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NotTaR of Television Sets : Width and height change with warmup
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Since both width and height are affected, this points to something common
like the low voltage power supply. If there are any indications of hum
bars, first check the main filter capacitor(s) or substitute a known
good one. There might even be other symptoms like faint retrace lines
on at least part of the screen.
Start by monitoring the B+ to the flyback (feeding the HOT) to see if this
drifts at all. If it does, then there is probably a low voltage regulator
problem - bad capacitor, resistor, or chip. Use freeze spray to narrow
it down. If this is solid, then there could be a high voltage drift but
this would be somewhat unusual without other symptoms (like arcing) since
the HV is nearly always tracks the low voltage supply.