Path: utzoo!censor!geac!torsqnt!lethe!yunexus!ists!helios.physics.utoronto.ca!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!spool2.mu.edu!sdd.hp.com!news.cs.indiana.edu!bronze!commgrp@silver.ucs.indiana.edu From: commgrp@silver.ucs.indiana.edu (BACS Data Communications Group) Newsgroups: comp.sys.ibm.ps2.hardware Subject: Re: IBM 8513 monitor problems -- all 20 of them Message-ID: <1991Jan14.140609.9675@bronze.ucs.indiana.edu> Date: 14 Jan 91 14:06:09 GMT Sender: daemon@bronze.ucs.indiana.edu (Mr Background) Organization: Indiana University Lines: 51 huff@kuhub.cc.ukans.edu (Steve Huff, University of Kansas, Lawrence) writes: >> I fixed my blotches by rubbing a refrigerator magnet around the screen. >> Does this cause any harm to the monitor? >It shouldn't... that's basically a mini bulk eraser. >One other note: IBM told us (after replacing 22 of our IBM 8513 monitors) >the blotches would disappear after use. We're not holding our breath. Blotches are caused by permanent magnetization of the shadow mask (sheet of metal with holes in it, just behind the screen). The 8513 and all other modern color monitors and TVs have "de-gaussing" coils built in, which operate momentarily when the power is turned on. It's a coil of wire wrapped around the forward part of the tube, connected to AC power through a thermistor which increases resistance when it gets warm and limits the current. (Monitors of DEC VT-240 and some others use a relay instead of a thermistor.) A _very_ common failure in IBM 8512 and 8513 monitors is a shorted de- gaussing thermistor, which causes the internal fuse to blow. The device is a rectangular black plastic box with three terminals, and is located a few inches behind the green LED pilot light, near the de- gaussing coil connector. The monitor will work without the de- gausser, but may develop blotches of bad color. Check to see if the IBM replacements have had de-gauss thermistors removed. In that case, use a handheld de-gaussing coil (available at electronics stores). The proper procedure is to place the monitor in the position in which it will be used, turn the monitor OFF, move the coil in circular motion around the screen (plane of coil parallel to screen), then draw it away from the screen _before_ turning it off. De-magnetizing requires an AC magnetic field which is gradually decreased. Using a permanent magnet to de-gauss a color CRT will make the problem worse. Unconfirmed report: The de-gaussing thermistor in IBM 8512, 8513 and 3192 monitors is identical to the one in NEC 1401 monitors. It is NEC part number 38112026, "positive thermistor degaussing control," $2.50 from: NEC Home Electronics 1255 Michael Dr. Wood Dale IL 60191-1094. BTW, monochrome CRTs require no de-gaussing because they have no shadow masks. -- Frank Reid reid@ucs.indiana.edu