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Sinclair ZX81 Teardown

Bill Detwiler cracks open the Sinclair ZX81 (a low-cost home computer released in 1981) in this TR Dojo teardown gallery.
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Topic: Hardware
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1 of 57 Bill Detwiler/ZDNet

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

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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.

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

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Along with the actual Sinclair ZX81 manual, the sell of this machine included a book of game programs.

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

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The Sinclair ZX81 weighs 12 oz. and measures 6.6 in. long by 1.6 in. high. It has a 3.25 MHz processor, 1kB of memory (expandable to 16kB), and could store data to an external cassette tape.

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

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The Sinclair ZX81 does not have a power button. A channel selection switch is located on the bottom of this model. The ZX81 was designed to output video to standard UHF and VHF television sets, but not all models have this switch.

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

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The Sinclair ZX81 uses a touch-sensitive membrane for a keyboard.

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

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Using this edge connector slot, you could attach the external 16kB memory module or a printer.

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

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

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Along the left side are the TV output socket, EAR and MIC input/output sockets, and 9V DC power socket. The EAR and MIC sockets allowed you to connect a cassette tape recorder to the Sinclair ZX81 for storage. The ZX81 had no sound capabilities.

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

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The Sinclair ZX81 has only 1kB of memory--a small amount even in 1981. Luckily, this external module let's you expand the unit's memory to 16kB.

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

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

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This particular machine has the model number ZX81 USA. There's also a reminder that there are no "user serviceable" parts inside. Guess we'll have to open the case and find out.

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

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Luckily, the Sinclair ZX81 uses standard Phillips screws to hold the external case in place.

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

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The first step in cracking open the Sinclair ZX81 is to remove the external Phillips case screws.

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

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In addition to the visible case screws, there are several screws hidden under three of the rubber feet mounted to the base of the Sinclair ZX81. The first hidden screw is under one of the feet near the back of the unit.

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

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The second hidden screw is under the foot near left, front corner of the Sinclair ZX81's base.

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

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A third case screw is hidden under the foot near the right, front corner of the Sinclair ZX81's base.

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

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With all the case screws removed, we can lift the base of the Sinclair ZX81's case away from the rest of the unit.

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

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With the base removed, we get our first look inside the Sinclair ZX81. At this point, much of the internal hardware is still out of view. All we can really see is the underside of the main PCB.

The inside surfaces of both the top and bottom halves of the ZX81's case are covered with a silver, metalic coating.

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

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The underside of main PCB in the Sinclair ZX81 is bright red.

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

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A pair of Phillips screws hold the main PCB to the upper section of the Sinclair ZX81's case.

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

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With the two Phillips screws remove, you can left the main PCB away from the upper half of the Sinclair ZX81's case.

Two extremely thin ribbon cables connect the touch-sensitive keyboard membrane to the main PCB. You can detach the pair by gripping them close to the PCB-mounted connectors and gently pulling them free.

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

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With the main PCB removed, the only component left in the upper half of the Sinclair ZX81's case is the keyboard membrane.

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

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These two ribbon cables connect the Sinclair ZX81's touch-sensitive keyboard membrane to the main PCB. They are probably the thinnest ribbon cables I have ever seen. I thought they might rip as I pulled them from their PCB connectors. Luckily, no such damage occured.

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

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Using a touch-sentive membrane instead of a typwriter-style keyboard was one way Sinclair kept the cost of the ZX81 low.

As the membrane is glued to the case, I'm going to leave it in place.

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

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There aren't any chips located on the underside of the Sinclair ZX81's main PCB, but there are a few interesting components.

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

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I'm not exactly sure what purpose this large metal strip serves. Both ends are soldered to the main PCB and there's a cardboard strip positioned under it--to prevent it from touching any of the other solder pionts. Judging from the strip's shape, it appears to touch the inside of the case's lower half.

I've seen photos of other ZX81 PCBs that lack both this metal strip and the silver-colored coating inside the case.

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

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As with the thin metal strip running along the bottom of the PCB, I'm not exactly sure about the purpose of this large metal fin. On the other side, it's attached to the same point on the PCB as a positive voltage regulator.

And the metal strip, shown in the previous photo, is bent over it.

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

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Photo by: Bill Detwiler / TechRepublic
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Photo by: Bill Detwiler / TechRepublic
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Even when compared to computers from its time, the Sinclair ZX81 has a remarkably simple PCB. There are either one or two memory chips (one on this unit), the 3.5MHz processor, an 8kB ROM, and an uncommitted logic array (ULA) chip.

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

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The Sinclair ZX81 had 1kB of integrated RAM. The memory was provided by either one 4118 chip or two 2118 chips. This ZX81 has a single Mostek MK4801 / MK4118 chip.

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

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This Mostek 8kB ROM (MK36809N-IRL) was used to store the Sinclair ZX81's  BASIC interpreter.

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

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This Sinclair ZX81 has a Zilog 3.5MHz Z80 processor (Z8400A PS Z80A CPU 8229). Nippon Electric Company (now more commoly known as NEC) also made Z80A CPUs for the ZX81.

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

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The Ferranti uncommitted logic array (ULA) chip (ULA2C184E 8226) handled several functions for the Sinclair ZX81, such as detecting keyboard activity, synchronizing the display, and processing the audio signals used for saving and retrieving data from the cassette tape.

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

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The Sinclair ZX81's ASTEC (UM 1082 LA2/3) RF Modulator can generate signals for both UHF and VHF televisions. Being manufactured for sale in the United States, this unit is likely set for VHF.

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

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

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Photo by: Bill Detwiler / TechRepublic
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On top of the Sinclair ZX81's PCB, the thin metal strip and larger metal square meet. The metal strip is bent in such a way that when the ZX81's case is closed, the strip should touch both the square and the plastic case's inner wall.

An MC7805 positive voltage regulator is also mounted to the same contact point as the metal square.

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

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

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Along with the four main chips, the Sinclair ZX81's main PCB is filled with resistors and capasitors.

That about does it for the ZX81, let's look at the 16kB external memory module.

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

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As I noted earlier, the Sinclair ZX81 has 1kB of integrated RAM. You could expand the unit's memory to 16kB with this ZX 16K RAM module, which connected to the unit via the edge connector.

Like the ZX81, the RAM module's case is held in place with standard Phillips screws. There are two screws on the front.

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

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There are also two screws on the back of the Sinclair ZX81's ZX 16K RAM module.

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

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With the four case screws removed, you can remove the back half of the ZX 16K RAM module's case.

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

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A custom-cut piece of cardboard protects the solder points on the PCBs inside the ZX 16K RAM module. You don't see this on modern-day computers.

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

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The Sinclair ZX81's ZX 16K RAM module conains two PCBs folded on top of each other. The PCBs are attached to each other with a short, bendable ribbon cable. You can lift both PCBs away from the front half of the plastic case as a single unit.

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

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More cardboard is located inside the front half of the ZX 16K RAM module's plastic case.

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

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Photo by: Bill Detwiler / TechRepublic
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Unfolding the PCBs inside the ZX 16K RAM module, we can see the actual memory chips.

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

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The Sinclair ZX81's ZX 16K RAM module contains eight chips with the markings ITT 8225 4116 4NS.

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

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On the other PCB inside the Sinclair ZX81's ZX 16K RAM module are six Motorola chips with the markings SN74LS157N I8222.

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

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Photo by: Bill Detwiler / TechRepublic
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In addition to the other two chips types, the Sinclair ZX81 ZX 16K RAM module contians a single chip with the markings 74LS393N 8223 SA.

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

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Along with the three types of chips, the Sinclair ZX81 ZX 16K RAM module has several resistors, capacitors, and other electronic components.

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

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Photo by: Bill Detwiler / TechRepublic
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With a price under $100, the Sinclair ZX81 was less than half the cost of other home computers of the time, such as the Texas Instruments TI-99/4A and Radio Shack TRS-80 Model III. And, you can clearly see that emphasis on low-price design features, such as the membrane keyboard and 1kB internal memory. But despite the ZX81's meager price tag, the device doesn't feel "cheap" and was very easy to disassemble.

For more information on the Sinclair ZX81, check out the following links:

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

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