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Repairing a Unicomp keyboard

June 28, 2018 - Reading time: 4 minutes

Recently the keyboard I use most of the time, a full sized Unicomp, began developing dead keys. Sometimes they would contact and sometimes not. This rapidly became an untennable situation and so I pulled off the relevant keys to see if anything was obviously amiss. The springs themselves looked ok, so I assumed that the rocker which they're mounted on had broken. With the passage of enough time plastic becomes brittle and can break, especially when there's a lot of vibration going on as will happen during typing.

Opening up the casing with a 5.5mm socket I noticed a lot of small round bits of plastic falling out. At first I thought they might be some vital components, but on close inspection they were all irregularly shaped and didn't look like anything machine manufactured. I'd never deconstucted this type of keyboard previously, and searching for more information it turned out that these were the plastic heads of the rivets which hold the metal backplane on, many of which had fallen off. So what had happened was that the plastic had become old and brittle and the summer heat had probably caused the backplane to warp and break them off. With the backplane no longer properly held on there was nothing other than some plastic and rubber for the buckling springs to hit against, causing the keyboard to "go mushy".

So this was going to be a bigger job than I had thought. Fortunately there are quite detailed howtos online for how to remedy this type of calamity.

Being fairly expensive you might think that the manufacturing quality of the model M type keyboards is top of the line. But actually it's not. The Unicomp keyboards I use are closely based on the original IBM keyboards from the first generation of personal computers in the early 1980s. They were built to be mass market items, mostly sold to businesses. As such the build quality is not all that different from the Commodore Amiga which I was using at the end of that decade. Although it's quite thick the casing it's not all that solid and makes a lot of creaky noises if you carry the keyboard around (just like the Amiga did) and using plastic rivets is also decidedly cheapskate.

The way to fix my problem was to completely deconstruct the keyboard, drill out the plastic rivets and replace them with 8mm M2 bolts. Known in the trade as "a bolt job".

Content Warning: Explicit photos of keyboards follow.

With the casing removed the keyboard looked like this. I took photos at each stage mainly as a reference so that I could hopefully put things back together in the same order.

Pulling off the keys is straightforward and the metal backplane could then be removed by using a soldering iron to melt away the few remaining rivet heads. Also the USB cable was unplugged and its ground lead unsoldered. After that the small control board can be unscrewed and pulled out. The plastic matrix and its rubber covering can then be easily removed. I also carefully removed all the key springs. Those are ultra delicate.

So then you have the plastic key holder - for want of a better term - which is the thing which needs drilling. Ideally I would have used a small hand held drill but I didn't have one of those and instead used my usual large and heavy industrial grade one. This makes the drilling unweildy, but with some amount of patience it works.

Reassembling the Unicomp keyboard with 8mm bolts is a very fiddly operation at first. The key springs are exceptionally easy to disturb, and if any of them are missaligned then the corresponding key won't work and the repairs would have been in vain. For this you need a very steady hand, so avoid drinking a lot of coffee before you do it.

The result then looks like this. For reference there's another Unicomp below. It's the smaller "space saver" type.

And the nuts on the backplane look like this:

I didn't drill out the rivets on the bottom row, because the plastic lip along the bottom was no thicker than the 1.6mm drill bit, so it was pointless trying to drill into it. Hopefully there should be enough bolts to secure the keyboard though.

When adding bolts to the backplane I rocked it back and forth and if the key switches are working normally then the springs should also rock up and down. If there were any springs which weren't rockin' they could be twiddled (that's a technical term) with "the chopstick of death" (in my case the whittled end of a jostick) until they snapped into position.

Then it's a matter of laboriously pushing on the keys again, reconnecting the control board and resoldering the USB cable ground lead.

And amazingly it all worked. No more duff keys.

These days it's unusual for any consumer electronics to be repairable. This is one of those rare examples where it's still possible to mend it yourself in a quite straightforward way if you know how and are prepared to handle some fiddlyness.


The blog of Bob Mottram, a Free Software hacker and maintainer of the Freedombone project.

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