2.7 Degaussing
The effect of stray magnetic fields on the color purity of an ITC depends
on the orientation of these fields with respect to the ITC. Color
displays have a magnetic shielding system, consisting of a magnetic
screen, internal to the ITC, and the shadow mask together with its
mounting frame, to combat stray magnetic fields.
To be effective, the shielding system must be degaussed by applying a
strong alternating magnetic field which decays gradually and symmetrically
to zero.
This is achieved by using dual, positive temperature coefficient resistors
(PTC) in conjunction with a degauss coil on the ITC. When the mains input
voltage is applied to the display, the peak inrush current through the
coil is limited by one half of the dual PTC, while the other half is
heated by the current flow in it. Equilibrium is achieved when the heat
in one half of the PTC is sufficient to prevent current flow in the other
half, and consequently in the degauss coil.
When degaussing is required, for example, if a display is relocated, the
heating associated with a degauss action means that another degauss action
cannot be started until the circuit has cooled down.
Allow at least 30 minutes with the display powered off, before starting
another degauss action.
IBM PS/2 8511 and 8518 Color Displays HMS
Degaussing
¦ Copyright IBM Corp. 1992.
2.7 - 1
