Degaussing Color Monitors
                                    by
                               John Bachman


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     Rainbows have their place.  In the sky is great; right above a pot of
gold is even better, but a monitor screen is a terrible place for a rainbow. 
Monitor rainbows can be impossible to remove unless the technician knows
proper degaussing procedures.  

     Occasionally, a color monitor develops a discoloration or color purity
problem on part or all of the screen.  Typically, the discoloration follows a
semicircular pattern around one side or corner with several bands of different
color distortions, like a rainbow.  Sometimes the discoloration involves the
entire screen, but that is rare.

     This rainbow may be caused by an externally induced magnetic field that
has permanently magnetized some material in the monitor.  The three CRT
electron beams are guided to their appropriate phosphor dots by a magnetic
deflection system.  The beams pass through a shadow mask near the phosphor
surface assuring that the red beam hits the red phosphor, the blue beam hits
the blue phosphor, and the green beam hits the green phosphor.  If some nearby
component (frequently it is the shadow mask itself) has become sufficiently
magnetized then the beams receive an undesired deflection, and will not land
on the appropriate phosphor, or will land partly on one color and partly on
another.  The result is an impure color that arcs around the magnetized
component - a rainbow. 

     Motion is an important clue to this problem.  If the rainbow moves it is
not being caused by a permanent magnetization but by some alternating magnetic
interference in or near the monitor.  If the rainbow does not move then it may
be caused by permanent magnetization and degaussing is necessary.  Moving the
monitor to another location will determine if the interference source is
internal or external.  Does the rainbow disappear when the monitor is moved? 
If so, the magnetic interference is coming from something near the monitor's
normal location.  If the rainbow travels with the monitor the problem is in
the monitor and degaussing may be required.

     Degaussing removes permanent magnetization by introducing an alternating
magnetic field that is stronger than the offending permanent magnetization. 
This field will energize the magnetic domains of the material and induce an
alternating magnetic field.  Then, if the amplitude of the the alternating
magnetic field is gradually reduced to zero, the material will be
demagnetized.

     The easist way to degauss a monitor is often to let the monitor do it. 
All modern color monitors have built in degaussing coils and circuits.  Take a
close look the next time you open one.  There will be a coil of wire wrapped
in tape or other insulation surrounding the CRT faceplate.  Usually, it is
coiled around the CRT behind the mounting ears.  That is the internal
degaussing coil.  It is a coil of wire connected to the ac line through a
thermistor current limiting circuit.  The thermistor has a low resistance when
cold and a higher resistance when warm (typically a 10:1 ratio).  It is in
series with the degaussing coil so that when started cold a large current will
flow through the coil and then will decrease to a low value.  The internal
degaussing coil thus automatically degausses the monitor every time it is
turned on.  This degaussing takes place while the monitor screen is blank so
that the resulting discoloration during it is not visible.

     Unfortunately, design limitations reduce the magnetic field strength
available from internal degaussing coils.  That limits the amount of permanent
magnetization that can be neutralized by the internal degaussing.  If a
monitor has been strongly magnetized internal degaussing may not be enough and
a rainbow results.

     Manual degaussing requires a degaussing coil.  You may have to search a
bit to find one, but they are available.  One source is Apollo Wholesale in
Irving, Texas, 800-488-2782.   Many homemade coils exist, but be careful with
them.  There are line voltages involved; isolation, insulation ratings,
spacings and fusing must be considered in any degaussing coil design. 
Manufactured degaussing coils cost less than $50.  Why take a chance?

     The basic principal involved in operating a manual degaussing coil is
the same:  introduce a strong alternating magnetic field and then slowly
reduce it's amplitude to zero.  This is accomplished by holding the degaussing
coil near the monitor (it can be on or off) flip the degaussing coil switch to
on and slowly move the coil away from the monitor as smoothly as you can. 
When the coil is at arms length from the monitor flip the switch off.

     When you hold the degaussing coil close to the monitor and turn it on
you have introduced a strong magnetic field.  Then, as you move the coil away
the magnitude of the field at the monitor decreases.  When you have the coil
at arms length from the monitor there is no perceptible magnetic field at the
monitor and you turn the coil off.  The rainbow is back in the sky where it
belongs.