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Have you checked the tube sockets? Predictas are prone to having internal breaks in the metal parts of the socket. If sync is fed to the vertical grid but the socket grid is open then you'll have no sync to the tube.
Another thing that's worth while is to check the sync waveforms with an oscilloscope....If the sync separator is not opperating properly then there may not be valid signal for the Osc to lock to.
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