I like this idea the most, but I swapped the chips and didn't get any change. Could they both be borked? What symptoms should I be looking for if U301 is fried, too?
Regardless, I've ordered some replacements. Always good to have spare 8520s around 
Maybe you'll need to poke around with an oscilloscope and look at the levels on the port.
There's an A2000 schematic here, but unfortunately it's resolution isn't that good.
http://amiga-manuals.xiik.net/schematics/index.phpTry the A500 schematic as I would guess it's probably the same around the ports.
