A connection is a connection, so electrically DuPont cables will work well enough.

I'd have two concerns though:
- flatcables are flat, headers+dupont connectors less so. It's entirely possible it won't fit with the bulkier solution. Maybe 90 degree headers would improve matters, but whichever way you turn it, it will be bigger. Of course a Model M case is huge, so it might not be an issue, but do check before (de)soldering anything.
- DuPont stuff is great for prototyping, but those connectors don't grip very tightly. I'd be concerned that over time the connectors might vibrate loose. I'd prefer a connector that grips a lot tighter. One simple, cheap option might be an analog CD audio cable (if it has four wires). Also not *the* tightest, but at least one big connector for all four - if it fits.

To clean that kind of stuff I use a soft rubber eraser, usually when you find that kind of stuff in some place of the keyboard, you'll find it in other places, so take a look at the rest of the keyboard and "erase" any track that's not light grey and clean it with alcohol..
Then to fix the traces, in theory a "conductive ink pen" or another kind of conductive ink is suposilly the best solution, but I've never had to rapair tracks untill now so I don't know if it works first hand.
Edit: Looking close to your first photo, I'm almost sure that you have more of that in membranes, I think that you'll have to disasemble it completlly and get some small screws to reasemble it.
This video explains it fairly well, https://www.youtube.com/watch?v=GeA5IhjKk5k, and notice that the membrane of the keyboard on that video also has that black stuff on the tracks. 😀

dionb wrote on 2022-01-31, 10:15:

A connection is a connection, so electrically DuPont cables will work well enough. […]
Show full quote

A connection is a connection, so electrically DuPont cables will work well enough.

I'd have two concerns though:
- flatcables are flat, headers+dupont connectors less so. It's entirely possible it won't fit with the bulkier solution. Maybe 90 degree headers would improve matters, but whichever way you turn it, it will be bigger. Of course a Model M case is huge, so it might not be an issue, but do check before (de)soldering anything.
- DuPont stuff is great for prototyping, but those connectors don't grip very tightly. I'd be concerned that over time the connectors might vibrate loose. I'd prefer a connector that grips a lot tighter. One simple, cheap option might be an analog CD audio cable (if it has four wires). Also not *the* tightest, but at least one big connector for all four - if it fits.

Thank you, those were both good points and exactly what I was looking for. Things that someone more experienced might think right away.
In my case the specific jumper wires and pin headers I had were really, really tight. You feel that it'll break if you pull it from the wire and not the plastic part, probably cheap stuff not exactly to the specs but it helped this time. So I decided to let it with the jumper wires, especially since access is only 4 screws away.

You were spot on about the height thought, on the board side there was no problem as the ribbon was attached vertically and now the jumper wires sit even lower. On the leds side I had to bend the header and make it "angled" before soldering it on to bring the total height at about the leds level. It worked out nicely and I'm happy!

Hoping wrote on 2022-01-31, 10:33:

To clean that kind of stuff I use a soft rubber eraser, usually when you find that kind of stuff in some place of the keyboard, […]
Show full quote

To clean that kind of stuff I use a soft rubber eraser, usually when you find that kind of stuff in some place of the keyboard, you'll find it in other places, so take a look at the rest of the keyboard and "erase" any track that's not light grey and clean it with alcohol..
Then to fix the traces, in theory a "conductive ink pen" or another kind of conductive ink is suposilly the best solution, but I've never had to rapair tracks untill now so I don't know if it works first hand.
Edit: Looking close to your first photo, I'm almost sure that you have more of that in membranes, I think that you'll have to disasemble it completlly and get some small screws to reasemble it.
This video explains it fairly well, https://www.youtube.com/watch?v=GeA5IhjKk5k, and notice that the membrane of the keyboard on that video also has that black stuff on the tracks. 😀

Luckily the rest of the keyboard is clean, even the other end of the ribbon cable looks fine. It was just the end on the leds side, not sure why.
The problem with these ribbons is that once oxidation appears there is not much material to scratch and the trace breaks easily. In this case the thick trace was completely cut inside the plastic, not just on the exposed part.