There is another U800 replacement developed by "Dr. Ram", built on Lafay's work and has several improvements. He calls it the RU800. Search for it on the TekScopes group on groups.io.
But it's very unlikely it's U800 and it would be unwise to mess with it as a first troubleshooting step. U800 is the horizontal output driver and the screen shot from the OP clearly has an issue on both axes.
Really... Start with the simplest possible causes before diving deep. The screen shot is what it looks like when the beam finder is pressed. Check the beam finder button and associated circuitry.
If you press it and nothing happens at all, it's stuck on. First thing to check is if the button is mechanically stuck or broken internally.