LK402-AA Keyboard Fix

After several years of just sitting on the shelf, the “LEFT” key on one of my LK402-AA keyboards went dead. Completely dead – no matter how hard you hit it, it just wouldn’t work.

So here is a guide on how to fix a dead key in a LK402-AA (and perhaps other LK4xx keyboards?)

What You’ll need:

  1. A screwdriver,
  2. A copper or other conductive, adhesive tape or foil.  I used 3M p/n 1181 1/4″ copper foil:

    Step By Step Procedure

Step 1: Identify the keyboard

Step 2: Pry open the top lid. You may and will need to use some force. Old plastic becomes brittle, so keep an eye on how many of the small clips you are willing to break before finally going in.

Step 3: The keyboard should look like this. It is time to remove the key matrix, the black plastic plate holding all the keys. Pry it carefully, it is held by several vertical snaps or holders, their locations are visible if you look at the bottom side of the keyboard. As you release it from one snap, it does not easily snap back, so you can move to release the next one. You WILL need to use some force. My matrix did not snap, be careful not to snap yours

Step 4: After removing the keys, a rubber blanket is now visible. It is easily removed.

Step 5: After removing the rubber blanket, the construction of the keyboard is visible: a transparency with key ‘pads’ and conductive traces, a thicker, white plastic insulator, and a second transparency with ‘pads’ and conductors. The transparencies and insulator are held together at the top, and can be easily turned over like pages in a book.

Step 6: Flipping over the top transparency, the white insulator is now visible.  I had to remove it as well, as the fault turned out to be in the bottom transparency.

Step 7: Fault identified: bottom transparency.  A conducting trace has become corroded and disconnected. It is a trace that creates a contact between the RETURN key and the LEFT key. Because LEFT is the last key in the conductive chain, the LEFT key is the only one that has stopped working.

Step 8: Using the conductive tape (copper foil), I created a conducting path between the two key pads on the bottom transparency:

This is how it looked after placing back the insulator and the top transparency:

Step 9: The rubber blanket, key matrix, and top cover can now be assembled back in place. Done!

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