Instructions
1) Detach all wires from the larger of the two connectors on the cable. I found they just pulled out easily.
2) Identify and label +12v +12v -12v +5v +5v GND POWER_ON wires on the cable using this page http://en.wikipedia.org/wiki/Molex_connector
3) short out the power on and GND wires to turn the PSU on, or add a switch.
4) Tape up the ends of the remaining cables or pull them out.
5) Solder on the wire ends to the DIN plug using the diagram in Zetr0's post.
6) Test the wire at this point to make sure the point to point connections are correct and there are no shorts. (I shorted out a PSU doing this live, slipped, and had a nice firework, so avoid doing a live test)
7) Plug Din into spectrum, molex into the pico PSU, power brick into Pico PSU. Test spectrum works.
8 ) Make some holes in the box and put the PSU and connector in it.
I thought about using an ATX PSU, but the -12V worried me a little. As far as I know -12V was removed from the ATX specification years ago. Certainly every ATX power supply I see at work doesn't have it.
I can guess that the RS232 port on the +3 needs -12V but I don't know if anything else needs it. If it's only the RS232 that won't work without it, then no problem I guess...
However if you can find an old AT PSU this has all the voltages you need and is easier to turn into a speccy PSU anyway as it has a mechanical on/off switch rather than a standby power supply and input lines etc.
Comments
As a bonus it generates less interference than the original PSU, so you'll get a better RF picture.
Following Zetr0's advice here
http://www.worldofspectrum.org/forums/showpost.php?p=621908&postcount=5
You'll be needing:
1x ATX Pico PSU
1x 12v power brick
1x 6pin Din 240 degree (http://www.ebay.co.uk/itm/230785373343)
1x ATX 20 Pin To 24 Pin Female To Male Power Adapter Cable Connector (http://www.ebay.co.uk/itm/230705294292)
1x Box (e.g. http://www.ebay.co.uk/itm/270905519779)
Instructions
1) Detach all wires from the larger of the two connectors on the cable. I found they just pulled out easily.
2) Identify and label +12v +12v -12v +5v +5v GND POWER_ON wires on the cable using this page http://en.wikipedia.org/wiki/Molex_connector
3) short out the power on and GND wires to turn the PSU on, or add a switch.
4) Tape up the ends of the remaining cables or pull them out.
5) Solder on the wire ends to the DIN plug using the diagram in Zetr0's post.
6) Test the wire at this point to make sure the point to point connections are correct and there are no shorts. (I shorted out a PSU doing this live, slipped, and had a nice firework, so avoid doing a live test)
7) Plug Din into spectrum, molex into the pico PSU, power brick into Pico PSU. Test spectrum works.
8 ) Make some holes in the box and put the PSU and connector in it.
[EDIT] added step 3.
I can guess that the RS232 port on the +3 needs -12V but I don't know if anything else needs it. If it's only the RS232 that won't work without it, then no problem I guess...
Mike
However if you can find an old AT PSU this has all the voltages you need and is easier to turn into a speccy PSU anyway as it has a mechanical on/off switch rather than a standby power supply and input lines etc.
And an additional step (will also edit post above)
Remember to short out the power on and GND pins to turn the PSU on, or add a switch.