Thanks [Jenny List] for the writeup.

Reading through the comments I'd love to acknowledge any other people who had made insane contraptions like this. If you end up writing them up, please let me know I'd love to read them and I'll link to them from here.

To respond to the people writing things like "I'd use product X instead of going to all this effort", the answer at the time I constructed this was simple: cost.

I had pulled this awesome keyboard out of a dumpster and said to myself "man, I'd love to actually be able to use it". I had no Apple hardware, no sensible way to acquire an ADB-to-USB converter (I believe they did exist at the time and were quite expensive) and knew nothing about custom keyboard controllers, had no usable microcontroller (I hadn't encountered Arduinos yet, so electronics programming was either a Lego RCX or stuff hooked up to a computer's parallel port, neither of which would work here.) and no idea how ADB, PS/2 or USB worked, so weighing up all the available options, the simplest solution to my problem seemed to be to graft in a donor keyboard controller.

The total cost for this project was:

1. $7 for the donor USB keyboard
2. $20 for the USB hub (I can't remember how much it cost, but I purchased, literally, the cheapest I could lay hands on. $20 seems about right.)
3. a couple of dollars for the matrix wire, solder, solder wick, etc. (all of which I had on hand already)
4. a couple of dollars for the USB-B port (I believe I had to purchase one new)
5. Every other piece of hardware (The Apple Extended Keyboard, USB-A ports, PCB scraps, the yellow and black wires, the thick black cables, etc.) was recycled.

My time, given that I was a university student, was free.

So overall the total cost was about $30 + a few weekends of my time.

I also wanted to remap some of the keys (specifically the ctrl, option and apple keys) so they matched the order they'd be in on a PC keyboard, so any converter would had the modifiers ordered ctrl, alt, win, not ctrl, win, alt, so I would have needed software hacks to make the keys match where my fingers think they are. I was running Windows at the time and I'm not sure it's possible to switch the modifier keys without third-party software.

If I'd done this today, I'd have found or built a USB-C hub that could use any port as the upstream port so I could have had USB-C ports on both sides and used a Teensy or equivalent to run the original matrix in place of the original controller chips. This would have made this project a mostly-software project, not a crazy hardware project. I'd also have ended up chasing the ideal of having this daisy chained with a Thunderbolt monitor or USB-C power supply.

Someone commented on the longevity of the joints and hot snot. I can only talk to this particular keyboard, but I've had zero problems over the past decade, so as far as I can tell, there's no issues with longevity. Note that this has been bounced around a bit, travelled and gone through several house moves. I'm not gentle with my electronics, even stuff I've made myself, so if it was going to fail, it would have by now.

The construction is a little eccentric, mainly because of the specific parts and stuff I had available. The yellow and black wires are multi-strand wires recycled from somewhere and are relatively flexible. All the other wire is solid-core wire and relatively inflexible. All wires are point-to-point wires between legs of the various keyswitches. Some of the soldering is a bit dodgy, but it's all solid. There are resistors at the board end of all the black wires and those are all anchored with way too much hot snot, most of that hot snot is globbed over other wires, so even if it pops off the board itself, the wires should keep it from moving around too much.

Version 2 of this, i.e. proper matrix with diodes and a Teensy, will require adding diodes to every single keyswitch, so there will be a lot more potential for movement there, but I'll be adding a lot of diodes and the matrix should allow me to tie the diodes together, so they should be no more delicate than any other hand-wired keyboard.

And the last comment was about harvesting it for parts and building something new. The goal here was, from the very beginning, to have an Apple Extended Keyboard I could use on my PC. I agree that this keyboard could easily be stripped for parts and make an awesome custom keyboard, that's not the objective here.

Thanks for the write-up, thanks for the comments and thanks for the reminder that all the bits I need for version 2 are currently in my possession - all I need to do now is build it.