[drh4683]

Interesting. I'll have to keep checking around. If I turn down the brightness control, the contrast and turn all the screens and background controls fully CCW, the raster is still present, a drab green image is what appears, slightly below average viewing brightness.
I worked on a CTC-19 a couple years ago that had little control of brightness that I couldn't cut off as well. That set turned out to have a completely hacked up B-boost circuit with parts that didn't reflect any schematics. I changed it all back to factory and it worked perfect again.