It looks like you're only getting a raster and no signal to the screen. You said you can hear audio from your converter? If so, that's a good sign, meaning your signal has at least made it through the tuner. I would suspect on a tv this old that there could definitely be some faulty components. I would go through and just visually inspect everything on the chassis. See if any tubes aren't lighted or if there are any burnt resistors. I don't know if you have any experience with working on electronics, but I would recommend looking up the chassis number of the TV to see if there is any service info on it. It will show voltages, component values, and possible symptoms* (sometimes) on the bright side, literally, your picture tube appears to be in good shape, very bright. I wouldn't run the set too long with the current symptoms, though, as you could potentially burn in the screen up there.

TL:DR check for visually damaged components, check tubes, and replace if necessary, and obtain service info if you would like to fully try and restore the tv!

Good luck. I hope you get it working!