Keytronic Keyboard Repair - foil discs

[david__schmidt]

I recently adopted a Franklin Ace 1200 whose keyboard had failed entirely. Early Franklins used the typical Keytronic foil-over-foam capacitive design, so I set out to fix it with inspiration from Geoff Harrison:
http://www.solivant.com/sol20kbd/

I read several sites that had recipes for the mylar/semiconductor foil to go on the bottom of the key switches, and experimented with many household items I had lying around. I had (very) little luck with foil-esque potato chip bags, tape-over-aluminum foil, shiny wrapping paper, foil cupcake wrappers, and the like. I just couldn't source one of those "space blankets" some folks talked about. Well, I found the next best thing that I bet everyone has some of: anti-static bags. The outside is the perfect film for the job.

I mirrored Geoff's manufacturing techniques except that I made the foil and foam sandwich together in a big sheet by gluing the foil to the foam before cutting it out. I then glued individual foam discs to the little stiff plastic discs saved from the original keyboard with contact cement.

I found 7/16" diameter to be a bit big for my keyboard, so I used a 3/8" hollow punch to cut out my parts.

After more hours than I care to admit to... the keyboard lives again!

 

[mbbrutman]

David,

Excellent work. There is a company called Mil-Key that sells the original foam and conductive discs, but their pricing is very high for a hobbiest repairing a few keys on a keyboard.

I had not thought of using a static bag before, and I like your manufacturing technique - do it in a sheet before cutting. The only part that is missing is the type of foam that you used .. foam varies so much and in this case the foam makes a big difference in the tactile force provided by the keyboard. Did you get to a craft/fabric store and just try a few different types?

 

[david__schmidt]

David,Excellent work. There is a company called Mil-Key that sells the original foam and conductive discs, but their pricing is very high for a hobbiest repairing a few keys on a keyboard.

Ah, good to know that some still exist out there. My pricing would be high (i.e. "you can't afford it") if someone asked me to do this for them. ;-)

I had not thought of using a static bag before, and I like your manufacturing technique - do it in a sheet before cutting.

Right. Messing with tiny little plastic discs is hard on the eyes and tough with fingers full of rubber cement, it turns out.

The only part that is missing is the type of foam that you used .. foam varies so much and in this case the foam makes a big difference in the tactile force provided by the keyboard. Did you get to a craft/fabric store and just try a few different types?

You are exactly correct, and I did not. I started experimentation with two layers of double-sided foam tape (the stuff you use to hold stuff on a wall). It is dense foam, and provides a fairly mushy/thuddy feel. The foam I ended up with was just some thin (0.15"?) dense packing foam that likely came on a roll, used for wrapping some components I had bought. In rummaging through the attic, I found some very similar stuff my wife had bought to pack dishes in. It ultimately has the same thuddy feel that the double-faced tape did, but it turned out to be easier for me to deal with.

I have a feeling that the original foam was able to collapse completely when making contact, rather than providing resistance as mine does now. So a more authentic restoration would probably be to source much more compliant foam, which would in turn make it much harder to cut down to the correct width, since it likely wouldn't be thin to begin with...