at least five feet away before cutting power to the coil.
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Move slowly when using the hand-held degaussing coil. Do not use a degaussing coil on the back or inside of a CRT device because more damage can occur. A small magnet can sometimes be used to repair the screen by holding it just close enough that it starts to affect the image, then moving the magnet in widening circles. A magnet can damage a screen even more, however, so it should only be used if there are no other options.