This is in direct relation to an HP Pavilion a1440n desktop computer.
The Pavilion a1440n motherboard, has a 24-pin ATX main power connector.
Shown here on HP Support > Pavilion a1440n > Motherboard Specifications,
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00590499&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&product=1843644Scroll down the page to the motherboard illustration, and the photo below it.
In the illustration, the 24-pin ATX main power connector is marked -
ATXPWR. (Right upper side. Next to the Floppy drive connector)
You can see where the 24-pin ATX main power connector corresponds, in the photo.
This is a closer look at a 24-pin ATX main power cable, it's respective connector, and the corresponding connector on the motherboard.
(Not the Pavilion a1440n motherboard that is shown)
http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24The two wires you want to jump across are the Soft Power On wire, (PS_ON), and ANY Ground wire.
ALL wires with Black insulation are Ground wires.
The Green wire is the Soft Power On wire.
This is a DC circuit. A flashlight using AA, AAA, C, or D cell batteries is one example of a DC circuit.
Direct Current.
Using just one battery for an example in a flashlight, power goes from the Negative terminal of the battery, through the slide On/Off switch on the side of the flashlight, and up to the Negative side of the flashlight bulb.
Power then goes through the filament of the flashlight bulb, to the Positive terminal of the battery, completing the circuit.
By using a jumper wire from the Soft Power on wire, to ANY Ground wire, you are completing a DC circuit.
The 5 Volt Standby circuit.
When a Power Supply is plugged into power, there is a 5 Volt voltage present in the Power Supply.
By pressing the Power On button, you are making a momentary contact, which completes the 5 Volt Standby circuit.
This 5 Volts 'excites' the Power Supply, and turns the Power Supply on.
(Actually you are momentarily closing the Power On switch. The Power On switch is located inside that plastic Power On button.
The Power On switch is a Momentary Contact Switch. The two contacts touch BRIEFLY, then spring pressure moves the contacts apart.
This is an example of a generic ATX Power On switch, that I have found fit's many computers,
http://www.directron.com/atxswitch.html )
Most of the time the Ground wire (Black) that is nearest the Soft Power On wire, (Green), is the Ground wire that is used.
Just down for ease of connecting.
You can use ANY Ground wire.
Looking at the photo in the middle of the above Playtool link, you are looking at the FRONT of the connector.
Note the metal square terminal connectors, in the socket holes.
These metal square terminal connectors, connect to the power wires coming into the 24-pin connector's plastic body.
This example resemble those metal terminal connectors, except they are square in shape, while this example is round in shape,
http://www.mcmelectronics.com/product/28-9410&mybuyscid=12972124402The power wires come in the BACK of the connector.
The power wires are insulated.
This means that to connect a jumper wire to the metal terminal connector at the end of the power wire, the jumper wire has to go pretty far down in the socket hole.
The bare end of the jumper wire, has to go down in the socket hole far enough, to touch the metal terminal connector
Looking at the photo all the way to the right, in the Playtool link, where the power wires go down into the Back of the connector, is where the jumper wire goes.
The jumper wire goes into the socket hole, WITH the power wire, and the Ground wire.
Right next to the power wire, and right next to the Ground wire.
(Green Soft Power On wire {PS_ON}, and ANY Black Ground wire)
I use a jumper wire that is insulated, just like the power wires that go into the 24-pin ATX main power cable
Approximately 3 inches long, 1/2 inch of insulation is stripped off of each end of the wire, and the wire is bent into a U-shape.
Most use a paper clip, with several winds of black electrical tape in the middle, and the paper clip is bent into a U shape.
Choice is up to you, but please read on before you begin.
The U-shaped wire/paper clip is turned upside down.
The legs of the wire, or paper clip, go down into the socket holes.
Right next to the power wire, and the Ground wire.
The 24-pin ATX main power cable is still plugged into the 24-pin ATX main power connector, on the motherboard.
(As shown in the photo to the right, in the Playtool link)
I would suggest having the Power Supply unplugged from power, then insert one leg of the jumper wire into the socket hole with the Green Soft Power On wire.
Plug the Power Supply into power.
Now BRIEFLY touch the other leg of the jumper wire, into the socket hole of ANY Ground wire.
(ALL Ground wires end in one terminal point. Doesn't matter which Ground wire you chose)
You are making a BRIEF contact. No more than 2 seconds.
(Count, 1001,..1002)
You MUST ensure that the bare ends of the jumper wire, or the ends of the paper clip, TOUCH the metal terminal pins down in the socket holes.
(Green wire socket hole, and ANY Black wire socket hole)
Otherwise you are Not making a contact.
Advisement:
There will be a spark!
Warning you in advance.
Hence you may wish to wear a protective glove, on the hand that holds the jumper wire.
You may also wish to use an insulated wire, instead of a paper clip.
However,........the voltage we are talking about here is 5 Volts DC.
Two AA, AAA, C, or D cell flashlight batteries produce 3 Volts DC.
[The Power Supply in your Pavilion is an SMPS.
Switched-Mode Power Supply.
It converts AC {Alternating Current} electricity from your household, school, or business into DC electricity. (Direct Current)
There are three main DC Voltages produced by the Power Supply.
A) 3.3 Volts. (3 point 3)
Orange insulated wires are 3.3 Volts
B) 5 Volts
Red wires
C) 12 Volts
Yellow wires
ALL Black wires are ground wires.
If you use the jumper wire on the Soft Power On wire, and a Ground wire, AND the Power Supply comes on, you have a bad Power On switch.
If the Power Supply does Not come on, you have a bad Power Supply.
No if's, and's, or but's about it.
Apologize for the length of this solution, but I wanted to make it as plain, as if I were sitting next to you explaining this.