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So you definitely have drive. It is curious that it is giving a more negative grid voltage but it is a less dangerous issue than the grid being less negative.
Horizontal output screen grid(pins 1-7) and boost are important to know. If screen is high it means the flyback is drawing too much current, if it is low it means the flyback is drawing too little current (and cathode emission is being absorbed primarily by the screen grid which is bad for the tube).
When diagnosing horizontal knowing that the grid is not too positive tells you the osc is working, and the screen grid and boost voltages are the more important for troubleshooting.
If you can't see the hiss you could find it by ear...get a few feet of small plastic or rubber tube or hose. Hold one end of the hose to your ear and move the other one around in the HV cage like a stethoscope and listen for the loudest hiss.
Sometimes sets develop HV breakdown in strange places ...a friend brought me a tube color 13" Zenith once that the screw that held the HV rect socket to the insulator cup had arced through the bottom of the cup to the chassis. You couldn't see the arcing but it was loud enough to find. Had to drill out the carbon tracked plastic, fill it with silicone and shorten the screw to repair the issue.
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