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Its always easy to measure the screen, if its high the tube is overloaded, if its low its lack of drive.
I would agree the ring test can be pointless unless the primary is shorted. I took apart several rca ones to repair and rewax them, they all had the same failure, the hv winding was arcing from the top of the doughnut to the bottom with a big carbon trace from one end to the other.
Zeniths seem to fail less because they are potted in that hard epoxy.
From what you are saying I would try changing it.
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