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It sounds like possibly excessive beam current in the CRT is dragging the HV down. This can happen if the G1s in the CRT go excessively positive. Could you post the part of the schematic showing chroma outputs, vid output and CRT? Does the 3AT2 get real hot when this happens?
You may already be familiar with a problem chronic to RCAs of the CTC-15, 16, 17, and 25 lineage. Two ground lugs on the far right end of the chroma board go open, killing the heaters of the 6GU7s, causing their plate voltage to go sky high, which is direct-coupled to the CRT G1s. Beam current skyrockets, dragging the HV down, causing a symptom like you describe in your Zenith.
Or maybe it's something else altogether.
Bill(oc)
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