Sinclair ZX Spectrum 128k+2A/+2B/+3 PSU Build

I like to keep an assortment of working power supplies for various vintage computers in an accessible manner, alongside all my test equipment, so I can easily power up pretty much any computer or board that I might need to.

However, I’m often reluctant to use most original power supplies, as they are inefficient, unreliable, usually difficult to service, and sometimes even downright dangerous. Some PSUs are generally okay – those with only an AC or unregulated DC output, and later switch-mode PSUs – but many period PSUs can cause equipment damage, as they can fail in a manner that causes them to output overvoltage.

Luckily, power electronics have advanced significantly over the past few decades, and retro computing is such a lucrative market that several manufacturers have started to produce modern-made switch-mode PSUs for vintage computers. However, at the time of this build there weren’t any commercially-available, fully-compatible replicas of the original PSU for the Sinclair ZX Spectrum +2A/+2B/+3. So, I decided to make my own.

An original Sinclair ZX Spectrum +3 PSU.

The original PSU (shown above) is pretty chunky, and weighs in around 2KG. The later +3 PSU, with the highest load ratings, is a switch-mode PSU so is fairly reliable, but I keep mine with my boxed Spectrum +3, so could benefit from a separate test PSU.

After some research, I found a thread online which detailed someone else’s attempt at making an equivalent PSU – apparently, a power brick for a specific model of Cisco network router (model #34-0874-01) has all the required voltages, in excess of the required current ratings (+5Vdc @3A, +12Vdc @2A, -12Vdc @0.2A).

The Cisco 34-0874-01 PSU, before I butchered it.

I won’t quote the contents of the thread word-for-word as I’m not the one who came up with the ideas involved, but the conversion is relatively simple: cut the original block connector off the output side; replace it with a 6-pin male DIN connector, and wire the pins up to correctly match the pinout required by the Spectrum +2A/+2B/+3; wire up the ROF signal for either permanent power-up, or install a power switch.

DIN connector pinout, as seen from the SOLDER SIDE of the connector (image credit: SpectrumForEveryone).

This process didn’t take too long to do. I bought myself the appropriate base power supply, a good-quality metal 6-pin DIN connector, and I ended up installing a rocker switch too, which involved cutting a small hole into the case of the supply.

It’s extremely important to triple-check that the DIN connector is wired up correctly, as if it’s wired up wrong and you plug it into a machine, it could cause serious damage.

The finished product, a modern-made ZX Spectrum +2A/+2B/+3 PSU.
The finished product compared to the original PSU.

The outcome was very successful: the new PSU weighs in around 500g (compared to the 2KG of the original), includes a power switch and detachable IEC cable, is significantly smaller in size, is much more efficient and reliable, and supports more load – all for around £25.00 all-in. I’d call that a good bargain.

Time for testing!

The new PSU working well with my own Spectrum +3.

Published by themightymadman

A conscientious, intelligent and committed graduate engineer, with excellent interpersonal skills, an eye for detail and a keen interest in hardware design, mathematics, and software development.

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