This is a good place for an analyst like a Sencore VA62 etc.
You can try lifting one end of the video detector diode & driving it with
your video source (the RCA plug). Remove the 3rd IF also ( 6EJ7 IIRC ). One volt P-P is pretty much standard. This will divide the chain in half & tell you
whether to look to the left or right of the diode. Divide & conquer
BTW analysts are almost dirt cheap now. The sencore will do a LOT
more than most. If you get one be SURE it works, has all the cables,
& both the quick set up guide and full manual.
73 Zeno
LFOD !