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Make sure the HV is adjusted so that exactly 1 volt drops across the 1K resistor in 6BK4's grid circuit when pix is dark. As brightness increases, the voltage should decrease to about 0.5-0.7 volts. Tab books' RCA color TV service manual volume 1 by Carl Babcoke explains this operation thoroughly.
HV value alone is not a good way to adjust shunt regulators, as it ignores current through the 6BK4 which is a better indicator of correct range of regulation as brightness changes. Visual check by dropping brightness to black screen while watching the cathode glow brighter in the 6BK4/6EL4 tube. That may indicate excess current.
I have found on my CTC20 Rupert that .01 mf (1 kV?) disc cap with the spark gap was leaky and allowed HV to vary so much that focus would not track with brightness. All it takes is one good and random HV snap then that cap is toast, even with a spark gap. Good luck
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"When resistors increase in value, they're worthless"
-Dave G
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