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Sinclair ZX80

The ZX80 was released in 1980. Here for the first time was a simple and most important of all affordable personal computer that anyone could buy and use. Granted, the 1K of RAM limited what one could do straight out of the box, but with an optional RAM expansion unit of 16K fitted the little machine became quite capable.

Modern Replica - First Build


Here is a .zip file containing a variety of games and .O/.P conversion utilities I collected from all over the Internet. The .zip file contain games for 1K, 3K and 16K RAM configurations. All rights belong to the original content creators. The files are merely mirrored here for convenience and no copyright infringement is intended. Here's some more software for your ZX80. Visit the ZX Resource Centre

Grant Searle produced a gorgeous and extremely accurate replica of the ZX80. It is an impressive feat of reverse engineering. Visit his site for more information. I have managed to purchase a ZX80 replica kit. The board and keyboard quality is top notch! Some of the components provided were incorrect though. A 5.5MHz resonator was provided instead of a 6.5MHz one. Also missing was a 74LS04. Instead, the kit provided a 74LS08 which is wrong. One or two incorrect resistor values were also provided. Check the ZX80 schematic and board photo below during building to confirm component values and placement. NOTE, Grant is not involved in the production of these kits.

Here is very well designed back porch generator using a 555. The designer also provides a link to an excellent description of the circuit's operation.

This back porch generator works extremely well and produces a crisp, bright picture! The transistors can be substituted with BC550.

Picture from ZX80 on monitor using the 555 based back porch circuit below.

Unbuilt kit

Some components provided were incorrect, but the quality of the board and keyboard is excellent.

Placing Chip Sockets

The pads are tiny so use plenty of liquid flux. It helps a great deal.

Placing Chip Sockets

Chip socket placement completed. The one RAM socket is the wrong way round. (doh!)

First results on screen

Running with no back porch and 6MHz crystal instead of 6.5536MHz crystal, so video quality is still poor


The keyboard membrane has a small bump on each key and simply sticks to the PCB.

Detail of chips

One or two equivalents used but overall everything works.

Back Porch Circuit

The 74LS74 based back porch circuit is shown. Poor performance!

No Back Porch

The video output has no back porch typical of a stock ZX80

Back Porch Added

The newly introduced back porch can be seen in the video signal using 74LS74 based circuit. Allthough the back porch is present, video quality remained poor.

Video with back porch

Some improvement using 74LS74 based circuit. Not quite as much as I expected.

Without back porch

Dark video, but my monitor seems to still handle it well.

ZX80 Schematic

The schematic for the ZX80 that'll make it possible to find the VIDEO and SYNC signals for the back porch generator.

Complete board

Top right is the game controller. This is hardwired to the cursor keys. Bottom left is the 16K RAM pack.

Closeup View

To the left is the back porch generator

Front View


Running ZX80 KONG

Kempston Interface

Compatible with ZX Spectrum, ZX81 and ZX80

Kempston Controller Interface

The Kempston compatible joystick I constructed. The newly constructed RAM pack is to the right and interfaces with the joystick controller.

Game Pad

With Up, Down,Left, Right and Fire buttons

555 Back Porch Generator

Works very well!

Complete Setup

Ready for Pacman or KONG

Inside the original ZX80 (left) and my build (right) right-click open in new TAB for full resolution

original ZX80 board photo by Grant Searle

Modern Replica - Second Build

I built a second copy of the ZX80 from separate components I purchased. This board follows the original 1980 PCB layout more faithfully. Consequently, an adapter is needed for the 2732 EPROM to adapt it to the mask ROM expected by the motherboard. The keyboard membrane is smooth and shiny, which I quite like. It also replicates the original look more closely. ZX80 KONG and Pacman available here.

Video Output Modifications


Running inverted video R32=330Ω and R30=1kΩ.

The composite video output of the ZX80 isn't exactly to standard. Below is a view of the video signal into a 75Ω monitor. The signal is about 760mV peak-to-peak. It can be seen that the sync pulses are far too big relative to the peak white. The current standard values are R32=330Ω and R30=1kΩ. Better values are R32=820Ω and R30=470Ω. Video is also being run in inverted mode by connecting point A to C instead of B near IC9. Running inverted video obviates the need for a back-porch generator since the back-porch is now at black level.

Modification for inverted video

Video signal using R30 and R32 values as below

R30 and R32 (standard values)

Much improved video signal with new resistor values. Image is now bright and crisp with excellent contrast.

ZX81 Upgrade

It is possible to turn a ZX80 into a fully compatible ZX81 by upgrading the ROM to 8K BASIC and adding a small amount of additional circuitry to handle the generation of the display while the CPU is busy. It is also necessary to use a ZX81 keyboard overlay since the keyboard mapping is different to a ZX80. This will allow a ZX80 to function as a ZX81 supporting both slow and fast modes. The additional circuitry is called an NMI generator. Visit Grant's excellent site showing his NMI generator design as well as Martin's site showing his design for an NMI generator PCB. A ZX80 replacement ROM with 8K BASIC is available from here. Ask the seller to program the ROM with the appropriate ROM image. The keyboard overlay was printed on a colour laser printer.

New Software

New software on cassette for the ZX80 is sold by Cronosoft.

Downloads and Links

ZX80 Resource Centre

ZX80 books and documentation (zip archive)