I recently bought an almost working Amiga 500. When I got the unit in, I found that it mostly worked, the floppy drive spindle needed greasing, and the ram expansion just needed re-seating... but the "Left Amiga" key was broken off, and both the "Return" and "Enter" key did not work.

I just disassembled the keyboard and put a new plunger in for the Amiga key, and I cleaned some dust from the key switch side of the membrane. I saw no outward sign of a problem with the enter keys plungers, or contacts. But I did see some evidence that at some point something a little sticky was spilled into the keyboard, there being a residue on the metal plate under the numeric keypad side of the keyboard, and when I lifted the membrane that side was a little sticky coming off.

I looked at the circuit board under magnification and don't see any outward signs of something burned or shorted. The fingers on the membrane that go into the connector on the board look,.. uniform.. black, but I assume that's the same material as the contacts for the keys.. there doesn't seem to be any damage to the fingers.

As I've found with other vintage computers, I don't assume I'm the first person to take this keyboard apart, so it may be theres a broken trace somewhere on the membrane..

Update: I used the Amiga Test Kit and found that all keys are working except "Enter" and "Return". I also measured voltage at their respective Row pins on the controller chip. When nothing pressed they read zero volts when the enter or return key are pressed their respective y row pin goes to 5V.

What might the problem be and how should I debug it? I can put a scope or analyser on it if I need to.