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The show on the screen is Yellow Submarine.
I have added a photo of the switch for the screen grid voltage. In normal light (picture A) one can not see the grime trace. But with flashlight (picture B) it is visible. It is only a small trace. But it acts like a Zener diode. With a usual Ohm meter, all things seem to be OK. But when some hundred volts are present, the trace become electroconductive. Below a hundred volts the trace acts like an insulator. So the trace keeps the voltage of around 100 volts, equal whether there are 200 volts or 700 volts at the switch.
Eckhard
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