This idea was too good not to do. So I did it.
First, I cut the old power cable from the power brick. That gave me a cable with the right sort of connector on one end, and eight wires on the other end. The wires were: black, black, blue, red, red, orange, green and white. That slightly surprised me, I wasn't expecting a white. Obviously, the usual power colour conventions aren't being followed. So I traced each wire to the connector - the connector is documented on the power brick, thus:
On my cable, black black and blue were ground, red and red were +5v, orange was +12v, green was -12v and white was on/off. WARNING - if you do this yourself, check the colours, maybe Cisco change it from time to time. Also check that the pinouts on the plug are the same. If they are different, then do what I did, and identify each wire with a continuity tester.
Then I took a 20-way power extender, suitable for a PC power supply, and cut it in half. On that, the black wires are ground, yellow is +12v, red is +5v, blue is -12v and green is on/off.
I soldered the Cisco cable ends to the extender power ends, wrapped it up in insulating tape, and now I have a cable that will let me use any PC power supply as a Cisco power supply.
And there's an idea for a business for you. If you go onto Ebay, you'll see tons of Pix 506e units for sale, at £25 or less, being sold without the PSU. That's probably because the PSU is a lot more likely to fail than the 506e itself. You'll also see power supplies for sale, at £25 to £50. WARNING - there's two kinds of units sold as 506 power supply; one has a white 3x2 plug, and won't fit in the 4x2 socket of the 506e, and the other one has a black 4x2 plug, which is the right one. The Wrong Sort of power supply is, of course, quite cheap. And a Pix 506e including power supply, will probably fetch about £50 - £80, looking at Ebay today.
So the business would be; buy any Pix 506e that comes up on Ebay without power supply for £25 or less. Buy a few cheap power supplies. I notice that many of the Dell ones on Ebay have a 4x2 way plug that looks just like the Cisco one (but I doubt if they can be used for this, because they won't offer the range of voltages that the Pix wants). This must mean that the plugs aren't unique to Cisco, so you can probably buy them somewhere. A standard PC power supply will cost under £8.
Wire the power supply to the plug. If you cant find a source for the 4x2 plugs, I'd wire a 20-way to the 506e, so that you can plug the PC PSU straight into that. Check that the Pix works, reset the password as per my previous blog, and you can sell a working Pix 506e, with "Special power supply", for, say £50, or £70 with a 12 month guarantee, and the cost to you was about £35. You should be pretty safe to guarantee it for 12 months, because as I said, the only failures I've ever had with Pixes are with the power supply, and you'll replace that for £8. The Pix is obsolete, but I can tell you that it works as well as it ever did, which is to say that it works superbly and the only failures I've ever had are with the power supply. One caveat; it only has two ethernet interfaces, so you can't use it to set up a DMZ, whereas the Cisco ASA 5505 with a base licence can be used to set up a DMZ.
It will give a throughput of 100 mbps, which is enough for any home user or small business. The competition is the Cisco ASA 5505 (150 Mbps, and judging from Ebay, they also suffer from failing power supplies, because there's tons available without PSU) which costs £300 new, £160 on Ebay.