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Does the bar stay fixed in the middle of the picture, or does it roll very slowly up the screen?
A rolling black bar is caused by 60 Hz hum getting into the signal path somewhere. The roll is caused by the slight frequency difference between the 60Hz line frequency and the 59.94 Hz vertical scan rate. Look for a bad cap or H-K leakage in an IF or RF stage ahead of the video detector.
A bar that stays fixed across the screen while viewing an NTSC signal would indicate a vertical sweep frequency signal getting into the video somehow. Look for a bad filter or bypass cap on a supply that is common to both the vertical sweep and the RF/IF stages.
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