Jurassic Tech: The Sinclair ZX81



Sinclair ZX81, RF modulator & Mac mini.

The Sinclair ZX81, released as a solder-it-yourself kit in 1981, reflected its times. It was a bridge between the computer kits that became popular in the 1970s (beginning with the Altair) and the ready-to-run PCs that were soon to follow.

Released the same year IBM unveiled the original IBM PC 5159 — that was August 1981 — the Sinclair ZX81 especially foreshadowed the small, inexpensive computers we see today.

It was mini-er, though not mightier of course, than Apple’s first Mac Mini of 2005. Weighing in at just 12 ounces, the ZX81 was much smaller, though a bit longer.

The tech world of 1981, to anyone who doesn’t remember it, would certainly seem like a strange alternative reality. IBM, fearful of losing its big computer advantage in businesses, released its IBM PC that year in response to guerrilla employees sneaking Apple IIs running Visicalc into offices. And Radio Shack was selling its TRS-80 — or the “trash 80,” as most of us called them.

The Sinclair ZX81, at $99.95. was the second kit from Sinclair Computers. And it cost half as much.

What a deal it was. The kit consisted of a printed circuit board, four chips and a handful of other components. Even a person with modest soldering skills like I had could put it together in an hour or so.

Sinclair ZX81.

It shipped with 1K (1,024 bytes) RAM and a NEC ZX-80A microprocessor that was more or less equivalent in power to the processor in the TRS-80. It also included a version of BASIC baked into firmware.

In my pics, you can see the RF modulator I used to connect the ZX81 to a TV set I used for its display — we’re talking 64×48 pixel resolution here.

You see, the ZX81, designed in the United Kingdom, had an integrated RF modulator that wouldn’t work with the NTSC TV sets we all had in the United States.

The exposed fingers of metal on a connector in the back were for expansion — you could add memory and other capabilities that way.

Here’s a view at the bottom of the unit.

Sinclair ZX81 bottom.

The side of the unit featured a 9V power jack, video out to the RF modulator and earphone and microphone jacks. The earphone and microphone jacks were not for sound, though. You used them to store and retrieve data from tape — ordinary cassette tapes.

Sinclair ZX81 side with ports.


Sinclair ZX81 rear.

The ZX81′s keyboard, as you can see in the photos below, served triple duty. It let you input letters and numbers, some graphical patterns and some Sinclair BASIC keywords — those were associated with each membrane key.

Sinclair ZX81 keyboard.

Here’s a closer look.

Sinclair ZX81 keyboard closeup.

The ZX81 was resource poor and slow even by 1981 standards. But it was an interesting diversion at the time and, I believe, well worth the $99.95 plus shipping I spent on it.

  • Monk Canafax

    Kit? I bought mine fully assembled from large chain store for $99! Later added the 16KB Ram pack for another $50.

  • Jeremy Lesniak

    It looks like a postage scale!

  • Todd Ogasawara

    Monk: There was an assembled product in the U.S. sold as the Timex Sinclair ZX81 (vs. Sinclair ZX81 without the Timex branding). That is the model you bought. It became available after the kit version

  • Berick

    The ZX81 became the center of an ecosystem, with add-ons, software, magazines and user groups. There were even first-person shooter games.(Sinclair also later made the more-powerful Spectrum.) I remember seeing the user groups still scheduling meetings into, I believe, the mid-90s, with talks about connecting your ZX to hard drives and such.

  • Todd Allcock

    I had the pre-assembled Timex version with the 16KB expansion pack. It’s amazing to remember that people used to be able to write decent functional software measured in KB rather than MB!

  • Todd Ogasawara

    FYI: I found the ZX81 assembly instructions and schematic diagram. You can find an addendum with photos of the document at: http://bit.ly/rs2mJc

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