Hi, my first post, so pardon me if this topic has been covered already.
Some history first. Back in the day my first computer was an Atari 400, so I was very accustomed to the feel of a membrane keyboard. A few weeks back I was able to buy an Atari 800 (untested) at a yard sale. I figured if anything was wrong with it, I could probably fix it, since they were built like tanks and I'm pretty handy.
When I powered her up, I found that only the first row of keys worked. I figured it was dirty connections to the keyboard so I took it apart cleaned the connectors, re-assembled and tested. No change. Hum...?
Next I figured I'd disassemble the keyboard, a Mitsumi. I did that and found the spring loaded, silicone pusher contractor to the membrane keyboard underneath. Everything looked good, but a partial reassemble only reactivated a few more keys.
I checked your forum and found that this keyboard has a chronic history of membranes going bad. Drat. I didn't want to fix or replace a membrane assembly. On a lark I figured I'd test the membrane by itself, bringing me back to my 400 days. Lo and behold, every membrane switch provided that satisfying Atari bloop from the speaker when pushed with my finger. I realized the plunger assembles were not providing enough force on the silicone nubbin to actuate the membrane switches. I tried carefully stretching the metal plunger springs to get more force on the membrane, but this only improved a few more keys. How to get more force on the membrane????
I then looked at the silicone part itself. It was top to bottom symmetrical. Hum, what if I added a twist to the part putting the top nubbin into the bottom nubbin location and visa via putting a twist into the side silicone springs increasing the spring preload of the side silicone springs. I did this to a few. Eureka! Each switch I changed in this manner worked perfectly. I carefully did this to all the plungers, re-assembled and viola! The keyboard works perfectly now!
I was able to get the process down to 5-10 second for each key. It was easiest if I pushed the printed side of the key up, and used my thumb and a small screwdriver to flip and reseat the part. They don't seat in the spring perfectly square, but they still do the job. Photos shown below.
Perhaps, this is all you need to do to your 800 if you are having keyboard problems with a Mitsumi keyboard. Now the disclaimer. I am not responsible if you damage your keyboard or computer by undertaking this hack.
PS If this hack has not been done before, and a process title has not been claimed for it, I declare this hack to me the "Murphy Twist"!