NotTaR of Television Sets : TV shuts down with dark picture or when ..  
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TV shuts down with dark picture or when changing channels

This may happen at any time or possibly after being on for awhile in which something heats up and drifts out of spec.

The low voltage regulator may be letting the voltage rise excessively. Then, a dark picture or video muting during a channel change triggers the X-ray or power supply overvoltage protection.

Monitor the output of the low voltage power supply B+ to see if it is stable as the brightness/scene changes.