I had a set like that, it had been recapped before I got it. turned out one of the coupling caps was attached to the wrong term on a term strip that fed the video amp tube. Like the ohters have said I would go with a video injection to isolate the loss of the signal. I use a BK 1076 video analyst.
here is a video of the defect I found.
https://www.youtube.com/watch?v=qkJ3...CVtyRrqXVtll-g