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

SPECTRUM START SCREEN

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.

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