Sinclair ZX Spectrum Power Supplies – What Types Are There (Part 1)?

Above is the inside of a Sinclair ZX Spectrum, the Spectrum Plus. It has a refurbished Issue 3B PCB. This Printed Circuit Board (PCB) was repaired, refurbished and enhanced by PopeyMon Games and Fun, including the DC-DC Mod that makes the power supply circuit more reliable.

It is also installed with new, quality Vishay electrolytic capacitors and a modern switched voltage regulator. The heat sink has been removed, because the original 7805 regulator and Sinclair power circuit and supply design was the source of most of the excess heat.

PopeyMon has refurbished and sold many Spectrums of all types. Another example of a 16k Spectrum is shown in this article here. These machines are sold directly if you use the Contact details here or via the PopeyMon eBay shop. I have games, joysticks, interfaces and more and will happily put a bundle together for you.

There are several types of Sinclair ZX Spectrum. The Spectrum Plus is simply a new case for the same PCB as the 16k and 48k rubber key Spectrum.

So it follows that there are several different types of power supplies for the Sinclair ZX Spectrum. The Sinclair UK1400 PSUs for the 16k, 48k and Spectrum Plus are interchangeable.

The Spectrum ranges from the rubber key 16k and 48k models during the Sinclair ownership of the Spectrum brand, as shown in the photo above, through the Amstrad ownership of the Spectrum brand (after Alan Sugar’s Amstrad bought the rights to the Sinclair Computer brand), to the present with the new Spectrum Next.

This article talks about the first Spectrums of the Sinclair era. This includes:

• the rubber key Sinclair ZX Spectrum with 16k of RAM
• the rubber key Sinclair ZX Spectrum with 48k of RAM
• the hard key Sinclair ZX Spectrum + (Plus) with 48k of RAM and black case
• the Sinclair ZX Spectrum 128 (a.k.a. “Toastrack”), although only briefly, in relation to the earlier PSUs. A separate article is forthcoming soon.

The power supply (PSU) shown in the photo above is a UK1400 model. They may also have found their way to be used on a Spectrum Plus, although the official Spectrum Plus PSU comes in a different case that matches Rick Dickinson‘s superb design of the Plus machine itself.

The rubber key PSUs and the Spectrum Plus PSU are interchangeable, because the Spectrum Plus is the same machine inside, with a new case, keyboard and reset button. They all have the same polarity, voltage and current delivery.

Model name: UK1400

Input: 240V AC

Output: -12.5V DC approx., unloaded (unregulated), maximum current rating 1.4A (1,400mA and hence the name UK1400)

Polarity: Centre Negative (not positive) Outer Positive (not negative as on most barrel connectors)

As noted elsewhere, the polarity of these PSUs is unusual – centre negative, outer positive. It is essential that this is correct, and many so-called “replacement” PSUs available have centre positive. Getting this wrong could seriously damage your Spectrum!

Years ago, I bought one from a bad seller on eBay who claimed it was compatible. When I challenged him after receiving it, he said it was suitable. BUT IT WASN’T. Don’t worry, I did not put my beloved Spectrum in danger. I got a refund. He changed his mind when I challenged him with the technical facts.

The UK1400 power supplies are not suitable for any other model of Spectrum – not the 128 “Toastrack” and not the (Amstrad-era) Spectrum +2 Grey. This is despite the fact that the DC (computer side) barrel connector will fit on these later models of Spectrum.

Why? Because the power requirements of the Toastrack and Grey are higher than the 16/48 and Plus. They require more current than the UK1400 can deliver.

This model is called the UK1400 because it delivers a maximum of 1,400 mA (milli-Amps). Or put another way, 1.4A (1.4 Amps).

There was a potentially very serious fault with the design of the UK1400 (see Safety article at the link), and there are several different designs of the UK1400. Below is a comparison of two PSUs that show one design of the case and of the internals:

Both of these examples are an implementation of a full wave bridge rectifier with single smoothing capacitor. You can see that even within the same design, there are variations:

1. Ignoring the rubbing off of the red paint on “ZX POWER SUPPLY”, the labels are different between the two PSUs. To give just two differences, although there are more, one has one style of the Class 2 symbol (the square inside a square), but the other a different style. One has the model number UK1400 underlined, but the other does not. While these may be superficial differences, they show variations even before you get inside the PSU. It is believed that different contractors came up with different designs to deliver the specification given by Sinclair in their contracts – something evidenced by the fact that there was a recall (see the Safety article).
2. Inside the PSU, there are variations even within the same design. The design in these two examples has the PCB mounted at right angles to the transformer, whereas other designs has the transformer mounted top-down on top of the PCB. The internals of the case are different between designs, so the designs are not necessarily interchangeable internally, because the mounts and orientations are different.
3. The side-mounted PCB on the left-hand-side (LHS) is different to the PCB on the right-hand-side (RHS). One has the components on side one side, and the other has them installed differently. The capacitor is mounted differently too.
4. The transformer blocks are of different manufacture. One has foam on top (believed to avoid hum when operating), the other does not.
5. The RHS PSU shows the notorious problem of the strain relief breaking. The previous owner has just “tied a knot” on the inside of the case. This is not good. It can be repaired using a clever design with heat-shrink tubing, or a replacement DC cable could be installed by someone who takes care. I have done this.
6. Both of these PSUs are faulty and have failed the PopeyMon functional and/or PAT (electrical safety) tests, so are not for sale. Only when they have been fully repaired and tested would they be put for sale. I have sold many of these.

So you can see that:

• There are a variety of designs of the Sinclair ZX Spectrum over the years.
• There are a variety of power supplies for those designs of Spectrum.
• And even within those types of power supply, there are variations.

Collectors love variations. They show the history of the Spectrum, its quirks and foibles, and people absolutely love the Spectrum. I am only showing just one implementation for one design of PSU for the 16/48 Spectrums, which I already have said, is also suitable for the Plus. There are other designs, which I have not shown here.

Once again, please see my other article on the Safety of these models. There are many of these power supplies still lying in people’s lofts, or kept nice and dry in an original box, or at the bottom of a wet shed.

Before use, in any of these cases, they need to be checked for safety, as well as functionality and that they are delivering the right voltage levels, current and polarity. So please note that they must be checked before use, and check the Safety article.

If you are looking to buy one or want one checked, please get in touch. I will happily sell you one – fully tested and working – for the Spectrum 16/48k/Plus, for the grey +2 or the black +2 or +3. I also have a lot of other Spectrum parts, working computers and games for sale. I will happily put together a custom bundle for you.

I will be writing more about the Spectrum power supplies in the near future. Watch this space! Part 2 is now here.

I would like to credit and highly recommend the excellent videos by “JoulesPerCoulomb” on Youtube. “Greetings and good time of day…” to whoever this mysterious but extremely helpful man, is. His video on power supplies was a brilliant starter a few years ago to the many hours of research that I have conducted on Spectrum power supplies, and which I present in this series of articles.