Sound like caps on the crt. Different set up but, I just had 1 go out on me that was doing the same thing.
Looks like the guy who sold it to you either didn't know or sold it just in time(for him!).
Easy enough to fix if that's the problem. I'm assuming that the computer(jamma board?) is still running just with no picture.
Pessimist: Oh, this can't get any worse!
Optimist: Yes, it can!