The first Apple USB keyboard I have is the one that came bundled with the imac G3. It was bondi blue, had a USB hub with 2 ports on both sides of the keyboard. It also (like the ADB keyboards that came before it) had a power button. I don't quite remember how this keyboard came into my possession, but when I tried to use it, some buttons wouldn't work. Particularly, the ESC key and probably the first few function keys next to it.
Getting this unit open wasn't too hard: just have some patience and remove 50 something phillips head screws. What got my attention was that there are no longer keyswitches under each key. (not in the sense as in my description above anyway). It has been replaced with 3 sheets of what looks like "acetate transparency sheets" we used to use to project presentations. the top and bottom sheets had some circuitry on it, very similar to a flexible PCB. The middle sheet had holes punched on it. By now you'd have figured out the rest. The user would "press" a key and those keys would line up where the holes are. Top sheet contacts with bottom sheet and voila the key is "pressed"
Unlike PCBs, the "circuitry" on the top and bottom sheets seem very fragile. The former owner may have spilled some cola into it (or maybe just water). Parts of this "circuitry" is now a different colour from the rest. Poking my ohmmeter between 2 points that's supposed to conduct shows no connection.
My original solution to this was to take some aluminum foil. carefully cut it to the right size, and then along with some cardboard, get some nearby screws to "press down" on this. This solution did work.... for a while... eventually the cardboard would flatten out and what was a tight fit is now loose... sometimes the keys wouldn't connect anymore. I have opened this unit up again to replace the cardboard.
Now... many months after that, a good friend of mine acquired a powermac G5 and suspected that the power supply may not be good. While working on this unit, he let me use a white Apple USB keyboard (not the aluminum ones)... I was trying to get front row to appear by pressing the keyboard shortcut command-ESC. This did not work. After some more testing, I worked out that the ESC key, along with F1 - F6 are not working. I was ready to try my aluminum foil solution again. Unfortunately for me, the connection was broken over a wide area .. almost 2 inches wide. I could not get the foil and cardboard to be pressed down enough to make a lasting connection. Then I remembered:
Surely I can use this to repair the dissolved circuitry. A trip to the local electronics store snagged me the last unit in stock.
I had another identical keyboard at home waiting to be fixed. This is how the membranes looked like after opening. Note that the fluid has seeped up deep into the keyboard due to capillary action:
After using some baby wipes to clean out the fluid, you can see the damage that was done:
In the above picture, some of the lines are darker. Testing these with my ohmmeter reveal that they no longer conduct. After using the conductive pen on it and being careful not to bridge connections that aren't supposed to be:
An xacto knife can be handy in scraping off accidentally bridged connections. After letting the ink dry (did not take more than 5 minutes), I tested the unit before putting all the screws back. I also took the opportunity to clean the insides of some nasty looking stuff (mostly crumbs and spilled drinks). Keyboard is all working again.