Two Designs of the ZX81 PSU

This is another article on Sinclair power supply units (PSUs) in the PopeyMon Games and Fun series of articles. This one is on ZX81 PSUs. I have another set of articles on ZX Spectrum power supplies for all models of the ZX Spectrum.

There are at least two types of ZX81 power supply. There may be more, but I have not come across any more than just the two in the photo. If you know of others, please get in touch:

  • the UK700, which can deliver a maximum of 700mA (0.7A)
  • the UK1200, which can deliver a maximum of 1200mA (1.2A)

In the photos above, there are two different designs of the ZX81 PSU. On the left is the UK700 and on the right is the UK1200.

The cases of both UK700 and UK1200 are the same. The UK1200 for the ZX81 is almost identical to the Spectrum UK1200 that I show in one of the Spectrum articles. It uses a different connector, so extra care must be taken to check polarity. I have also not tested this out, so please do not do this unless you are sure what you are doing.

The capacitors and the circuits are quite different.

The UK700 does not have four separate diodes to create the full wave bridge rectification. It uses a special component and has two smaller 1000 uF capacitors. No fuse is visible on the mains side of the transformer, although this may be hidden.

The UK1200 has four diodes for the rectification, and one larger 2,200 uF and one smaller 1,000 capacitor. The fuse is visible on the mains side.

The UK700 may struggle if you are using peripherals with your ZX81 that use more current than the ZX81 on its own, but the UK1200 should be able to handle peripherals better.

I have used both the Sinclair 16k RAM Pack and the Memotech 16k RAM Pack successfully with both the UK700 and UK1200.

But I would be concerned as to whether the UK700 would be able to deliver the current for a ZX Printer. The UK1200 would be preferable. I have yet to test this though, as I don’t have a working ZX Printer.

Please see the warning below about the dangers of power supplies. Even those that deliver low DC voltages to the computer, still involve mains voltages which can kill.


In any case, whether you do understand or not, it is wise to check the safety of any PSU before use. Why?

Because 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 ZX81 polystyrene and box, and lovingly cared for during the last 35-40 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 even been dropped from a great height onto a stone floor. Or dragged along tarmac by your delivery courier.

You just don’t know!

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

PopeyMon Games and Fun has refurbished and sold many ZX81 and Spectrum PSUs.

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

Thanks for coming to PopeyMon Games and Fun.


  1. I can confirm (when I purchased mine back in the 80’s) that the larger 1.2 amp power supply for the ZX81 was sold with the printer, that needed extra power to run.

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