I remember you that preregulator circuits are mains power supply connected and that is very dangerous.
Please read safety procedure of Tektronix service manual before atempting further repair !
You absolutely need to use a good isolation transformer.
Now we know that the oscilloscope functions correctly when the -8.6V voltage is correct.
What is the current with 38V DC?
Normally, the voltage of the preregulator is of the order of 41 to 42V and the potentiometer has a greater adjustment range than +/- 100mV.
There is clearly something wrong!
There are two possible options:
- Or seek what is abnormal, but this seems not beeing very easy.
- Either reduce the voltage of pre regulator to 38Vdc, this seems to me easier.
But in the second case, the current will be higher in the pre regulator and there is a significant risk of overheating, and more particularly overheating of T933 (120-1439-00) that can become defective (isolation failure, short circuit between turns )
I always strongly recommend to install a fan following the diagram of 2236.
I did it on my 2235 to prevent failure of the pre regulator.
To reduce output voltage of the pre regulator, I would try to connect a 470K resistor in // with R922 (86.6K in your diagram).
If output voltage is lower, (as I think it would be), than measure this voltage and report the result here.