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It may be worth injecting composite video at various points between the detector and DC restoration diode (make sure to use a series cap around.1uF 600V to protect your video source). You may have to try s couple to get correct video polarity. If you get good sync injecting composite video of the correct polarity and better amplitude than the tuner/IF are making them the sync and osc system is fine and you need to trace the weak tuner/IF issue...if bad sync with good composite video then the sync separator or sweep osc is an issue.
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