Inside the Sinclair ZX81
Inside the Sinclair ZX81
aka the Timex Sinclair 1000 in the US
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In a previous post I wrote about the Sinclair 1000 (ZX81) that I picked up on Facebook Marketplace. I had never used one before and it was only $20 so I thought it would be worth the gamble.
Although I briefly got it to work, it seemed to stop working when I plugged in the 16K RAM expansion. I haven’t really taken another look at it, but I thought it would be worth opening it up to continue my “Inside” series and show you its guts.
This is an incredibly small computer. It’s tiny, even. It has a horrible membrane keyboard. I have no problem saying this is the worst keyboard I’ve ever used, by a large margin
.
I know these computers were very popular in England because of their low price. For many the ZX81 was their first introduction to computers. I don’t have that nostalgia for this machine so it is really hard for me to see much good about it. It seems more like a fancy calculator you can hook up to a TV more than a computer, but anyway, let’s take a look inside.
Here’s the bottom of the unit. It came apart easily by removing about 5 screws, some of which were under the pads.
Once the screws are out, the bottom case comes off easily. Here you can see the back of the circuit board in the top 2/3 of the case. An already tiny computer is even smaller!
Unscrewing a few more things allows the board to come out so the top can be viewed. As you can see, there is just not much here.
RF modulator (top right) - This provides the TV output. This computer can only output black and white with no sound.
FERRANTI chip is the Uncommitted Logic Array (ULA). This combined a bunch of other circuits and functions into one chip and is largely responsible for the ZX81’s small size.
The NEC chip in the center is the Z80 CPU.
The Sinclair chip is the ROM, primarily the 8K BASIC.
The Toshiba chip on the right is the 2K of RAM
.The connectors on the left are used to connect a cassette tape drive.
The ribbon cable on the bottom right connects the keyboard. It is very flimsy and feels like it could disintegrate at any time. This might explain why so many keys on this unit do not work.
Next up is the 16K RAM expansion module. It plugs into the back using the exposed connectors in the top right of the board.
This module stands up behind the computer and is notoriously fiddly. The slightest wiggle can cause the computer to reboot.
Inside the module are two circuit boards folded together which contain the RAM. I only see 15 chips in my photo, but there clearly needs to be 16
.
Although I have not been impressed by the ZX81, I do understand why so many are fond of it
. And why there are so many YouTube videos about how to restore them. Here are a couple from the Retro Shack YouTube channel, one of my favorites:
Yes, it is that bad. It makes the Atari 400 keyboard seem like a Selectric by comparison.
This is actually a US Timex Sinclair 1000, which came with 2K. The original ZX81 came with just 1K.
I am clearly wrong about this. Based on feedback, 8 of the chips contain the 16K of RAM. The remaining chips are logic chips to make everything work together. I might have noticed this if I had pulled the boards apart more to see chip numbers, but I didn’t want to risk damaging it.
I’ve been getting tons of feedback from people in the UK about this.
Outside of the context of the times, it makes little sense - but remember that for many in the UK this was their first ever computer. At a time when computers cost the the equivalent of a few thousand dollars in todays money (yet had no 'real' use for many), contemporary machines like the TRS-80 were simply not accessible to most people.
So when the ZX81 came along costing barely more than a tenth the price, it was a big deal. It was something that kids could ask for for Christmas, not an indulgence for adults. And with nothing to compare it to, most kids didn't mind the weird keyboard and all the other restrictions. Within months you could expand your machine with all sorts of cool add-ons - it welcomed hardware hackers - and some truly impressive software showed that you could do amazing things with it. Check out 3D Monster Maze which (again, for the time) was shocking.
It was a stunning bit of industrial design as well - the guy who designed it was from the same school of design as Apple's Jonny Ive, and with Sinclair he came up with techniques and tricks that made computers affordable before mass manufacture kicked in. Compared to the clunky, homemade aesthetics of 70's homebrew computers, this was a sleek bit of hifi equipment.
To modern eye, yes, it's primitive, tiny and awkward. A mass of compromises and very limited in capabilities. Yet it had a working BASIC, easily plugged into your TV and a tape recorder, and was supported by a vast industry of hackers, educators and user groups. It's worth comparing it to the Vic20 that came out a year later (and cost five times as much) to see what expectations of the time were.
I literally taught myself how to program on one of these $99 computers. And built a lot of hardware that attached to that external bus. 40 years later, I'm still making a great living as a Software/Hardware Engineer. -Matt D. Livermore CA USA