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Check for a video signal at pin 7 of the video detector tube 6AL5 with your scope while feeding a RF signal into the tuner adjusted to that station.
If you find it there check for it at the testpoint on the grid of the video amp 12BH7 pin 2.
If you find it there look for it on the CRT cathode pin 11.
I assume you tried varying the contrast control.
If you can't find a video signal at the video detector then try injecting a signal using a DC blocking cap (0.1 uf) of a 1-3Vpp signal at the plate (pin 7) of the video detector tube. This signal could be from a audio generator. ( I think someone suggested that before.) You should see something other than a blank screen.
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