This means that the size of the picture is not constant from top to bottom
(width changes) or left to right (height changes). Note that some slight
amount of keystoning is probably just within the manufacturing tolerance of
the deflection yoke and factory setup (geometry magnet placement, if any). On
a TV, this is only noticeable with scenes having straight edges (e.g., video
games) in relationship to the CRT bezel.
|NotTaR of Television Sets : Keystone shaped picture
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However, a sudden increase (and it will usually be rather substantial in a
TV) may indicate a problem with the deflection yoke.
An open or short in a winding (or any associated components mounted on the yoke
assembly) will result in the beam being deflected less strongly on the side
where that winding is located. Typical TV yokes have fewer individual windings
in parallel than high scan rate monitors so the effects of one such fault are
more dramatic. See the section: Deflection yoke
If the set has been dropped off a 20 story building, the yoke may have shifted
its position on the neck, of the CRT resulting in all sorts of geometry and
convergence problems (at the very least).