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Thats sounds like the high voltage is dropping on bright scenes. Does the picture bloom? like expand all the way around?
You really need to do check the high voltage. The other thing you can do is measure the voltage drop accross the 1k resistor in the cathode of the hv regulator tube. It should be around 1 volt on a dim scene and drop on bright. (900ua drop accross 1k = .9 volt) If its dropping to zero you dont have enough high voltage headroom and theres another problem.
This is kind of explained in the sams under horizontal sweep adjustments.
It could be something like a weak horizontal output tube, weak hv rectifier, low b+, horizontal blanking issue, something in general causing low output of the flyback.
That shut tube needs to be always be conducting some current to maintain a steady high voltage and focus voltage.
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