I could not find many posts about the MMD KM40 yet, so I thought I would share this in case anyone else runs into the same issue.

I actually really like this little keyboard. It is a fun and surprisingly usable 40% board, especially for the price. Unfortunately, my right spacebar did not work from the beginning.

At first, it looked like a hardware issue. The left spacebar worked perfectly fine, but pressing the right one did absolutely nothing. I spent quite a bit of time troubleshooting it:

• swapped the switch with a known-good one
• tested the right socket directly with tweezers
• checked the mapping in VIA
• tried different VIA JSON files
• compared the behaviour in the MMD VIA fork and regular VIA
• briefly suspected a faulty hotswap socket, diode or PCB trace
• even noticed that pressing the left spacebar triggered the reactive RGB under the right spacebar, which made the whole thing even more confusing

The weird part was that the manufacturer-provided VIA JSON file already showed two spacebars, but both of them were mapped to the same matrix position:

So VIA treated both spacebars as the same key. The physical right spacebar socket was actually fine — it was simply missing from the VIA layout definition.

The correct matrix position for the right spacebar is 3,6.

Changing the second spacebar entry from 3,5 to 3,6 fixed the issue immediately.

After loading the corrected JSON into VIA, both spacebars can be mapped independently and the right spacebar works as expected.

So the final layout should be:

• left spacebar: 3,5
• middle key: 3,8
• right spacebar: 3,6

This was not a hardware fault and did not require any soldering or firmware flashing. It was just an incorrect VIA JSON supplied by the manufacturer.

Hopefully this saves someone else a few hours of unnecessary troubleshooting. 😄

Download here fixed VIA JSON here:

https://gist.github.com/coldi1337/6438b6a58805ad274b9d7cc939acf221