What is it?

This is a self assembly Z80 based computer kit. It is compatible with the Sinclair ZX81 computer released in 1981. It has been designed using modern 74HC series logic to be fully compatible with the ZX81.

This version includes a built in 40 key keyboard using tactile switches. An overlay PCB sits over the switches to indicate all the additional functionality each key provides.

This is a tried and trusted design, I have been selling these for 5 years now. The kit comes with all new modern parts which should mean every kit built works first time.

The Z80 has now been discontinued, but I have secured stock from one of the last batches of Z80 CPUs. All kits will be supplied with Z80s from that batch, date code 2427. While stocks last as they say.

Why did you make it?

An original ZX81 is a very minimal design with all the logic combined into a single chip. This ULA chip is a common point of failure, as are the vintage ROM and RAM chips. This is an ideal drop in replacement for a dead ZX81 board, or an upgrade for a functional one. This is an opportunity to build and program one of the first home computers.

What makes it special?

The upgrades from the original design include a full 32K of RAM on board. A ROM chip with the option of eight ROM images, and is supplied with ZX81 Floating Point BASIC ROM.

Unlike the ZX80 and Minstrel 2, this includes NMI "slow" mode, so is flicker free and compatible ZX81 software, including many high resolution graphics tricks.

The video output has been switched from a UHF modulator designed to drive a TV set to a composite video output that should suit a variety of TVs and monitors. It does not include a colour burst signal, so may confuse some composite to HDMI adapters. Jumpers can be used to select 50Hz or 60Hz frame rate for PAL or NTSC regions, and normal or inverse video.

The video signal has been cleaned up to add the back porch to the video signal, which was missing in all but the last revision of the ZX81. It also adds pixel synchronisation to ensure even sized pixels and no glitches from adjacent inverted and non-inverted characters. These improvements make very good solid picture on a modern LCD, just like the original would have been on a 1980s black and white portable TV.

Power is by 9V DC 2.1mm jack. Current consumption is around 80mA. Jumpers select either centre negative (ZX Spectrum, Commodore 16, guitar pedals) or centre positive (everything else).

The kit includes a 7805 linear regulator. No heatsink is required for normal operation, but if you plan to add a lot of expansions, you could fit a 7805 switching regulator.

All through hole parts are used making this kit suitable for an intermediate level kit builder upwards.