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its best to only do one thing at a time on a working set. That way if something goes wrong its easier to find the problem.
I looks like no signal getting thru the video since the audio is working.
1st is the tube lighting up? sometimes pcb grounds can crack, Need to be gentle when removing tubes. I use a spray lube/contact cleaner on tube before removing just aim around the base and let it set. this way the tube comes out easy and less likely to flex an old pcb.
If it was mine I would start with signal injection at the grid of the video out and work back. OR scope the signal begining at the output of the video diode.
if you lack the equipment for the above you need to do voltage test and see if the video tubes (video amp and video out) have correct voltage.
as to why this would happen, hard to say, most likely a flakey pcb or if you inserted a shorted tube then maybe smoked a plate or screen resistor.
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