I've been blessed!

[old_tv_nut]

I'm confuseder and confuseder. Not only is there some drift with the bottom of the brightness pot grounded, but either way, the pot center tap does not want to be zero for proper picture, but something higher. The pot range is not -8 to +8 v as calculated in normal configuration, going from a volt or two negative to 11 or so positive.

Out of desperation, I swapped out the AGC tube - no difference. Also went back to the video board with the freeze spray, still nothing seems to be sensitive to cold.

[Alex KL-1]

Sorry if the following was said or asked before, but: the screens voltage also change with brightness change? Also, the potmeter itself perhaps is drifting or the circuit that fusnishes the negative voltage (is derived from...?)

[Penthode]

If the voltages are off on the the brightness pot, there are only a few components to check.

I am suspicious of the 11 volts positive. I would measure the DC voltage on the input of the delay line for if the 2ufd electrolytic is leaky, (C402).

The only other source of a higher than normal DC are the 1st Video Amp cathode resistors R 402 and R403.Try measuring the voltage at the junction of the two resistors and the cathode of the 6AW8 first video amp cathode.

Problem behavior question. I understand the problem is that the CRT is bright when first powered and gets darker over a period of 10 minutes or so? If that is the case, when power cycling, how long must the set be off to when it is repowered the return of the from cold high brightness problem is observed?

[old_coot88]

Then there's the old quick'n'dirty 7 seconds off test for borderline weak tubes. Although not for that specific purpose, the test might provide some clues.
With the set warmed up and in the fault mode, turn it off for 7 seconds, then back on. Does the fault remain once the tubes stabilize, or... ?