I'll be damned if you weren't right about C28, the coupling cap. I had that screwed up
. Pin 1 and 2 now give a change in raster when I inject there.
The picture below shows what the three vertical line pattern looks like on the screen.
You can see where the three lines are drawn on the screen, although they are distorted. Is there adjustments on the set that I can use to try and dial the picture in? The video is being injected just aft of the disconnected M4 btw.