The 512K sounds like it has a dry joint. If you don't hear a chime, a constant repeated ejecting sound from the FDD and a white line on the display (or nothing at all) then i'd suspect the solder joints on the analog board before replacing the caps. When I opened my machine up, I saw this in a number of places:



Repairing those resolved the problem. After discharging, inspect the analog board for any suspicious looking solder joints and resolder them. Also go over all the solder joints holding the flyback transformer in place as this is a high tension and high voltage area that has been known to fail, producing some interesting anomalies including the one you've described.

I'll bet that after some small, cheap repairs that 512k will work along with the rest of them.