The arrow-down key of my Amiga 1000 keyboard (Mitsumi made) didn't work. So I dismantled the keyboard and discovered that one of the legs of the key was broken. The legs are soldered to the PCB, but are quite easy to remove. The mechanism can then be opened and repaired.
These pictures are not mine. I took them from https://geekhack.org/index.php?topic=19761.30
I took the oportunity to remove all keycaps and clean them. I also retrobrighted the housing of the keyboard and the space-bar. They were yellowed a lot.