AE#73 Repairing A TRS-80 Model III Keyboard

My parts Model III had a completely non-functional keyboard. Here we diagnose and repair the problems. This procedure can be used on similar computer and synthesizer keyboards that use carbon coated contacts

[@bobweiram6321]

I would add a tiny bit of lightbulb grease on the metal contacts to prevent corrosion. For punching out the foil, I would remove the backing and stick it onto a clean UHMW cutting board and punch it out. The adhesive won't stick well to the UHMW, allowing for easy removal. I believe, however, there's a flesible graphite base paint.

[@dmuntz]

I'm working on a Model III with some non-functioning keys, but this machine has switches that are the same as the Model I (2 pinouts, and a funky switch mechanism. For all but one, I was able to get the switch working again, but one of them is holding out. When I say the switch mechanism is "funky" it has two exposed copper lines and then a Y-shaped piece that "springs" out above the two copper lines. Pushing the key down pushes the Y-shaped piece closer to the exposed copper lines and lowers the resistance. I'd say the switch was designed to enable the detection of the depth to which the key was pressed.

[@artifactelectronics]

That almost sounds like a touch or velocity sensitive key.

[@Daveyk021]

Anything under 1000 ohms is good. Clean the metal in the bottom, and lightly "sand" the carbon pad on printer paper. Get the resistance to under 100 ohms, most under 100 ohms. Should be good for many years. One could also change the pull-up resistor pack, RP1, on the mother board, to 2K or maybe 4.7K, and the keys will be a lot more responsive. When the keys don't work, they measure about 6K-ish. If RP1 was a 20K Pull-up, thy keyboard will without taking it all apart.

[@jimbakerforlinux]

Hey Arti. Well, you've given me the push that I needed to get my Model III back in shape. It quit working not too long ago and I was very sad... I am trained in the art, so to speak, so I'll follow your lead and get mine up and running like new again! Thanks again for the push! Jim

[@artifactelectronics]

It's great fun getting it work again, but after that challenge is met my own interest tends to diminish rapidly

[@jimbakerforlinux]

@@artifactelectronics Hey Arti. Yes, I hear you... I can see how that would be the case. I got mine running again. Yippee!

[@tonygriffiths2485]

Wow ..... THAT was a lot of fascinating work. the sort of thing a service engineer doesn't even want to consider. Just swap it out. Well that is today. Back in the day keyboards were very expensive.

[@RockwellAIM65]

Keyboard repairs were also expensive... for the reason you mention.

[@darkwinter6028]

The reason that metal polish turns dark is it’s a very fine abrasive slurry; and what you are seeing is the metal particles that it has abraded off of the surface (it doesn’t take much to darken the swab).

[@darkwinter6028]

I’d put a cutting mat behind the tape before hitting it with the punch. Not only would it help the punch work; it would protect the desk...

[@FireballXL55]

I have used aluminium foil and a paper hole punch on some industrial keyboards.

[@SidneyCritic]

You might be able to punch disks cleaner with a paper punch that punches pages for a ring binder folder. Just open the bottom of the punch and look down the punch hole from the bottom, and that lets you line up the metallic foil tape with the hole.

[@1959Berre]

Different size, too large.

[@MxArgent]

I took in a III and 4 at VCF Midwest in a trade, and hoh BOY are the keyboards cumbersome to get at. Entire bottom row on the III wasn't working so I pulled it and attempted to treat the keys using a bit of deoxit on a cotton swab. 40 solder joints desoldered and replaced for nothing...and for some reason the rubber domes were a lot more fiddly on mine than yours. I spent at least ten minutes trying to get each one reseated. Ended up just shrugging and doing a temporary part out to help on troubleshooting my IV's floppy drives since it's in better shape under the hood (and I pray it stays that way). I really wanna get the III going eventually, though, since it's practically immaculate.

[@MxArgent]

@Barry Manilowa Thanks for the pointers! Good to know just replacing the switches is an option - if I'm gonna have to desolder 'em all, I wanna do something that I know'll work! The III itself has been lying dormant - I've been working on the 4, which has suddenly developed vertical deflection issues. (Oh boy)

[@1959Berre]

Happy New Year. Keep the good stuff coming. Why didn't you clean the carbon disk with IPA as well? That usually is enough to get those contacts back in shape. You may avoid ripping traces or pads if you add some fresh solder prior to removing the old solder. The solder will flow easier, putting less stress on the traces.

[@artifactelectronics]

And a Happy New Year to you too. I did give the disks a final IPA cleaning just before I inserted them in the switch housing but neglected to show that part.

[@gettingpast4391]

You do not need to remove the keycaps. Doing this risks breaking the plungers

[@douro20]

Going to put a hard drive in your other Model III?

[@artifactelectronics]

Sounds tempting, maybe I'll try the cheap IDE interface.

[@RockwellAIM65]

Dude. Spin the q-tip on the contacts.

[@gettingpast4391]

You do not need to remove the keycaps. Doing this risks breaking the plungers