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If there is an AGC control try adjusting it. Also look at video anp tubes, their base voltages, and if it uses a SS video detector don't rule out it being weak or bad.
How does your HV compare to the schematic? A bad/marginal yoke could drag down the HV. It is normal for hold to be disturbed by height/linearity adjustments. The collapse to line is likely a bad spot in the pot you were turning...You might want to try cleaning it.
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