Keys not working is a common problem with old MSX computers. Most times a good cleaning is all what's necessary to get them working again.
So how do you do that?
First make sure your MSX is disconnected from the mains.
First you have to open the MSX. Flip the MSX over so you see the bottom. Then remove the two screws. Flip the MSX over in it's normal position. Next gently slide the top towards you and up. The top comes off that way.
Remove the screw for the ground cables.
Gently disconnect the connector for the keyboard ground cable.
Flip the keyboard towards you on it's back. Then remove all screws.
Once the screws are removed you can flip the keyboard back. Gently so the keys part seperates from the membrame part. Clean the membrames plastic surface with ethanol. Also between the sheets. You can seperate them. In my case they where fixed together with tape. Make sure when putting them back they align.
The plastic base plate with the keys attached is free now. First remove all springs, be carefull, so you don't loose them. Then remove all metal parts. Under each key there is second spring. So you have at least 2 springs per key. Finally you can gently remove the keys with a small screwdriver.
There was a lot of dirt under my MSX keyboard.
In the middle of the cursor key there is a small square fixed with a screw. Remove that as well.
Keep all parts together.
Give the keys and plastic base platea good cleansing. I used a sodium percarbonate solution to clean the keys and Dreft for the base plate.
Reassemble the keyboard.
A nice and clean keyboard!
Turn the keyboard and put the smaller springs back.
Reassembled, connect your MSX and test if all keys are working again.