What Are The Common Problems With The Rubber Key Sinclair ZX Spectrum and Spectrum +?

The Rubber Key Sinclair ZX Spectrum

In 2022, the Spectrum will be 40 years old. It was originally released on St. George’s Day, 23rd April in 1982.

So what are the most common problems with the first two models? This article looks at four of them:

  • The Keyboard Membrane
  • The TV Display
  • The Electrolytic Capacitors and Power Circuit
  • The Spectrum will not boot to the white screen and Sinclair Research prompt

Before we start, please be aware that I offer a basic test, assess and first hour of repair service. If it takes longer, then I will let you know my assessment before doing any further work. This service is available for all models of Spectrum, and at the same cost – click here for more details.

Let me know what you are looking for, by getting in touch via the Contact form or contact details at this link here. I give discounts if you buy a bundle of products or services.

The Keyboard Membrane

A New and Old Keyboard Membrane on a Sinclair ZX Spectrum + (Plus)

By far the most common problem on a Sinclair ZX Spectrum and Spectrum +, is the keyboard membrane. You might be able to get the Spectrum to display the Sinclair Research prompt (see below), but the keyboard may not work at all, or some keys work and others do not.

This is because the keyboard membranes dry out, become brittle and fail with age. It is rare to find an original one that is fully working these days*. And if the membrane is working at first, it will probably not be long before it starts to fail when the Spectrum is started up again, with some keys not working or the ribbons breaking connections with the Spectrum’s printed circuit board (PCB). The ribbons may break when you open up the Spectrum.

In the first photo above of inside a Spectrum + case, the old membrane is shown on top of a new one. The new one has been fitted to the top of the Spectrum + case. The new one is shiny. The old one is dull. The second photo below shows the completed process of fitting the membrane.

The new membrane fitted to the Spectrum +

At PopeyMon, I offer a service to source and to replace faulty membranes with new ones and to service the keyboards on the later models of Spectrum. There is a service that I provide for each model of Spectrum here – click the link:

*There is one exception – those produced by Samsung were higher-quality and last longer. But even these can fail and must be checked.

This procedure is particularly tricky on the Spectrum + and on the Toastrack to get right, and to make sure that all keys are working. The membranes are also more expensive than for the rubber key models. And hence the price difference with the service between the rubber key and +/128k. A photo of the new membrane for the rubber key, being fitted for a PopeyMon customer during a full repair and refurbishment, is shown below.

New Keyboard Membrane

While I have your Spectrum, I can also perform other repairs and refurbishments. Read on for more.

The TV Display

A Sinclair Spectrum Running Manic Miner on a Modern TV using Composite Video TV Signal

Another common problem is that when the Spectrum was brought out, it was common for home computers (called “microcomputers” back then) to use the domestic TV to output their display, and to not use monitors.

TVs in 1982 used an analogue radio wave signal, which was normal for broadcasting TV in those days. There was no digital TV signal. And the TVs were cathode ray tube (CRT) designs.

So inside the Spectrum, there is a modulator box that converts the Spectrum display to Channel 36 UHF (Ultra High Frequency).

But in 2022, this is a problem because modern TV designs do not use CRTs. And the analogue TV signal has been replaced with a digital TV signal.

If you have an old TV, this is not such a problem. But CRTs are getting old and can cause problems with reliability and safety. They are high voltage and high current.

Some modern TVs allow you to easily tune to analogue signals. Others have a complicated menu system to navigate. And other TVs don’t allow you to even tune in to analogue at all.

Modern TVs also expect certain characteristics of the signal from the old computers and the computers just cannot live up to the expectations.

So while you can do it “the old way”, it is advisable to use the composite video signal that was always generated by the Spectrum and then converted by the RF modulator. You just ignore the RF output and by-pass it.

The composite video modification is another service that I provide at PopeyMon. You can buy it from the shop by clicking here.

The Electrolytic Capacitors and Power Circuit

An Issue 3B Spectrum PCB Repaired and Refurbished by PopeyMon

A common problem in old electronics is that electrolytic capacitors can fail. Age, storage conditions, usage patterns, heat and other factors can mean that they can dry out and deteriorate from their function of storing charge. This can impact on the performance of other components in the Spectrum, and even stop it from working completely.

Of particular concern inside the Sinclair-era models of Spectrum is the DC-DC conversion power circuit on the right-hand-side of the PCB in the photo above. This power circuit is notoriously unreliable and has caused many repairs to be done. It has also damaged other components, such as the lower RAM chips that are required for the system to start successfully to the Sinclair Research prompt.

The power circuit was revised and revised again in later issues of Spectrum PCB. When Amstrad took over the Sinclair brand, it was removed completely from the Spectrum PCB and put in an external PSU.

This wise decision by Amstrad did not solve all of the issues in the later models of Spectrum, but it solved some of the problems on the computer PCB. I have other articles on this website about the models of Spectrum and a whole series of article about the PSUs. I also sell all models of Spectrum and PSUs in the PopeyMon shop.

Why have I mentioned the power circuit in relation to the electrolytic capacitors? Because the power circuit is essential to the operation of the booting of the Spectrum. If the power circuit is not producing the correct voltages, then your Spectrum will not start up properly, if it starts up at all.

So if the capacitors in the power circuit are failing, then this can mean that the power circuit is not working correctly and your Spectrum fails. It can damage components that are particularly difficult to replace.

It is therefore wise to perform checks before starting up a Spectrum. I can perform these checks and more for you, when you send me your Spectrum.

I offer a service to replace all electrolytic capacitors with brand new and quality new ones – click here. While this may not fix all types of problems, it can prevent some problems from occurring now and in the future.

The Spectrum Doesn’t Boot to the Sinclair Research Prompt


If the Spectrum does not start up to the white screen and Sinclair Research prompt, then there is a problem. When the Spectrum starts up, it performs a self-test and if the test is successful, you get the prompt. Possible causes could be the power circuit as described above. It could also be a host of other problems. If you cannot get all keys to work, then it is probably the keyboard membrane, as described above. If you cannot get a TV display, then the section above describes possible causes.

I offer a basic test, assess and first hour of repair service. If it takes longer, then I will let you know my assessment before doing any further work. This service is available for all models of Spectrum, and at the same cost – click here for more details.

Let me know what you are looking for, by getting in touch via the Contact form or contact details at this link here. I give discounts if you buy a bundle of products or services.

Thanks for coming to PopeyMon Games and Fun for all your Spectrum needs.

How Many Sinclair ZX Spectrum Power Supplies Are There?

Two Designs of the UK1400 PSU

This is another article on Spectrum power supplies in the PopeyMon Games and Fun series of articles.

In the photo above, there are two different designs of the UK1400 power supply unit (PSU). This was for the 16k and 48k rubber key Spectrums. As described in more detail in Part 1 of the Series and Part 3 of the Series, the UK1400 can also be used on the Spectrum Plus.

In the photo are two similar designs. But they are not identical. Can you spot the differences?

Spectrum UK1400 with Transformer on Top

Several other designs also exist, including designs where the transformer is on the top of the PCB, instead of next to it (see photo).

The cases can also differ on the inside, meaning that different designs may not be interchangeable. There are also variations in the capacitors, in the number of capacitors and even the polarity of the wires to the DC cable can be swapped, with respect to the colouring on the wires.

So what seems to be the same PSU from the outside, can be quite different on the inside.

Unless you have owned the PSU ever since buying it, you may not know what has happened to the PSU since it was manufactured. It may have been looked after in the original polys, and lovingly cared for during the last 35 years. Or it may have lived in a damp cellar, being damaged in bad conditions and affected by temperature variations and damp. It may have been dropped from a great height onto a stone floor.

You just don’t know!

And there is also the official Sinclair Safety Warning for one of the designs to consider, as described in this article here.

So using and tinkering with power supplies can put your Spectrum and more importantly, your health and safety, AT RISK!

You should check your power supplier is the right one, and that is safe for you and your Spectrum.

So unless you know what you are doing, please seek professional help.

PopeyMon Games and Fun has refurbished and sold many Spectrum PSUs.

Please get in touch and we can provide a refurbished power supply or repair yours.

There are more types of Spectrum PSU:

Please click on the links for more info on each power supply, and which can be used with each model of Spectrum. The general advice is that as each Spectrum was released, the power requirements increased. So earlier PSUs tend to NOT be compatible with later Spectrum models. In any case, the power connectors changed on the Black +2 and +3.

Thanks for coming to PopeyMon Games and Fun.

Spectrum +2 (Black) PSU

How Dirty Is Your Spectrum?

Above is the inside of a Sinclair ZX Spectrum +2 that I bought online. This is one of the models that Amstrad produced when Alan Sugar bought the Sinclair computer brand from Clive Sinclair. It is a later issue 4, also known as the +2B.

The ad for the computer did not have photos of the inside! I would describe it as an insect zoo. The other machine from the same supplier was not as bad, but it was still pretty bad.

It is the worst example of a machine that was not looked after by its previous owners. Many others that I get hold of, are in much better condition, and some are in near mint condition. The condition of a Spectrum is a major factor on the price, as well as whether it is fully working or not.

It is testament to the quality of manufacture by Amstrad that this filthy machine, when cleaned up, worked! It did take a lot of work though!

I now use this machine as a test machine, and have fully cleaned it up and fully tested it, with repaired tape drive and fully repaired and cleaned keyboard.

  • How dirty is your Spectrum?
  • What is the worst condition that you have seen one in?
  • Was it worse than the Spectrum above?
  • Did it still work?

Tell me in the comments below or get in touch at the contact details here!

I always try and test the computers that I buy, fix them and in some cases, fully refurbish them. That depends on what my customers want.

I sell them with accurate descriptions, as much as is possible.

If there are still any outstanding issues, I list them, so that my customers know in advance of buying. Many online sellers are not so honest.

If there are then any problems, all I ask is that they contact me to rectify them problems first.

Get in touch at the Contact details here. I will happily sell you parts or whole machines, or anything else Spectrum that you are looking for – games, joysticks, replacement cases, replacement keys, and so on. Just ask and I will check my stock.

Refurbishment and Upgrade of A 16k Rubber Key Sinclair ZX Spectrum

Above is a Sinclair ZX Spectrum. It is a 16k version, with the rubber keys. This Spectrum has an Issue 2 board that was manufactured in 1983 here in the UK. The board came to me in a bad way, and I have spent considerable time repairing, refurbishing and now upgrading it to the full 48k of memory. In the early days of the Spectrum, there were both 16k and 48k versions produced. You could then get them upgraded.

So for decades, this machine had remained a 16k. So I had a good play with the 16k games like Jetpac and Artic Galaxians after refurbishing it, but I could not play the 48k games until I had done the upgrade.

I took the 16k Sinclair ZX Spectrum to my workbench. The refurbishment has:

  • replaced the old case with one in good condition
  • replaced the keyboard membrane with a brand new one. The original membranes dry out over time with age and heat. It has been nearly forty years since this Spectrum was made, so the original was not working.
  • composite video modification for composite video output instead of old analogue UHF radio wave output on Channel 36
  • “tuning” in of the video output using the trimmers on the board, to give a vibrant picture with bright colours on the composite output
  • new shiny modulator case for cosmetic use mainly now after the composite video mod
  • heatsink and voltage regulator removed and replaced with modern “cool running” regulator
  • all electrolytic capacitors replaced with high-quality Vishay and in similar blue to originals. Over time, with heat and with age, these decay and can damage other components if they go bad,
  • foam to protect the new keyboard membrane,
  • replaced the ULA with the best ULA for an Issue 2 fitted,
  • mandatory modifications made to DC-DC converter circuit. This improves reliability of the notoriously unreliable circuit that produces the voltages for the different components on the board.

Above is the 16k Spectrum after it has been upgraded to 48k. This involves adding the extra RAM chips and some logic chips and adding a link to make sure that the upper RAM is used. I also had to replace one of the IC sockets. I checked the levels on the board with a meter before powering it up. Then I fully tested the machine using a Diagnostic ROM. All tests passed! So I decided to load Mikie, a 48k game, for a quick go!

SUCCESS! And I got further in the game than I had ever done before – level four I think it was! It is a great game, written by the legendary programmer Jonathan (Joffa) Smith. Sadly Joffa is no longer with us, but he produced some superb games, mainly for Ocean and Imagine – Hyper Sports and Green Beret being two of my favourites. How Joffa managed the technical feats of making the humble Spectrum do what he did, was amazing.

So I now have another fully working, refurbished, 48k rubber key Spectrum!

I am regularly selling Spectrums and when this website is fully up and running, you will be able to buy them here!

Get in touch and let me know what you are looking for – I am always happy to put a custom bundle together for you. I have many happy customers who are happy to endorse my work, as shown on this website.