These projects are based on the ZX81 Computer.

The Euro-ZX81 is my first design which I made several years ago. At the time my most important concern was to create a working ZX81 as at the time I didn't have many proven ZX prototypes yet. In fact, first I made the original ZX97, then next came a simple ZX80 -> ZX81 design which I finally split up more evenly into the Euro-ZX81 which consists of two 10x16 'euro-size' PCBs. They are piggy backed on top of eachother into a nice space-saving design.

Next came the Micro-ZX81, which I designed a few months ago. Unlike the Euro-ZX81, the Micro-ZX81 is not based on a ZX80 -> ZX81 type of design, its design is actually derived from the ZX97. An advantage of this is the fact that because of including Wilf Rigter's excellent ZX97 timing circuits, no capacitors are used for timing adjustments. Another feature of the Micro-ZX81 is it's size! The dimensions of this single PCB design are 10 x 8.7 cm, or about 3.9 x 3.4 inch. This is made possible because of using mostly SMD technology with double-sided PCB layout and double sided component placement. This combination saves a lot of space! However, please be aware that using this kind of design density, it will require great care in building. The biggest problem we will encounter when building such a PCB is the fact that many vias need to be made underneath the already very small SMD ICs which also are situated very near to the PCB. After soldering in the vias, we need to apply quite a lot of soldering skills to get the IC's properly situated. Mostly it will mean that the legs of the ICs need to be bent to come closer to the PCB pads in order to be soldered. So use some caution and prepare to be very precise in your work when choosing this project!

Now, about the ZX81 Issue 4 project. This is my latest design for which I had formed plans recently in 2006. I chose for this name because as far as I am aware of, the original ZX81 was only made in three revisions, Issue one, Issue 2 and Issue 3. I gave my Issue 4 PCB this name because this PCB is intended to be built as a replacement for the original ZX81 board when it's ULA has broken down and no replacement is available. The ZX81 was sold in great numbers, and ULA failures, due to cost reduction in the manufacturing process which amongst other problems caused a high operating temperature, are also well known amongst people familiar with the ZX81. One tip for owners of working ZX81s: use a heatsink with plenty of heat conducting paste(because of it's not very straight surface) on the ULA, do be aware that opening the ZX81 might cause more problems in itself such as broken keyboard membrane strips. The best solution for this problem is desoldering the original keyboard connectors from the ZX81 board, carefully soldering some flatcables to them, shortening the membrane strips, carefully inserting them back into the connectors with wires and then glueing the connectors solidly to the upper shell of the ZX81 case. In this way, you ensure that opening the ZX81 case will not cause any future stress. To get back on subject, this Issue 4 PCB is intended to replace a broken original ZX81 PCB and to actually be built inside an original ZX81 case.(in my opinion a very cool design by Sinclair) Amongst other things, it has the same dimensions, same screw hole positions, same connector locations. For this PCB I have used a similar design to the Micro-ZX81, except not using SMD but conventional DIP ICs. However, it was necessary because of limited space(about 10x14cm) to design a double sided PCB. Otherwise it would have been impossible. This project is now completed and tested. (14-10-2006)