Back when I was a poor university student, I came across a bunch of Apple peripherals in a dumpster, this included this Apple Extended Keyboard, a couple of mice and one of their smaller keyboards.

I'd never really spent any time with Apple hardware (my family is firmly IBM PC compatible) so I'd never typed on one, but it felt nice and after opening it up, I concluded that it would be possible to make it a USB keyboard by the simple process of completely re-wiring the matrix to suit a USB keyboard controller and wiring all of that together.

I was technically right, it was possible to do all of that, but it was a heck of a lot more effort than I'd expected and produced a far inferior result than I'd hoped for.

The parts required were pretty trivial:

1. A USB keyboard (I got what was, at the time, an unbelievably cheap one for $7)
2. A USB hub (this was so I could replace the ADB port on the far side with two USB ports)
3. Some USB connectors (I bought the USB-B one and the dual-USB-A one came from a PC case)
4. Wire (some is breadboard wire I purchased for the project, the rest is misc wire from ... well anywhere I could pull it from)
5. USB cabling (all recycled from the $7 keyboard's cord)

And the process was simple, but time consuming:

1. Map out the entire matrix of the USB keyboard (it had a bunch of bonus keys that the hardware recognised but didn't have a physical key, presumably for international layouts)
2. "Apply" that onto the AEK's PCB
3. Wire between the new controller, USB connectors and hub
4. Cut out some of the standoffs and supports in the base so the wires would actually fit
5. Expand the far-side ADB hole so two USB plugs would fit
6. Glue everything down (hot snot FTW)
7. Plug it in and enjoy it

The "application" of the new matrix onto the PCB was, by far, the most complicated and time consuming part.

I did this by doing three major operations: de-populating the board of anything that got in the way (essentially all the controller chips) cutting traces on the board where they didn't fit and wiring between pads to build out new bits of matrix where needed.

It was rather time consuming.

I can't remember exactly how I did it, but I do remember that I couldn't remove the metal plate that holds the keyswitches in position, so I couldn't remove the wire links that allowed some of the traces to jump over other traces, so I had to cut those traces by cutting the copper on the board. I estimate that I made well over 100 cuts to split the nets up into the few bits that corresponded with the matrix of the USB controller.

While I was doing that, I was also wiring between keyswitches to link the various chunks of net together to form the matrix the USB controller was expecting. I think I marked each keyswitch off as I wired it in by putting a black sharpie mark covering the copper next to the pad.

Each net got either a yellow or black wire attached to it which then connected it to the relevant pad on the USB Keyboard controller board. The USB keyboard I bought was a cheap membrane keyboard so it had some black substance on the pads which I had to scrape so I could solder to them.

The black (row?) nets also got a resistor in series with them (harvested from somewhere) to simulate the resistance of the membrane traces. (This was probably unnecessary, but I didn't want to risk it)

Finally I wired the original LEDs up to the LED pads on the controller board, wired in the USB hub, glued everything down closed it up, plugged it in and marvelled at how awesome it was to type on what was one of my first a mechanical keyboards.