[DaveWM]

this is with just a raster, no signal correct?
I would scope the video lead to the CRT cathode/Grid look for a horz rate blanking signal.

next I would look a the boost voltage filter cap, make sure you have the right value and its not open, scope the boost voltage (use a 10x prob), see if its clean at the screen grid of the CRT. I don't have that model but my predicta show boost 400v at the CRT screen grid, and uses reduced boost voltage to control the brightness, if you don't have well filtered boost there perhaps that could be an issue.

something is blanking a the horz rate, it has to be the grid/cathode/screen of the CRT. should be easy to locate if you look there.