[old_tv_nut]

Update: No joy yet.
I started chasing the brightness drift this afternoon The suspect resistors measure OK cold, and after the brightness has drifted, freeze spray shows no effect on any components on the video board.

Cathode voltages drift by 10-15 volts, and adjusting the brightness control for the same variation shows the same brightness variation as the drift.

I noted that the far end of the brightness control goes to a negative grid bias voltage developed by the horizontal output tube, but changing the HO tube and/or the H driver made no difference.

I backed up and measured the 385 V supply, and it is low (as low as 330) and varies with beam current. If I understand the HV regulator properly, it should keep a constant load on the HV with brightness changes and therefore the 385 supply should not vary drastically. So, the regulator may be wonky or just acting wonky because the B+ is low.

I think my next move is to replace the 5U4's in the power supply, but I don't have any at the moment.