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I just looked at the schematic. The brightness control varies the voltage at the control grid of the CRT, not the cathode as in many other sets. As Notimetolooz said, check the brightness circuit. If the 330K resistor going to ground is open or grossly increased in value, that could make the brightness very bright.
One question to ask. Can you adjust the brightness so that the screen goes dark?
It also looks like the path between the plate of the video output tube to the CRT is DC coupled. Check that circuit as well as it could be throwing the DC bias off at the cathode of the CRT. There are also a lot of inductors in that circuit. A simple resistance check could find out if one of them has opened up.
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