Replacement PSU for +2A or +3 Machines

(Thanks to Neil Beadle who kindly provided the procedure and pictures upon which this article is based)

As part of the re-engineering effort that gave us the +2A and +3 machines, the original DC-DC converter circuit that made it from the Issue 1 Spectrum all the way to the grey +2 was removed, with a shift in philosophy away from generating all required voltage rails on the PCB to having them supplied directly from a multi-voltage PSU.

Sadly after all these years, a lot of such machines that have been dug out of wardrobes and attics are now missing their power supplies. On earlier models, substituting an off the shelf 9v 2A supply (centre negative) is relatively easy, but the +2A and +3 PSU’s are much harder to replicate.

The standard approach to this in the past has been repurposing a PC ATX power supply, but these are quite bulky and not really suitable for the space challenged user. However it has been found that a particular Cisco router PSU is easily adaptable, and more important both small and readily available on eBay.

Tools required

  • The PSU itself (part number: Cisco 34-0874-01)
  • 6-pin DIN male plug
  • Rocker switch (optional)
  • Soldering iron

The PSU Itself

The PSU is about the size of a regular laptop PSU, and carries the following voltages and ratings:

  • +5v: 3A
  • +12v: 2A
  • -12v: 0.2A

These more than exceed the requirements of the +3, which makes greatest demands on the standard PSU’s, needing 700mA from the +12v line.

(The +2A supply’s +12v line is only rated for 200mA, hence why it’s a bad idea to use one of these on a +3.)

The PSU has 6 wires within its cable which are colour coded as follows:

Black (2 wires): both ground
Red: +5v supply
Orange: +12v supply
Green: -12v supply
White: ROF (Remote On/Off)
Blue: RTN (Return for ROF signal)

The white and blue wires must be soldered together in order for the PSU to supply voltage, but they may be connected by means of a rocker switch also.

Rewiring the PSU

Cut the 6 pin block connector from the PSU lead, and expose the 5 wires we will need for the Spectrum side of the connection (both black wires, then red, orange and green). Using the following diagram, solder these to the 6-pin DIN plug (solder both grounds together):

Plug pinout as seen from solder side of plug.

 

It’s recommended that you insulate each connection with heat shrink tubing.

 

Installing the PSU Switch (or just being lazy)

Now you can now open the PSU itself by removing the two rubber feet at the mains input end and removing the screws underneath, the rest of the PSU case is held together with clips.

Next locate the white and blue wires running from the PCB to the 6-way cable. Cut these, leaving a decent amount of length on each so that they can be either joined together and insulated, or connected to a rocker switch to allow the PSU to be switched on and off without the need to unplug it.

The first photo shows a suitable switch along with a matching hole to be made in the PSU case to accomodate it. The second shows the white and blue wires (ROF and RTN) that must be attached to the switch, or optionally joined together and insulated if you do not want to install the switch. The third photo shows the switch in situ.

  

Finally, reassemble the PSU, test and enjoy!

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