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Since brightness is the issue, I would focus on the brightness circuit.
you tried a new video out tube (weak would reduce brightness)
so next look at the brightness control which sets the bias of the video out tube.
this is done via a voltage divider of 3.3meg and 330k with a source voltage of 135v
The next logical things to check would be:
135 source (stacked B+ supplied by the audio out tube acting as a voltage divider). that's a little low but prob not enough to cause a problem. A sub of the audio out tube may increase the 135v source.
The 3.3meg/330k voltage divider is where I would look. Since its unlikely the 330k has dropped (which would need to be the case for it to cause the low brightness) I would test the 3.3 meg (OUT OF CIRCUIT). If it has drifted high significantly that would reduce the voltage available to the high side of the brightness circuit.
The trouble with going strictly with the sams for voltages is there are a lot of variables so they can only be a guide. After that you need to understand how things work and use reasoning to try and narrow down the problem. Like if brightness is low look in the brightness circuit.
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