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The area you may need to look at more carefully is the video amp stage. Is there a video signal at the output (plate) of that tube....? If so then trace along the network between there and where the signal reaches the cathode of the picture tube. Possible culprits would be any defective component in the network of peaking coils, capacitors, resistors and whatever.
Also--heaven forbid--is the possibility of a heater-to-cathode short in the picture tube itself. Although in a series-wired set this would likely lead to a raster full of horizontal hum bars....
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