Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!ncar!midway!gargoyle!ddsw1!obdient!vpnet!cgordon From: cgordon@vpnet.chi.il.us (Crash Gordon) Newsgroups: comp.sys.ibm.pc.hardware Subject: Magnetic burn on VGA monitor Message-ID: <27694db9-562.3comp.sys.ibm.pc.hardware-1@vpnet.chi.il.us> Date: 14 Dec 90 22:55:04 GMT References: <33951@netnews.upenn.edu> <13416@encore.Encore.COM> Lines: 36 It's not actually the TUBE that's magnetized. Nor is is the PHOSPHORS. There is a piece in the color CRT called the "shadow mask". (Monochrome CRTs don't have a shadow mask.) The shadow mask is a perforated sheet of foil stretched just in back of the phosphors; the electron beam passes through the shadow mask just before striking the phosphors. What has happened is that the shadow mask has become magnetized, and is deflecting the beam as it passes through. The "neato" effect is caused by a magnetic field deforming the electron beam. This can be a lot of fun to play with, but do it with a cheap TV, not your PC monitor!. Most (if not all) color monitors include a degaussing coil, since CRTs tend to magnetize themselves slightly during normal operation. Since this normal residual field is weak, the degaussing coils are also pretty weak. But they can take out a heavy field if enough cycles are used. The coil is activated through a thermistor, so that the appropriate type of magnetic field is produced. (Degaussing requires an alternating magnetic field with a slow decay.) The thermistor shuts down the degaussing coil as it (the thermistor) heats up. This means that simply cycling the power to the monitor won't force more degaussing -- you must leave the monitor off for a minute or two to allow the thermistor to cool. Probably, your monitor problem will clear up in a few weeks, as the internal degaussing coil does its thing. The bad potential is that if the field which caused this problem was strong enough, it may have actually bent the shadow mask. In such a case it would be time for a new CRT. I doubt that yours is such a case, since an accidentally magnetized ruler probably wouldn't produce a strong enough field. ----------------------------------------------------- Gordon S. Hlavenka cgordon@vpnet.chi.il.us