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[ The PC Guide | Troubleshooting and Repair Guide | The Troubleshooting Expert | Troubleshooting Specific Components | Troubleshooting Monitors | Image Quality Problems ]

There is an irregular splotch of color on the screen, visible especially when a white background is displayed

Explanation: A stationary part of the screen appears discolored. When a solid white image is displayed on the screen, this part of the screen appears colored, and the color and position stay constant over a period of time. This is more likely to occur at a corner or edge of the screen.

Diagnosis: This problem is caused by magnetization of the metallic elements (in particular, the shadow mask or aperture grill) in the surface of the CRT screen. The magnetic fields cause the electron beams that comprise the picture to be distorted, causing colored areas to appear on the screen. The magnetization normally occurs from interference created by an electromagnetic device, but can also occur if the monitor is shocked or moved suddenly. The screen must be degaussed to get rid of the pattern. Degaussing is the process of demagnetizing the surface of the CRT screen.


  • Remove any magnetic or electromagnetic devices from the proximity of the monitor. Chief culprits are unshielded speakers (shielded speakers designed for use with computers are generally OK), motors and other similar devices.
  • Many monitors automatically degauss the screen each time they are turned on. You can tell if your monitor does this by listening when it is turned on. If the monitor buzzes for a few seconds and then there is an audible click, that is degaussing in action. If the monitor is warm, you can see the screen image warble as the degaussing is performed. If your monitor automatically degausses, then after removing the source of magnetic contamination, you may not need to do anything to remove the colored spot. After turning the monitor on and off many times through normal use, the spot should disappear. This should take a week or two.
  • Some monitors have a manual degaussing button. Try pressing this button occasionally to remove the color problem.

Warning: Do not degauss the monitor many times in rapid succession, as this may cause damage to the degaussing circuit on some CRTs.

  • You may want to use a manual degausser, which is an electromagnetic device you wave over the surface of the CRT to remove the magnetic field. These devices can be purchased or rented but should not be necessary with most modern CRTs. If you use a manual degausser, be very careful to keep it far away from magnetic media, which it can destroy very easily.
  • If you cannot get the color to go away, most decent repair shops should be able to take care of the problem for you for a nominal charge. Some color distortion problems can be more difficult to get rid of than others.

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