Rachel Has Problems

The developments of UK personal home computing in the 1980s was fast paced with new machines being introduced year on year. Few are as recognisable as the Sinclair ZX Spectrum, arguably today it could be considered an icon.

History

The story of the ZX Spectrum, and Sinclair, has been told numerous times online by far better writers than me, so I will do a short summary. The ZX Spectrum wasn't actually Sinclair's first computer released to the public, that came two years earlier (1980) in the form of the ZX80. Available in both kit and assembled form, it was one of the first computers released to UK consumers at a price of less than £100 (£79.95 for the kit, and assembled for £99.95, that's £331.63 and £415.59 in today's money) ¹.

A year later, following on from the ZX80, Sinclair released the ZX81 again in both kit and assembled form. The ZX81 was sold at an even lower price of £49.95 for the kit, and £69.95 for the assembled version (£242 and £339 respectively in today's money). Sinclair sold thousands of these computers despite only having 1kB of RAM ². Apple's 2 Plus, and Commodore’s PET both outperformed the ZX81 with 16kB of RAM, but with prices in excess of £400 these machines were out of reach for much of the UK market ³. Sinclair had found a gap in the market, and produced computers affordable to the working class.

The ZX Spectrum

It's Friday 23rd April 1982, Paul Mccartney & Stevie Wonder are top of the UK charts with Ebony And Ivory and a bitter dispute between the British and the Argentineans is playing out. Sinclair launch the ZX Spectrum at a cost of £125. Available in both 16kB and 48kB RAM varients, the design was distinctive, an all black case with rubber keyboard and a rainbow motif ⁴.

Demand was huge, selling over a million units within the first year alone ⁴. It spawned the creation of thousands of video games many produced in bedrooms up and down the country, the ZX Spectrum sparked interests in computer science for many, and showed countless more what computers where capable of.

Developments after 1982

In 1984, Sinclair would launch the ZX Spectrum+, more of a design change than anything with a new studier case and keyboard design, under the hood the electronics was largely the same as the original Spectrum. In 1985, the ZX Spectrum 128k was released which upped the memory to 128kB RAM and introduced a re-worked "operating system" supporting both 128kB and 48kB of running modes. However in 1986, Sinclair sold its computer rights to rivals Amstrad following on from financial troubles. Amstrad would continue to produce Spectrum models up to 1992, at this point more advanced 16bit computers were dominating the computer market and cheap games consoles were starting to become available⁵. After 10 years, it was all over. All in all it is estimated that over five million ZX Spectrums were sold in its various forms making it one of Britain’s best selling computers ⁵.

2025

OK, you're probably thinking what's this all about. Why have I just randomly summarised the story of the ZX Spectrum? Why am I so fascinated by this computer when I wasn't even born until the late 90s? You see, the thing is, I appreciate retro tech... some more than others I admit. I think it's important to look at these historical developments to really see just how far we've come and how they made things work in the past. I've previously used a ZX Spectrum+, but never an original ZX Spectrum. I'd never even seen one in person until December 2024 when I became the owner of a ZX Spectrum... and no, this isn't a recreation spectrum.

My ZX Spectrum

This is my ZX Spectrum that I purchased second hand on eBay in December 2024, and it's in superb condition! The same can't be said for the original box, which is falling apart.

After finding a television and connecting it to the RF port on the Spectrum I powered it up and I was greeted with "© 1982 Sinclair Research Ltd" it all appeared to be working fine. To be sure that all the RAM was in working order I used a diagnostics ROM to check the memory. Failed RAM chips aren't uncommon in these machines due to the parts used but I was pleased to see that all RAM was in working order, as was the speaker and keyboard. Excellent, time to take a look inside.

Inside my ZX Spectrum

My ZX Spectrum is an Issue 6A revision, the last revision of the "traditional" ZX Spectrum 48k model before production of the 128k. What is interesting about the later 6A issue boards is the use of the ZX8401 or PCF1306 IC which was utilised to control access to the memory chips, this IC was developed for the 128k Spectrum, but the later 6A issue boards also got to utilise this IC. The PCB is copyright dated as 1984, but each IC has its own date code as follows:

I don't know the details of Sinclair's supply chain, so I can't comment on how often they would receive electronic parts for the manufacturing process, however I think it would be fair to assume that this particular spectrum was manufactured in the first half of 1985. Regardless, that makes this particular Spectrum 40 years old this year, and it's still working!

All though small in comparison to other machines on the market at the time, it's worth quickly mentioning the RAM. Originally the ZX Spectrum was available in either a 16k or 48k variant. If you purchased the cheaper 16k model, your Spectrum was installed with the 16kB of "lower" RAM (The 4116 ICs), this RAM was responsible for the screen buffer and so any programs could only utilise the remaining memory. On the original issue 1 boards, there was no space to fit 32kB of additional memory so a daughter board had to be fitted. On subsequent board issues, the additional 32kB of RAM was designed into the motherboard. By ordering the 48k model, the Spectrum was simply manufactured with the 16kB of lower RAM for the screen buffer, and then an additional 32kB of "upper" RAM installed for user code (totalling 48kB). In order to keep costs down for the end consumer, after all this was the gap in the market Sinclair was filling way back to the ZX81, the additional 32kB RAM ICs used were faulty units. The 32kB RAM ICs are actually 64kB with two banks of 32kB, however some of these RAM ICs have a problem with one half (or bank) of the memory making that bank inaccessible for use. Texas Instruments and OKI would then both sell these ICs as 32kB RAM at a discounted price to which Sinclair made good use of. The TMS4532 RAM chips installed in this particular Spectrum are marked as 20NL4 indicating that the upper 32kB of the RAM bank is working.

The Plan

So you may well be thinking why, Why did I purchase a 40 year old computer? I've always been a techy person, geek if you like, and I want to see if it's possible to push the Spectrum to do things not envisioned in the 1980s. I am drawn to the designs and quirks of these old micro computers, those that really kickstarted the computer revolution. We all take our desktop tower PCs, laptops, phones and tablets for granted these days, but I think it's important to see where we came from. There is an element of preservation to this too, becoming the custodian of something ensuring it sticks around for the future (or something like that).

So, speaking of preservation...

What's been done so far

Admittedly my spectrum is in really good condition with no damage and little cosmetic defects to the faceplate of the keyboard. The only things I've done to this Spectrum so far is:

• Give the motherboard a light dusting and clean up the contacts on the interface edge connector
• Lightly clean up the keyboard membrane conductor tracks where they are plugged into the motherboard
• Given the plastics a bath in warm soapy water, same with the keyboard faceplate and the rubber key assembly.

As mentioned, I've also tested using a diagnostics ROM the functions of the Spectrum, everything appears to be fine.

What I'm planning on doing

Some would argue I should leave the Spectrum as is, however this is a later 6A issue board of which there are plenty of other examples, therefore I intend to carry out some modifications, these being:

• Replacing all the Electrolytic capacitors or "Re-capping." As far as I can tell all the capacitors on the board are original, there's some Philips capacitors installed which I'm not sure if they're newer replacements but all in all it's a worthwhile job as capacitors don't last forever.
• Modifying the TV output to pass through the modulated video signal and "bypassing" the RF modulator. At the time of manufacture, composite video inputs on domestic televisions was not necessarily commonplace, however these days with the close-down of analogue television, more and more televisions are not coming installed with analogue tuners. Composite inputs are commonplace and can be easily converted with adapters and thus I wouldn't have to rely on using a television, any monitor would suffice. There's also noticeable interference to the RF output from the Spectrum itself which may be resolved when using video output unless the interference is being inferred onto the electronics prior to the video modulator. I haven't looked into this in depth.
• Replacing the 7805 voltage regulator with a 5V buck converter. This will mean I could do away with the heat sink, or if not a buck converter replacing with a new 7805 as the original regulator is surely getting tired.

I plan to document this work on the blog so keep checking back for updates, you could even subscribe to the RSS feed!

I hope you found this post interesting, you can provide any feedback or comments under the associated Mastodon and BlueSky posts sharing this blog post.

Rachel x