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Have you replaced the electrolytic and paper dielectric capacitors?...That is a must on tube gear as the originals have failed or will fail and when they do they can take out other parts that are much MUCH harder to find.
If it has been recaped do you have a proper analog RF NTSC signal source and have you confirmed it works on a newer (80s to present) working TV?
If those things are good have you cleaned the contacts in the tuner or tested tubes in the tuner IF and video stages? If that is good wiggle tubes in the order of video output, video amp (if there is one between the detector and output), highest number video IF down to lowest, then tuner....when you wiggle (you may need to pull from socket and reinsert) a tube you should get disturbance to what sounds like a blank raster. The area between the last tube to cause disturbance and the first one not to is the trouble spot.
Another possibility is if the AGC pot is set too low....too low and you get no signal as you increase you get more signal till the sync separator overloads and sync is lost audio buzz and negative picture sometimes accompany that. Correct AGC adjustment is turn up till it starts to overload then back down just a bit below the overload point...If you touch the AGC, mark it's current position first so you can go back to it if needed.
I can expand on any of these topics if you want/need.
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