ZX81+35 is a clone of the Sinclair ZX81, started in 2014, and finished in 2016, that uses modern (SMT) components on a small (10x10cm) PCB, it only uses commercially available parts, so no ULA, and works with the original ZX81 ROM code.

Previously (five years ago I named it ZX14) but after two years working on it, I changed my mind and renamed it to ZX81+35 to clarify that my copy was first designed 35 years after the original

There are three versions of my clone, the first prototype was called ZX81+34 (Rev 1.0) and a prototype was build and tested. This PCB needed many patches, for example the 5V and GND signals on the micro-USB port was inverted, and the footprint for the crystal oscillator I chose was wrong.

The second PCB I made and tested was called ZX81+35 (rev 2.2) and was tested in 2016, and only needed a minor patch to work, and some patches to improve cassette loading.

The third version was never tested, but all improvements found were implemented in the layout, and patches tested on the Rev 2.2 PCB. this latest version was rev 4.0 This version fully worked as a ZX81 clone, but was not released because I could not get the turbo (220% overclocking) to work with a stable picture, so I stalled its release, and eventually I gave up on getting turbo mode to work. And started to work on a color home computer called RhoCoCo, information on that one can be found here: https://revspace.nl/Designing_the_RhoCoCo_Retro_Home_COlor_COmputer_hardware

A complete step by step description of all improvements I did for Rev 4.0 upto september 2016 can be read here: https://revspace.nl/ZX81PLUS34_ZX81_clone

Recently (june 2019) I became aware there is interest in my project, so I'm willing to release the latest schematics, BOM, gerbers and drill files here, so anyone can pick this up and create his own version. For now I uploaded the files I have, some are on an old laptop, so will take some more work.

I updated the older BOM so its compatible with revision 4.0

I also designed a keyboard overlay for a keyboard with 40 6x6mm buttons mounted in a 1/10" raster. I designed a (single sided) keyboard PCB for it, but never tested it, as my protoype keyboard was simply build up on veroboard, Iḿ reasonably sure though that the keyboard PCB is compatible with rev 4.0, if I can find the old layout and check it for compatibility with Rev 4.0