Troubleshooting NEC stereo monitor

[ESigma25]

I somehow lucked into a NEC CM-1915A, a composite stereo monitor from 1985, but it's got some weird problems I don't really understand? I am getting voltage to the tube it seems, it blinks when powered on and off, but I get absolutely no picture whether I have a video input plugged in or not, and no sound either.

Any ideas of what I should check? I've never had a CRT give me no picture at all before.