M100 has mechanical Alps switches with metal contacts inside. It is most likely just oxidization inside the switch, or a corroded trace on the keyboard pcb. Both fixable problems. And the fact that it's only a single key and not a group of them, suggests the problem is most likely entirely inside the keyswitch and not a corroded pcb trace. That makes it easy.
The first thing to try is just deoxit. Pull the keycap off (with a keycap puller, so it's pulled straight up, no prying from the side) and spray some Deoxit right on the stem, tap the key a bunch of times, let it sit an hour or two, tap the key a bunch more times. If it was oxidization, the key should start working gradually better and better the more you tap it and the more time the deoxit has to work on it.
Don't worry about overspray, it's fine to run all over, it doesn't hurt anything and it dries after a while. You can turn the machine on and test the key while it's still wet.
If it doesn't get any better after that, then the next thing to do is remove the keyboard pcb and examine the traces and the solder joints for that key. But most likely they will be fine and you need to just replace the keyswitch. You can prove it before buying anything by just desoldering both the += key and any other key, swap the keyswitches, and if the += key works, then go ahead and buy a new keyswitch to replace the bad one.
You can buy them from ebay sellers usually, either reclaimed or even some new old stock.
You can even pry apart the bad switch and possibly repair it. Maybe you will discover that a metal finger is just bent or too dirty for deoxit to fix etc.
https://www.ebay.com/itm/333703869469 -- bkw On 12/4/21 1:30 AM, Martin Harding wrote:
Hi, Does anyone know how best to repair this key ? Thanks, Martin Harding Sent from my iPhone
-- bkw
