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From: eric@apollo.uucp (Eric Peters)
Newsgroups: net.video
Subject: Re: Magnetic fields and Color TV Warning/Questions
Message-ID: <2baaf68e.39c@apollo.uucp>
Date: Fri, 31-Jan-86 15:06:29 EST
Article-I.D.: apollo.2baaf68e.39c
Posted: Fri Jan 31 15:06:29 1986
Date-Received: Sat, 1-Feb-86 20:44:50 EST
References: <499@oliven.UUCP>
Organization: Apollo Computer, Chelmsford, Mass.
Lines: 34

Automatic degaussing has been around so long that folks have forgotten
(or never knew) that the shadow mask of a standard color TV becomes
permanently magnetized from stray magnetic fields, and it needs to be
"degaussed" occasionally.  The shadow mask is a thin sheet of steel
with a quarter million tiny holes in it right behind the TV screen.
Its purpose is to assure that only one electron beam can shine on each
colored spot on the screen.  How it does this is another story.  Even
the magnetic field of the earth will eventually magnetize the shadow
mask, especially if the set is moved around a lot.  It used to require
a service call to demagnetize it.  But in all modern sets there is a
"degaussing" coil around the face of the tube, and it is controlled by
a thermistor.  It's arranged so that when you turn the set on and the
thermistor is cold, it operates the degausser.  When the thermistor
warms up after a few seconds, it turns off.

When you put speakers, or any magnetic objects, near the TV screen, their
permanent magnets magnetize a portion of the shadow mask.  When it becomes
magnetized, the electron beams are bent as they pass through it, and thus
the beams don't get to their correct colored phosphors.  All sorts of
"wild" effects can happen to the color -- You saw one example.

Note:  A strong magnetic field can physically deform the shadow mask perm-
anently.  It is very thin and supported at the edges.  If it should become
bent, it cannot be repaired.  So don't fool around with strong magnets near
the screen of any color TV you care about.

Anyway, once you let the set cool off (to cool the thermistor), it will
automatically degauss the next time you turn it on.  Sometimes it can take
two or three cycles to completely clear the color.  You may be able to find
a serviceman's degaussing coil, which is much more powerful than the one
in the set, and do a very thorough job.  But the operation is a bit tricky.
You can actually end up magnetizing the shadow mask worse than it was!

Eric Peters  (...decvax!wanginst!apollo!eric)