What would cause an SE/30 CRT to gradually fade in and out?

[olePigeon]

I have an analog board that gradually brightens and darkens a CRT.  It's cyclical and very regular.

Any idea what might be causing it?

 

[TheWhiteFalcon]

Some kind of filter issue? Oscillations in the power not being blocked?

 

[uniserver]

i would probably hit the caps int he AB and PSU first...    also i seen strange things when UE8 gets all goo'd up.

 

[techknight]

Did you swap the CRT by chance? H-K shorts developing in the tube itself can do this. During the cycle, tap on the electron gun and see if it changes.