This Redditor Recreated a ZX Spectrum Using Old Soviet Schematics

The Sinclair ZX Spectrum was incredibly popular in the early ‘80s because it was pretty capable and extremely affordable. In countries like the UK, it was responsible for introducing large swaths of the population to computing — similar to the Commodore 64 in the United States. And, of course, Sinclair guarded their valuable intellectual property and wasn’t keen on sharing it. But the Soviet Union wasn’t known for respecting intellectual property and the Soviets reverse-engineered the ZX Spectrum to make a personal computer for the people. Redditor MadCatUS used some of the resulting Soviet schematics to build their own ZX Spectrum.

Many ZX Spectrum clones appeared in the Soviet Union throughout the 1980s and even into the 1990s. They were impressive feats of reverse-engineering in their own right, because the ZX Spectrum was a mysterious machine. The ULA (Uncommitted Logic Array) chip, in particular, was something of a black box, but was very important for functionality. To oversimplify things, it acted as an intermediary between the Z80 CPU and the rest of the hardware. Because the ULA chip was so difficult to reverse-engineer, many Soviet ZX Spectrum clones omitted it entirely and used more conventional chips instead.

That is exactly what helped MadCatUS build their own ZX Spectrum. They used the schematics for the Дельта‑С (“Delta-S” in English), which works without a ULA chip and is therefore easier to recreate today.

Working from the original Soviet Delta-S schematics, MadCatUS carefully reproduced the circuitry of the mainboard in KiCAD. While they were at it, they also threw in some upgrades, like a 128k RAM mod and floppy drive integration. Once they were happy with the design, they were able to have a prototype PCB fabricated. They then populated it with the appropriate chips — some are NOS (New Old Stock) from the Soviet era, while others are still produced and can be bought new.

The Sinclair ZX Spectrum was infamous for its atrocious keyboard and MadCatUS wanted something better, so they also designed their own keyboard with nice Cherry MX key switches. Finally, they modeled a 3D-printable enclosure to hold everything. It isn’t a direct copy of the Delta-S, but it has similar brutalist aesthetics. The big difference is an extension on one side to house the floppy drive.

This is still a work in progress and MadCatUS needs to do a little bit more work to polish everything up. But it is already functional and they can even run ZX Spectrum games on this fascinating historical recreation.