The flyback/LOPT transformer is known for failing on these and can result in a dark screen. I had to replace the one in my own Plus around 15 years ago.

You could also check to make sure the filament in the neck of the tube is glowing, it's not very bright but should be clearly visible in a dark room. You could scope the video signal pin on the connector to the analog board, a logic probe or multimeter will work in a pinch to tell you if there's something there.