Repairing Microbee No3 Keyboard

The third Microbee was now nominally “working” insomuch as it was booting reliably to the command prompt. Only thing is, I couldn’t test anything further without a keyboard.

The keyboard was partially working, and having removed the keycaps I could see a mix of 3 different keyswitches. White stemmed with a horizontal bar, Black stemmed with a horizontal bar and Black stemmed with a vertical bar. Without exception every single vertical barred keyswitch had failed.

The system, including a mix of keys. I have already desoldered most of the keys by this point.

Thankfully I wasn’t the first to have to fix these. Microbees have a certain reputation for keyboard failure. There is a great article on exactly how to fix them.

Out came the desoldering station, and without too much effort, I was able to get 39 of the keys out without difficulty. A small handful resisted so I left them for later. Conveniently, one of my sets of pliers was exactly the right width to grip on to the top of each keycap and remove them once desoldered.

My procedure was pretty simple. First, mark which keyswitches need desoldering with a marker. Desolder with the desoldering station, check the pins were loose, and if both pins were loose, gently lift from the frame with the pliers. Some needed an additional “suck” with the desoldering gun but most only needed one pass.

Four Down, thirty three to go.

Later, I would remove the remaining keyswitches by reflowing, desoldering and in one or two extreme cases, simply heating the solder joints while gently tugging with the pliers. Usually only one pin was still stuck so that got the recalcitrant switches.

Once all the switches were out in a batch taking them apart was a combination of terror, brute force and careful levering.

The procedure I adopted (Based on the article above) was to jam a thin flat bladed screwdriver down one side, next to the pins and lever the plastic away from the body, then jam it down besides the other pin. Finally wiggle the screwdriver sideways and down so it forced the middle area between the two already abused sections apart. Once done, you could force the screwdriver gently in further, then lever the back off the switch. At this point if everything is going right, the switch would come apart into it’s component pieces. If not, then either the plastic cracks or the back sproings off and the spring flies off, never to be seen again. Trust me, I speak from experience.

Thankfully I only lightly cracked two switches and I found the spring eventually.

Each switch consists of a body, a base of two gold plated contacts, a spring, a keystalk and a small carbon embedded contact on a strip of rubber.

Body, keystalk (with carbon contact in situ), base with contacts and spring

Once apart, I cleaned the two contacts with a soft ink eraser followed by the ever popular isopropyl alcohol. I then dragged the contact gently across a sheet of paper once or twice., followed with a dab of isopropyl. After that came reassembly followed by testing. Using a multimeter and a pair of alligator clips I tested each switch after reassembly. One or two needed a second pass, and maybe adjustment of the contact positions by gently heating them with a soldering iron. I could then straighten the pins in the softened plastic and reassemble.

A closeup of a base, showing the contacts.

Before cleaning, most had a resistance between 2KΩ and 10KΩ. After most were around 200Ω with arange from 400Ω to 80Ω. Basically a tenfold improvement in conductivity.

Once all the first wave were done (All 39) I reassembled a single key and soldered it in. Testing it worked perfectly! OK, the other 38 went back in. One turned out to be a stabiliser for the shift key so I pinched the keystalk from it for one that was broken, and used the broken one behind the shift with a jury rigged spring to provide counterpressure. (I’m not entirely happy with this. I may swap out the spring for a different type in the future,)

At this point I removed the remaining eight switches (as detailed above) and cleaned and reseated them too.

There was a little excitement when the bodged in 33k resistor array being used as a keyboard pulldown snapped a leg, (It mounts across two of the screws in a very annoying fashion) but was able to rejoin it.

Finally, I was able to test all the keys, plus loading software. Everything now seems to work correctly, within the limits of the system. This is a 16k computer with an old version of the ROMs, so compatibility is somewhat limited.

It lives! Pay no attention to the messy workbench 😀

Finally, I cleaned up the speaker and remounted everything in the lower shell.

In related news, some kind people are helping me source the one missing keycap for the CTRL key, and possibly an upper shell for it! Awesome!

Published by ilike8bits

I collect old computers and consoles

2 thoughts on “Repairing Microbee No3 Keyboard

  1. Hi, You can buy the SIL 33K resistor pack x 10 pin from Element14, well you used to be able to. I bought in bulk for a few cents each a couple of years ago when I was repairing Microbees . I think I have 100 of them, do you want a few ?

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    1. Hey Robert. Thanks for the kind offer! The resistor bodge was already in place when I got the system so I left it there. I’m 99% sure I have the right resistor pack in my parts bin (Left over from work on both the Amiga and the Omega) so next time I’ve got it that far disassembled, I’ll fix that.

      Cheers!

      Like

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