mrteaze... that answer was pure genius. I was on this problem all day till stumbled upon your precise, efficiently described solution. THANK YOU! I have my machine back. To help other peoples' searches-- I had Optiplex320 with a H305N-00 power supply, PC won't boot with power button, solid or flashing amber light.
For those that could use a few more words...
In order to start my PC I have to jump start the power supply. While its unplugged jump power supply green wire to black wire then while its running plug it into motherboard.
[Tip: use little thin jumper wire piece – on the 20-pin connector from power supply that has been pulled from motherboard, find green wire and jump to black wire on either side of it. Insert in back where wire inserts to connector. Check that power supply fan is running… plug carefully to motherboard… boot!…unplug jumper].
If I pull jumper wire before I boot with power button-- power supply will stop. But if I press power button the PC will start --and then I can pull off jumper wire.
Mind you the jumper wire will not start power supply unless I unplug the power supply connector from the motherboard first.