Science of Cambridge Mk.14

I recently picked up a nice Science of Cambridge Mk.14 computer from Ebay. While it wasn’t the cheapest purchase ever, it seemed like a reasonable price considering what they usually sell for. Also, there were photo’s of it working – Bonus! The package also included the cassette interface, optional VDU board and an add-on keyboard using proper tactile switches.


Having already used some of Sinclair’s early machines, I knew what to expect of it’s original membrane keyboard. ie, I expected it to be awful and I wasn’t disappointed. Sadly, even though the machine looked to be in good condition, the original keypad was utterly useless (More so than it should have been). A couple of keys didn’t work at all and those that did needed quite a bit of force to work and when they did the machine would register the key-press multiple times.

I plugged in the external keypad thinking I’d have to make do with do with that. It wasn’t much better with the similar problem of the machine registering multiple key-presses. So, to get a usable machine, I had to repair one of the keypads. There were a couple of options. Firstly, replace the original onboard keypad with a new membrane keyboard, fix the 3rd party one or remove the original keypad and solder in some tactile switches. I decided to go with the first option as it would keep the machine looking original and if I wanted to, I could still hook up an external keypad.

Removing the old keypad…

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As it was already beginning to lift in one corner, the top layer was really easy to remove. The second layer which is a sort of rubber membrane had pretty much turned to goo and needed quite a bit more work to remove. Once that was done I then cleaned up the board with some IPA.


To make the new keypad, I designed a layout in Illustrator which was then printed on a high quality laser printer and laminated. Once cut to size, I stuck aluminium foil squares to the back of each key location and then attached another laminated sheet using double sided tape. This sheet had holes punched in for each key. the whole lot was then attached to the Mk.14 board using more double sided tape.

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Final result. I have a Mk.14 with a keypad that works as well as it did when new. Ok, even when new these keypads were awful. This new one will let me use the machine but its a pretty horrible experience so I guess I’ll still have to repair the add-on keypad.

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Apple IIc Keyboard Repair

Apple produced three versions of the IIc computer during its production run. These are the A2S4000, A2S4100 and A2S4500. The last one (A2S4500) was the IIc Plus which was quite an upgrade over the standard IIc. It provided a built in accelerator, 800k 3.5″ floppy drive and the entire power supply was now internal although it was limited to 110v only which is a bit of an embuggerance for us in 220v land. The IIc plus and the A2S4100 both had memory expansion connectors that allowed the memory to be easily upgraded but the best feature was that Apple built the keyboard using really nice tactile Alps keyswitches which feel really nice to type on.

Sadly, the earliest version which also seems to be the most common was fitted with a not quite so nice keyboard. If you have an Apple IIc and you don’t like the feel of the keyboard and it’s not as easy to type on as you think it should be, the chances are that you have an earlier IIc with this keyboard. However, it might be possible to make it work better!

Apple made the decision to fit two layers of rubber between the switches and the key tops to stop dust and other bits from getting into the depths of the keyboard. Unfortunately, in the 30 or so years since the machines were produced, this rubber has probably gone brittle and made the keyboard more difficult to type on and even causing keys to stick.

So, on to the fix…

  • firstly, remove the 6 screws from the base of the machine (Two right at the back and the four under the keyboard)
  • Release the clip from the front edge and slowly lift the top up. There are another two clips over the disk drive so be careful.
  • With the top removed, lift the keyboard out, unplugging the ribbon cable.
  • Now, with the keyboard removed you will need to remove each key top. I do this using a flat headed screwdriver under one end and supporting the other end with my finger. The key tops just pull straight off but it is important to keep them level while lifting so as not to break the stem.
  • Once all the tops are removed, the top rubber layers should just lift off. You might even find that it just crumbles apart. The second layer is more durable and can stay in place.
  • I usually give the keyboard a good dust out with a dry paintbrush at this stage and also wash the key tops in some warm soapy water.
  • Once cleaned, simply replace the key tops and reverse the procedure to re-assemble the machine. This should be enough to make the keyboard work a lot better.

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Note: While the above procedure has worked well for me on a number of occasions, other people have still had issues with sticky keys and have had good results by using a light machine oil on the small metal clips on the top of each switch or even removing these clips altogether.


SDK-85 Restoration – Part 1

Released in 1977, the SDK-85 was Intel’s development board for their new 8085 CPU. Unlike the 8080 on which it was based, the 8085 only required a single 5v power source making it ideal for small systems. Along with the 8085, the board contains a hex keypad, 7 segment displays, 256 bytes of RAM (Upgradable to 512 !) and a small monitor program in the 2k of ROM. There was also an interface for connection of a teletype.

One of the nice features of the SDK-85 is it has a large prototyping area for adding your own circuits to upgrade the system. However a downside of this is that when these boards do come up for sale, they often have their previous owners experiments soldered in. This was the case on my SDK-85 that I recently acquired…


This board was purchased on Ebay and came with a couple of extra components & connectors in the prototype area. My plan was to remove these, clean up the board and add sockets for the additional driver & ROM IC’s…

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The images above show the stages of the board clean up and addition of the new sockets. I decided to also change the 40pin socket that had been fitted for the optional RAM expansion. Nothing wrong with it but just wanted one that matched the other sockets. The next step is to add some solderless breadboards on top of the prototype area. This will allow the addition of test circuits & experiments but can easily be removed if I want to restore the back to the original spec. I’ll cover this once I’ve ordered the breadboards 🙂