Your ZX has evolved!

It seems I am never quite happy with the placement of my ZX Evolution board. This time I have placed the ZXevo inside a Spectravideo case. Previously, I have already changed the SVI keyboard circuit into a ZX Spectrum keyboard matrix, so it was not too much work to connect it to the ZXevo. Also the RGB and Joystick connectors were ready so I did not have to make them anew. (Fortunately the connectors could also be squeezed through the openings so I did not need to resolder them.)

With that old attitude of "trying out gets the things done" without pre-planning and measuring too much, I found that the evo board seemed to fit nicely inside the case. There never was any question about depth and width, but the height was a bit worrying. The keyboard block is rather bulky, and the pictured configuration leaves very little room between the board and the keyboard circuitboard.

It sure looks messy. The boards are fixed into place with screws, and for these I drilled new holes directly through the bottom. The SVI case is well elevated so it does not matter much if the screws stick out slightly. The board inside is slightly raised with plastic pieces, as the existing screw holders might otherwise get in the way. Besides, there's a metallic nut in each of the holders, and it would be a pity if these created some unexpected contacts with the board circuits... So I also put some cardboard between the case and the board.

To get all the connectors visible from the backside, some more cutting would be required, but here at least the VGA and the SD card are useable. Also, If I want to use a better PS/2 Keyboard, it's within reach.

Here's some of the details:

1. The joystick connector. This fits nicely into the existing opening, and screws hold it in place. The board end is shared with the keyboard connector. I made that part to be two separate chunks, so that the joystick wiring does not hang around uncomfortably when separating the keyboard part from the case.

2. The RGB connector. It's a bit pointless to use the ZXevo with a stupid VGA monitor, so RGB is preferable. I had to carve the SVI "TV" opening a bit to make the DIN fit. Now it fits so snugly it does not even need screws really. The SD card can be seen to the right, sticking out from the opening.

3.The Reset switch. This is the one almost absolutely necessary thing to make when working with Evo if you don't want to connect a PS/2 PC keyboard (=boring). Without a separate reset switch (to Jumper 6) it's not really possible to get at the service screen again without pulling the plug! This is just a simple microswitch that sticks out from the Spectravideo power switch opening. I used a separate plastic piece screwed in place to the bottom to hold the microswitch in place.

Only the basic keys are connected here, as connecting all the cursor keys and function keys and such would require a bit different kind of thinking. There's a possibility of using some of the keys for the Evo jumpers (such as the reset above), as they do not interfere with the keyboard logic, but dismantling the keyboard block gets a bit tiresome after a few times so I did not take this route.

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.