[TV MAN]

As Mr. Fix says, this is a live chassis so without an isolation transformer, it can be dangerous to work on. There's good and bad to this circuit and problem. From your description, the problem most likely is in the video IF chain, low/missing voltage, bad tube. Do you have any tubes to substitute? That 5AM8 detector could be dead, which might have retired the set. You could be missing or have a low voltage to the tuner or a bad tube. They do go bad in tuners. Did you change any caps or resistors in the video IF section? I really can't recommend doing more due to some safety issues. The isolation transformer mentioned simply separates your set from the ac power line which would help eliminate electrical shocks (or worse) & fried test equipment.