I found a Cherry G81-1000 in the scrap recently. It's one of those old 101 key models with a metal board and oldish mechanical switched. Model M fans would probably disagree. But it is comparible with an IBM Model M in many ways. Maybe it's becasue Model Ms are still produced that, from what I saw, Cherry G81-1000s are offered for more on eBay. So they do seem to be of interest to other vintage computer fans, not just me.
To test it I first had to replace the cable. I decided for a cheap, black audio cable with two 5-pin DIN plugs. I say audio cable because hat's what it was advertised as. It has no shielding at all. Anyway, just becausew they used proper shielding back then doesn't mean I need it. Because I'm not involved with the computer keyboard fan bubble I had no idea where to find the pinout on ther PCB. Web search engines were of no help. But since a stub of the original cable was still on it, I knew which colour of the original cable goes where. I then checkerd three other (a bit newer) Cherry keybards and found them to use the same colours. I tried the same pinout and it works. I'll attach a diagram just in case anybody needs to do the same thing.
The keybnoard is generally working. But typing does not feel nice and some keys stopped working after a while. From their arrangement it is pretty obvious that one trace on the board must be broken. Here starts the bigger problem. Because of the way the board is constructed, the most of the traces aren't accessible. They are covered by a metal shet from the back and the switches from the front, which are connected to the metal sheet. I know how to repair traces of a keyboard matrix on such a flexible PCB. But if I can't reach them without destroying either be PCB or the switches, that knowledge is not worth anything.
During my research on those old boards I also found out why the switches look so different from MX switches and why they feel so weird. They aren't really mechanical switches, but rather Cherry MY switches, which use indivisual membranes inside. This explains why typing on it feels so bad. I did not find a way to repait the trace, yet. But knowing now that nor cleaning nor using the switches for a while will make them feel better, it does not feel worth putting more time into this.
I initially thought it might be a good idea to design a PCB with a key matrix that replaces the original switch PCB. The new one could be populated with MX switches and thus the entire device updated to an excellent version of itself. But there are free open source PCB designs out there that replace the entire inside of the keyboard. That would be the easier route. And it would upgrade the keyboard to USB at the same time. Although the Atmega used in those could probably be programmed to use a simpler serial interface as well.
It would be nice to continuie here with "So I ordered one of those, done up the keyboard and it's now my favourite one." But I have enough unfinished and unstarted projects already. So no, not right now.