Some PSUs need a small load in order to operate (they'll trip right off with out the load).  Here and then, some PSUs don't use the standard color coding.  See if there are any other wires with 5v, and short it to ground instead (the standby 5v supply can survive being shorted out).  If it fails to turn on after all this, the PSU may be dead.

 

5V on the green wire means it's turned off. How about the purple wire? It should also measure 5V, regardless of whether the PSU is turned on or off, as long as AC is going into it.

Was that Dell PSU made by Delta (triangle with ellipse inside)? They often require a load to turn on, if not on the +12V then on the +5V, and I have an ancient Delta that needs loads on both the +5V and either the +3.3V or +12V. Also are you grounding the green wire to a black wire or to the case? Some Deltas have the case grounded only to the AC ground prong, not to the black DC ground wires.

If the color coding is nonstandard and you don't know which wire is the Power_On signal, it's safer to use a 100-300 ohm resistor to ground any wire.

 

Well said, larymoencurly. I will add that I came across a Dell PSU that the ground for the 5vsb supply was not connected to the case of the PSU or the other ground pins; it had it's own separate ground pin right next to the green "turn on" pin. When plugged into the motherboard, the two separate grounds were shorted together by the motherboard. However, that meant that to turn on the PSU with a jumper, only one of the ground pins would work to ground the "turn on" (green) wire.

I really like the PSUs that need a load in order to run. So many power supplies have extremely aggressive bleeder resistors that consume a lot of power (one PSU I came across consumed ~30w just in bleeder resistors) and generate a lot of heat right next to the output capacitors, causing them to fail prematurely. I enjoy removing them entirely, and I've not come across any problems so far.

 

Thanks for the suggestions. The purple wire also has 5v. I tried grounding that also (via black wire) and still no go.

I checked and all the black ground wires are welded in groups to the PCB so I don't think a specific ground wire should be used.

The PSU is Dell but on the PCB it says "Wannien".

I don't have a 10 ohm resistor but I did try connecting 2 metres of nichrome wire between the red and black wires to provide resistance, still no go (the wire is rated at 19 ohms resistance per metre).

Any other suggestions?