Brightness/intensity and other CRT voltages are usually derived from a simple potential divider, IE a load of resistors and pots in series with a positive voltage at one end, and ground or a negative voltage at tother end.
So the brightness pot should adjust the cathode - grid voltage, grid should be at - something volts in relation to the cathode, adjusting the pot you should get - 20/30v or whatever to - 5v say, something like that.
To fault find see if you can find a pinout for your CRT, find the cathode pin, black probe of DMM on that, red probe on the grid.
Here's some bumf to give you an idea -
https://www.robkalmeijer.nl/techniek...e45/index.html
Andy.