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[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?