The 520 boards, if anything like the 575 boards, are not very complex (at least not as much as that of a IIci). Caps are the only SMT items that would cause a problem but I doubt one would have radically failed in as little time as a month.

Check the loom/board connector. Make sure 520 board really is all the way in.

Take a look at the ADB connector(s) too. It's a stretch but the connector might just be too loose to adequately contact the keyb pins.