Sunday, 13 January 2013

Work in progress: Spectrumvideo 48k

The 48K Spectra. Or the Spectrum-Video ZXSVI748
I happen to have a Spectravideo MSX 728 in poor condition and as it is unlikely to have any other future, I wanted to try to use its keyboard for some project. My thoughts went naturally to the ZX Spectrum. First I toyed with the idea of building a new case entirely, but pretty soon I was positioning a rubber-key Spectrum motherboard inside the Spectravideo case. 

The 728 case is huge, so it's not a big challenge to fit the board inside. I was pleasantly surprised to find out that the openings for connectors are already in very useful positions. 

Trying the motherboard for size, before removing the screw holder.
Only one screw holder has to be removed and something needs to be done to the power supply connector. The question at the moment is if all important peripherals work, despite the good fit. An Interface II type flat peripheral poses no problems. The vertical modules may be a bit more difficult, as they collide with the backside of the computer. At least I'd like to connect the Divide 2k11 to the finished computer.

Looking from behind: 9V, Peripheral connector, tape connectors, TV out.

The Spectravideo keyboard unit.

The SVI keyboard is a removable unit with a straightforward matrix-type connectors. The matrix wiring is very convoluted and it's not really possible to salvage any of the existing lines, so the simplest thing to do was to severe all connections between the keys and create new lines.

The existing connections are destroyed by sawing between all the key positions.
As can be seen, this is not yet the full treatment.
Again, I am torturing my woodworking saws: The result is ugly, but it works. It also feels like the most disrespectful thing I've ever done to someone else's design.

Another option might have been to do a new circuit board, which would have been a first for me. As the board is quite big and would have to perform well against wear in a mechanical/electrical role, I did not want to try such a thing as my first board project.

The ZX Spectrum membrane connections are first drawn for reference.
The most gruelling task is putting the new connections in place. The only way for doing this I could come up with was to join each connector with a small wire, trailing from hole to hole. After drilling two small holes next to each of the connectors, the wires could be then be laid underside the board. (I used a 1,5mm metal drill.) The wires are pulled to the topside, where they are soldered to the key connections. Each of the keys is connected twice, for the matrix columns and rows. 

Between the circuit board and the plastic keys there is the rubber mat
that holds the conducting pegs.

I now have the normal Spectrum keys working. The keyboard is connected and in place. The keyboard works very well with the Spectrum, and it is certainly nicer than the rubber keys or the Spectrum+ keyboard. The contacts work well even if I'm not a very experienced in soldering. There is enough leeway in the rubber "domes" underside the keys, so it does not matter that the soldering is a bit lumpy. This in mind I'd say the SVI keyboard is very good for this kind of mod. 

Testing the keyboard connections.
The neat things are yet to come. Now that I have the essentials working, I can start thinking how to wire all the SVI special keys and what they might do in the Spectrum context. To get keys like comma and period working directly on the Spectrum, I have to connect them both to the symbol shift and the relevant key at the same time. 

There's also plenty of room inside the case, so that's another possibility for some creative uses. I'm also thinking that the ZX Evolution board could go very nicely inside the SVI case.

Edit: A word of warning! I have suggested that building keyboard combinations (Backspace, cursor keys, etc.) would be a simple matter of connecting the key to the relevant inputs on the motherboard. However, it does not seem to be quite as straightforward. The Spectrum+ and Spectrum 128 keyboards, which have these kind of combinations, use a three-layered keyboard membrane. The functions of the membrane are not easy to replicate with wires. I may have ruined the ULA chip of my Spectrum by messing around with a poorly built "Delete" key circuit, although the cause is not absolutely certain. I need to learn more before I can suggest a solution, that is, if it is possible at all - the Spectrum+/128 solution is mechanical. I still think it should be pretty safe to create the normal Spectrum keyboard circuit, as I have shown above, without any combination keys. Of course, in any case it makes sense to turn off the machine when inserting the connectors.


  1. Menisiköhän noihin Spekun emolevyllä oleviin liittimiin ihan piikkirima?

  2. Itse itselleni vastaten: piikkirima sopii juuri oikeille kohdille näppisliittimeen.

  3. Kyllä, sepä juuri, ei sattunut olemaan käsillä niin yritin kikkailla osat naaraskappaleesta. Finaaliin tulee piikkirimaa..

  4. Wow! I had one like this (snif, snif)
    This was my first personal computer and I loved it

  5. For me the Spectrum was first :) I did have access to a Toshiba MSX around the later 80s. I remember that the BASIC was very fast and the manuals were very instructive.