I got a dead Mitsumi keyboard working this afternoon. If you have a Mitsumi keyboard where none of the keys work, or groups of keys don't work, then the problem is likely to be the connection between the keyboard PCB and the flexible membrane, and you might want to try repairing it as follows:
(1) Remove the keyboard from the console.
(2) On the back of the keyboard PCB, unsolder the two wires from the pins of the Alpha Lock key switch. Make sure that there are no blobs of solder left on the pins - they need to go through the PCB later.
(3) Make a note of the orientation of the two metal strips on the back of the PCB, so they can be refitted correctly.
(4) Remove all the screws from the back of the PCB. Note that there's one hidden under the ribbon cable, near the sticky foam pad - carefully bend the ribbon cable up to access it.
(5) Remove the PCB from the rest of the keyboard. If you've removed all the screws, it should come off easily.
(6) Turn the PCB over and look at the white membrane. Towards one edge you'll see a group of pads that make contact with corresponding pads on the PCB.
(7) Starting in one corner, carefully peel the membrane off the PCB. The membrane should be tacky, not firmly stuck. Flex it as little as possible to avoid cracking any of the carbon tracks. Once removed, avoid getting any hair or dust on the tacky side as this might affect operation of the keys.
(8) Get some electrically conductive silver paint (such as www.maplin.co.uk/p/electrically-conductive-silver-paint-n36ba) and carefully paint over each of the pads that make contact with the PCB, making sure you get right to the end of the track that whizzes off round to the keys. Give them a couple of coats, with 1/2 hour drying time in between. Make sure you don't get any paint between the pads as it will short them together.
(9) On the PCB, there is a white plastic strip over the pads that make contact with the membrane. Carefully peel this off to reveal the gold-plated pads on the PCB below. Make sure that this area is clean and smooth - the membrane needs to lie completely flat against it to ensure a good connection.
(10) Refit the membrane to the PCB. Check that it is accurately aligned, with the screw holes aligned and the black carbon circles directly over the gold 'maze' tracks on the PCB. Check that it is lying completely flat - no ripples.
(11) Get a small piece of thin, soft foam and cut to size to cover the group of pads connecting the PCB and the membrane. Attach in place to the membrane with double-sided tape. (The foam needs to compress to about half a millimetre or so or so when the PCB is fitted back to the rest of the keyboard and screwed down. A small piece of cardboard from a cereal packet would probably work instead of foam - just need something that will ensure that the pads on the PCB and membrane are firmly pushed together.)
(12) Refit the PCB to the rest of the keyboard, refit the screws and resolder the wires to the Alpha Lock key switch.
(13) Refit the keyboard to the console and see if it now works.
Edited by Stuart, Sat Sep 3, 2016 11:33 AM.