With the behavior you're observing, there could be a short circuit on the motherboard side of the 4-pin connector, preventing power-up when it's connected. You could check for indications of this with a multimeter. With the 4-pin lead disconnected (and not connected to the mains of course), measuring from one of the motherboard socket pins the black wires connect to across to one of the ones the yellow wires go to should measure quite high in resistance. (Either "black" pin to either "yellow" pin.) Depending on the exact design of the circuitry, several kilo-ohms upwards - if a continuous low resistance is shown, e.g. zero, a few ohms or tens of ohms, that would indicate a component in the core voltage switching regulator is shorted or leaky. That would most likely be a surface mounted Mosfet or diode, rather than something like an electrolytic capacitor, so not readily repaired. Such a fault could also damage the CPU unfortunately.
Edited by Platypus, 24 March 2011 - 05:41 AM.