Retro Teardown—ZX Spectrum

In this Retro Teardown, we will look at the most iconic computer of British History - The ZX Spectrum!

The Spectrum History

Probably Clive Sinclair’s most successful and famous invention is the ZX Spectrum. This was arguably the most popular computer in British history and is well-known worldwide. In its lifetime at Sinclair Research, the Spectrum sold over 5 million units and had various upgrades over a period of 10 years. It was initially released 1982 and was discontinued in 1992.
Before the computer was released, Sinclair had already had produced three machines: the MK14 computer DIY kit, the ZX80 and the ZX81. These were incredibly popular models and helped to drive interest in computers in the 1970s, particularly in Britain.

But on December 1st, 1981, Acorn Computers released the BBC Micro which was a powerful machine endorsed by the BBC through the computer initiate program. This aimed to put a computer in every school and get the younger generation involved with the emerging computer market.
This was a big problem for Sinclair as he felt that there was no open competition to the BBC bid and also realised that any machine bearing the BBC logo would have serious gravitas and advantage over any competitor. So Sinclair did what he does best and developed a machine that everyone could afford and use.

The result was the ZX Spectrum that sold for only £125 (whereas the BBC micro was £225) and took the public by storm. Yes, the BBC micro was important to computer education as well as other company’s computers but the Spectrum was the machine that really stood on top.
It was not long before hundreds of games (Digger Dan anyone?) where created by both professional companies and teenagers during late evenings. Despite the fact that Sinclair despised the game industry (as he wanted a professional machine) he really was the man that brought everyone affordable computing, gaming and “Jet Set Willey”.

Till this day Spectrums are still sold, software is still written and games are played. As I write this article at my computer desk I have a spectrum setup to my left. With a dual monitor system for my PC the left monitor is also a TV and is permanently connected to the spectrum, a 32K RAM pack, and a cassette recorder.

Read More

• ZX Spectrum - Wikipedia
• Spectrum Hardware Layout - L BREAK
• The ZX Spectrum ULA: How to design a microcomputer
• World of Spectrum

The Simplicity in the Spectrum

The ZX Spectrum has a minimalistic design and is incredibly small for a computer, measuring only 223mm x 114mm x 30mm (W x H x D). The Spectrum uses a rubber membrane keyboard that is famous for its poor user-response but, personally, I feel that the keyboard is perfect for the machine.

The front of the ZX Spectrum. Such a lovely face!

The spectrum is held together using five small bolts screwed into the back. The backside also has four rubber feet (mine have disappeared over the years), the spectrum logo with some information, a serial number, and a speaker grid cut-out.

The back of the ZX Spectrum

The Sinclair logo has to be the best logo ever!

The spectrum has only a few IO ports which are a generic IO port (which contains all the CPU control lines, datalines, and address lines), a mic and ear port (for tape cassettes), a power input jack, and a video output line. I have modified my Spectrum so that it bypasses the RF modulator and outputs composite colour, which not only allows the spectrum to use modern TVs but also has better video quality.

The parallel port—this could potentially damage the ULA

The Mic, Ear, and RF outputs

The Motherboard, Processor, and Memory

The Spectrum is a single-board computer which means that all the components needed to run the machine properly are on one PCB. This not only dramatically decreases the cost of production but makes the computer easier to build and repair.

The PCB on the Spectrum is held down with just one screw near the centre.

The ZX Spectrum motherboard. Note there is no top-side solder mask

The keyboard attaches to the spectrum via two flat ribbon cables

You may notice a piece of wire soldered to my Z80 processor. This was a last minute change added to the Spectrum on the older PCBs (this is a 16KB model, issue 2) as there was a bug in the ULA (Uncommitted Logic Array).

Here is the main processor (Z80 NEC) and the Spectrum ROM that holds Sinclair BASIC. You will also notice a few electrolytic capacitors and an inductor which is for regulation of the incoming 9V supply.

Above the Z80, you will see a large number of PCB contacts which is the generic IO expander which allows you to connect your Spectrum to external hardware such as printers and memory packs.

The processor (Z80) and the ROM (D23128C)

The spectrum I own has 16KB of RAM which is occupied in eight 16KB RAM chips that each store 1/8th of the data bus. This was a common technique for old computers because it was cheaper to use eight ICs each storing 1 bit x 16,384 instead of one IC holding all 131,072 bits. As time progressed and transistors got smaller on ICs, all the RAM could eventually sit on one chip. The RAM ICs are NEC 416C 16Kbit.

16KB of RAM should be enough for anyone!

My spectrum also has eight DIP sockets that are empty for a 32KB upgrade which would boost the system RAM from 16KM to 48KB. But for me, I get 48KB by using an external 32KB RAM pack that slots into the Spectrum's IO expander port.

In case 16KB is not enough, surely 48KB is more than plenty, right?

Power Regulation

The spectrum is a very simple machine that uses a single 7805 3-pin TO-220 package to regulate the 9V input to 5V.

The heatsink for the Sinclair is rather heavy and takes up a lot of space which is no surprise when the current draw is around 650mA. Considering that most of the Sinclair supplies state a voltage output of 9V (but is actually closer to 12V), the power dissipation of the 7805 is close to 4.5W which can get really warm!

That's a beefy heatsink!

ULA and RF Modulation

The spectrum uses a ULA (Uncommitted Logic Array) which is a device that consists of lots of different logic gates in silicon. The interconnections between the logic gates are defined by the customer so a personal IC can be designed (this was before CPLDs and FPGAs).
The Spectrum's ULA handles video generation, some memory operations, IO operations, and keyboard interfacing.

Unfortunately, for many Spectrum owners, the ULA is typically the first IC to die. With no ULAs being manufactured, they're becoming harder to replace. However, you can now purchase ULA replacements which use modern ICs to mimic the operation of the ULA and restore broken Spectrums (but the inclusion of modern tech in a Spectrum is a sin as far as I am concerned).

The Spectrum uses a Ferranti ULA 6C001EX-7 IC which sits in a 40 DIP package

The ULA outputs a composite video signal which is then modulated into an RF signal so it can be used with standard TVs on channel 36 (PAL). The RF modulation chip is an LM1889N which is further supported by the many components surrounding the IC, as well as the large RF ASTEC UM1233 unit.

The video section of the ZX Spectrum. Heavy use of analog components!

Here you can see my composite video modification which essentially bypasses the RF modulation stage and leaves the composite video unaltered. This allows the spectrum to be used on modern TVs.

The black wire bypasses the RF modulation stage to output composite video (composite video hack)

A note for those interested in performing a composite video modification: I fiddled with both the tuning capacitors and resistors shown below in a line. I kept adjusting them until the picture produced had a good white colour.

Two tuning capacitors and two potentiometers enable video adjust

The PCB Underside

I was shocked when I saw the underside of the Spectrum PCB. I have never seen such an old circuit that was really clean and well built. The top side of the PCB has no solder mask and tinning only, but the underside has a green solder mask. This is to protect traces when the PCB goes through wave soldering.

The underside of the motherboard. Incredibly clean for sure!

The images you see here of the underside are the true colour! Yes, the PCB really is THAT green.

Underside near the RAM

The Sinclair logo on the PCB

Summary

So that’s the ZX Spectrum teardown showing all the internal parts and the construction of the machine. For a product that uses six screws and almost no moving mechanical parts, I am truly amazed at how well this computer has fared. The lack of dirt or dust (unlike many of my other machines) suggests to me that this machine is going to last much longer than most computers of its time.

Many people (like me) love this machine while there are others who detest it. But one fact that cannot be disputed is that this is a truly iconic machine. Next to the Apple I and Commodore PET, the Spectrum will be famous for generations to come.