Hiya LG--
I had a scope from the '70s that had a similar problem. There was just a big green blob on the screen. While some photos of your scope would be very helpful, I can tell you that the problem I had with mine was a blown (and I mean BLOWN) capacitor on the timebase board. The entire area was corroded, but after cleaning it up, laying new traces, and replacing the capacitor it worked good as new. Some images of the front end of the scope (the screen) would be helpful, as would be photos of the inside. Do you know approximately what decade it's from? That would help us determine whether it might be (for example) a tube, a capacitor, or an IC.