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Hi
I think the fault has been pretty well covered. The old shadow mask CRT picture tubes had a very high precision etched (I think) mask to direct the three beams onto the correct phosphors.
This mask picks up a residual magnetic field from the earth's magnetic field after it has been left in one position for any length of time. If the set is moved the new field causes a slight deflection of the beams so that they don't quite impinge on the correct phosphors.
The correction for this is a multi-turn "deflection yoke" placed at the shadow mask. Each time the set is turned on a "pulse" of high intensity AC is connected to this yoke to "degauss" the mask. However if the set has been moved after a long period in one position it may take four or five re-starts to fix the problem. This degaussig or re-magnetising from the earth's magnetic field can also occur naturally if the set is left standing in its new site for three or four days.
Another point about the older shadow mask CRT's is colour rendition. The emition phoshors emit a much truer colour than the filter type LCD's. I don't know the parameters of the plasma.
Barry
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