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Because the voltage at the cathode of the 5U4 (pin 8) is high, I see one of two possible scenarios: either the resistive divider chain is open (R193A, R194 R210//R177, R193B and R193C) or else there is a short on the negative end of the chain (-120v to ground).
I suspect you may find an open resistor chain (bad connection or bad resistor). I have seen R193A open on a couple of these chassis. It is the big fella mounted on the rear chassis apron. In each case, I successfully replaced the one bad section with a big wirewound (ceramic tube type) mounted high up on the HV compartment cage on a home made bracket.
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