I've been blessed!

[Penthode]

So the question remains with the voltages applied to the brightness pot, which end is causing the drift? I would be useful if we could see the voltages either end a the pot upon power on and after 5 minutes or so when the frifting has stopped.

The resistor at the lower end of the brightness pot is still suspect.

[old_tv_nut]

I managed to get the voltmeter on the junction of R601 (470K) and R131 (120 ohms), which should be the same DC as the HO grid, and I measure -26 volts, not -40.

The in circuit resistance of R602 is between 300k and 400k (don't recall the exact number) not surpirsing because of parallel paths.

Now suspecting R165, 1M HO grid resistor, which appears to be under the chassis.

I have to drop this now because I have somewhere to go, but will get back to it