FAQ addition: Repairing Capacitive Keyboards

REPAIRING CAPACITIVE KEYBOARDS v1.0 7/10/97 Kai Kaltenbach ---> What are capacitive keyboards? The vast majority of capacitive keyboards were and are manufactured by Keytronics of Spokane, Washington. The mechanism is really quite simple. A keypress pushes a foil circle onto the surface of a printed circuit board, thus completing the circuit and registering the keypress to the computer. Capacitive keyboards are very reliable; however, they will fail over time even if unused. Each foil contact circle is backed by a cylinder of PVC foam. The foam is subject to environmental deterioration. Once the foam loses its elasticity or dissolves entirely, the keys will stop working because the foil circles will no longer make good contact with the PC board. Various classic computer systems, such as the Processor Technology Sol, exhibit this problem after their 20+ year lifespan. ---> How do I tell if I have a capacitive keyboard? Just remove a key cap -- if there is a separate spring under the key cap, around the plunger, it's extremely likely that it's a capacitive keyboard. They won't always say Keytronic on the outside of the keyboard housing. ---> What do I need to start a repair job? First you need a set of replacement foam pads. Each foam pad is a cylinder about 1/4" in diameter and 3/16" long. There is a semi-rigid clear plastic circle attached to one end of the pad, and a foil cylinder attached to the other end. The only toosl you'll need are a #1 Philips screwdriver and a very small standard screwdriver such as those pen-clip types frequently included in computer tool kits (plus whatever tools are necessary to remove the keyboard from its enclosure). ---> Where do I get replacement pads? If you're a perfectionist, you can order replacement pads from Keytronic Corporation (http://www.keytronic.com/). However, it's much easier to simply find a capacitive keyboard in your local computer junk/surplus store and strip it for replacement pads (see above for info on identifying such keyboards). Note: If you're stranded on a desert island with only your computer and a screwdriver, you can get the important keys working again by using pads from the numeric keypad, which is usually in good shape because it's not used very much. ---> OK, I've got all of the necessary items, what are the repair steps? 1. Remove the keyboard from its enclosure. Place on an open workspace. Putting a pad of some type underneath may help avoid marring of the keycaps, but in any case be careful to prevent static discharge if your keyboard model has ICs on the PCB. A ground wrist strap is recommended. Lock up the cats. 2. With the keyboard face-down, remove all of the screws on the back of the PCB (there are a lot!) Don't worry, nothing is going to fall apart when you remove the PCB. 3. Remove the PCB. Place to the side. If it shows any signs of dirt on the contact side, you'll want to clean it with a non-residue, non-lubricating contact cleaner. 4. Identify pads that need replacement. If you're replacing all of the pads, you can skip this step. First, look for any foil circles that are damaged. Then, while looking down the length of the keyboard on the back side, press each row of keys and make sure that each foam pad protrudes sufficiently and cleanly out the back of each key plunger. The most-typed keys are the first to go (return, space, A, E, etc.) When in doubt, replace a pad -- that's a lot of screws to remove to do this over again! Replace pads one at a time. 5. Carefully remove the keycap and spring. There are actually keycap removal tools available, but whatever you use, just make sure to pry sideways. 6. Holding the keyboard perpendicular to the table, place the Philips screwdriver into the back of the key plunger and push gently to pop the plunger out the other side. 7. Pick up the plunger. Remove the foam cylinder by inserting a fingernail under the foam cylinder's clear plastic circle. A tiny bit of leverage and it'll pop out. 8. Put the replacement foam cylinder in place. Gingerly use the very small standard screwdriver to pop the clear plastic circle into the plunger by pressing around its outside edge. Be careful not to damage the foam or the foil circle. 9. Replace the plunger in the keyboard chassis. 10. Replace the spring and keycap. Use the very small standard screwdriver to press on the edge of the bottom of the plunger to firmly seat the key cap. 11. Repeat as necessary. Note: If you're skilled at delicate work and have various sized tools available, you can probably replace the foam pads from the back without removing the keycaps, but this is not recommended for beginners.