Sinclair ZX81 Teardown

Sinclair ZX81 Teardown

Summary: Bill Detwiler cracks open the Sinclair ZX81 (a low-cost home computer released in 1981) in this TR Dojo teardown gallery.

TOPICS: Hardware, CXO

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  • In 1981, Sinclair Research released the ZX81 as a follow-up to their earlier ZX80. The ZX81 was manufactured by the Timex Corporation and sold as a kit (£49.95) or fully assembled (£69.95). In 1982, Timex started selling the ZX81 in the US for $99.95 as the Timex Sinclair 1000.

    Follow along as we take a peak at the hardware inside one of the first low-cost home computers.

    Photo by: Bill Detwiler / TechRepublic
    Caption by: Bill Detwiler

  • We purchased this Sinclair ZX81 from an online auction. The unit came in the original box and shipped with power adapter, 16kB external memory module, cables, and product documentation.

Topics: Hardware, CXO

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  • What a glide down memory lane...

    My dad brought one for me when I was 10 or 11. They ran an ad on Popular Science and offered "home delivery". Mine arrived late and broken. After boot, it lasted just a minute or so on (probably a fault in the power supply). We had no way of turning it back, so my dad "sold" it to a friend a bought me a Radio Shack Color Computer.

    Had wonderful days and nights programming "Enhanced Color Basic" with just 16K. (The machine is still with us and branded 4K).

    What an exciting times were does watching PCs unfold.
    • Phantom metal strip...

      Bill: I think the metal strip was a correction of the problem I've mentioned on my past post, where some models (I later found out through and old forum) gave up after only minutes turned on. My guess was the physical ground wasn't enough and the PC board was modified to to avoid this problem.

      Think of it as the "three rings of death" great great grand father.
  • Does that take me back

    I built one from a kit. I quickly discovered you could double the RAM from 1K to 2K. My fingertips still hurt from working that keypad.
  • My First Program...

    ...was written on this machine, when I was about 12 years old or so. We were too poor to own one, but an adult friend allowed me to write a checking account tracking program on it, so I was able to practice my BASIC skills which had only been tested on paper to that point. And it worked! And the keyboard was awful, as noted.
  • RE: Sinclair ZX81 Teardown

    The metal strip was intended to contact the metal coating of the back of the case to meet FCC requirements for radiative interference. This used to be a big deal with all "PC"s from the C64 to the H89 made with plastic cases. Note, too, this had nothing to do with "tempest" requirements at first as that came much later.

    The metal square served as the voltage regulators heatsink. Those were designed for 7-12 VDC input and could get quite toasty as they were linear regulators.

    The Ferranti chip was address decoding, video display controller, KB controller and so forth. It's an early version of what we would call an FPGA or just plain "Programmable Logic Array", but was laser programmed at the factory. Thus "ULA", since it could not be programmed in the field.
  • RE: Sinclair ZX81 Teardown

    Oh, note, the 74157 is a 4x2:1 mux. This allows 16 bits of address to be muxed to make the multiplexed address lines silliness of typical DRAM work. The Z80, like it's grandnephews the x86 chips. could be tricked into working with DRAM like this. Those DRAM chips are 16kx1 and had 7 address lines that take the A0/7 to A6/13 lines from the 74157 muxes and had a RAS and CAS line from the Ferranti chip and the other 74 series logic chips.

    The 74393 is a 4 bit counter that probably provides the DRAM chips their refresh and timing for proper operation. DRAM, unlike SRAM, has a lot of crazy signalling to make it work.
  • RE: Sinclair ZX81 Teardown

    I built mine from the kit and learned BASIC on it. I built my own 4K upgrade. I traveled a good bit back then and I regularly cracked open the TV connector in the hotels to connect the screen. I had a lotta fun but I missed a lotta TV back then.
  • Loverock Still Owns One

    It's his only computer.
    Ron Burgundy
  • RE: Sinclair ZX81 Teardown

    I owned a TI 99/4A which was well engineered. Perhaps a comparison would be benefic.
  • Quite clever ...

    AFAIK the address lines were scrambled so that the bank-by-bank DRAM refresh automatically fed out the video stream.
    I've just junked a cobweb-and-dust covered one to make space, but somewhere I still have a boxed unassembled kit from my old collecting days - a bit like keeping a mint stamp.
    • DRAM Refresh

      The Z80 was innovative in its time in that it included logic to automatically refresh DRAM on every OP CODE fetch. Built in. In later years designing systems that did not need this (real SRAM arrived) made timing for the program memory very tight.

      Whether or not the glue chip in the design took advantage of this is unknown to me.

  • RE: Sinclair ZX81 Teardown

    I purchased one at Long's Drug Store for $99.95 in late 1981. A short while later I bought a 16K RAM upgrade module, but it fell off my motorcycle on the way home, so I had to buy another one. After a while I had just about every accessory ever made for the T/S-1000 including a 64K RAM module, a real keyboard (that housed the computer out of it's case), a speech synthesizer, a modem, and my all time favorite - a Winkyboard that allowed programs to be loaded from a cassette tape about five times faster. It was the beginning of my computer obsession. I still have everything hoping that someday it will become valuable.
  • RE: Sinclair ZX81 Teardown

    My Mom came home one day with the groceries and in one of the bags was the Timex Sinclair--which she bought at the grocery store! It was a complete surprise.

    The thing I remember most about it was when you pushed a key it didn't type that individual letter but whatever function word that was printed on it.
  • RE: Sinclair ZX81 Teardown

    more classic Zdnet to go with the windows8 screenshots leaked.....<br>"Follow along as we take a peak at the hardware inside one of the first low-cost home computers."<br><br>'one of the first', You actually mean 'the very first' home PC and that was in the UK long before anywhere else in the world...long before the US users ever got near it, well second as you didn't build up the ZX80 the first real kit form from sir Clive, but rather Tear down the ZX81
  • Oh what Memories.

    I Still Have My ZX Model! It works and all!!
    I wrote my first programming on those Basic enabled keys!
    It was really great. I was too young to buy the kit model so my parents made the effort to buy the assembled model.
    Also my model was not the first ones and it had about 64K of memory.
    Thanks a lot for this article!

    Best regards,
  • Memory sharing

    One of the interesting things about it was the way that program memory and display memory (it was a character memory, not a full raster) were shared. Once your program reached a certain size--and that didn't take long--it would start stealing from the display memory. The display would get smaller and smaller. At that point, there were decisions to make.
  • Check those prices.

    I am sure the UK prices you state are wrong. At launch I thought the kit was ?79 or ?99 and aassembled ?30 more. I had just ordered the old ZX80 kit and I remember they sent me the new model AND refunded me the ?30. I was well pleased.
    My soldering was terrible and it didn't work until I removed about half the solder I had used! Then became a great programming tool. Ultimately lead to Sinclair QL, Psion organisers (and a bit of gap in the wilderness with MS) and now back to good tech with Apple!