This may be caused by a faulty or a badly soldered low voltage regulator.
You need to find a test point for the low voltage named "B+" and check if it is stable when the brightness goes down (dark commercial, channel change, brightness change).
If the "B+" voltage is not constant, this means that the part that regulates it (a regulator IC or associated components) is probably faulty or may be loosely soldered to the circuit board (loose solder joints may often cause such problems since the loose contact on the pins of a component becomes even worse when a component heats up.
It is not a bad thing to have a detailed schematic of the set - it will save you time when you search for the test points and components which need to be inspected.
regards
Triarcuate
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