|
I'm having a little trouble reading the schematic because of the resolution and I don't remember everything you've tried.
Have you checked the contrast control? It has four terminals instead of three so it's not as straightforward to check as a normal pot.
For it to affect the brightness the way it does it must be shifting the DC level at the cathode of the CRT. One side of the pot goes towards the plate of the video output tube which is at about 98V and the other towards the screen of the same tube which is at about 155V, if I've read it correctly. It looks like the cathode of the CRT is at about ~130V ? So, the DC level will shift some when you adjust it, but it should be over a limited range. around 130V. If the 4uF electrolytic tied to the screen, pin 8 of the video output tube, were leaky it could cause that level to be off which could cause a larger shift in the DC level at the cathode when you adjust the contrast.
Probably not much help, but just a thought.
John
|