HP desktop power-supply LED lights, but system shuts off/hibernates during startup

The "Problem" section below describes a startup problem my 9-year-old HP desktop developed last week. You may be seeing similar behavior in your HP desktop, or even in another brand. The "Solution" section describes a simple fix (cost: ------

Problem:
Our HP Pavilion a220n tries to start up but turns itself off or sometimes hibernates in the middle of booting up. Although I got it all the way to the login screen once, it then shut off again. It is not stable enough to use or to copy our files over to a new PC.

I've eliminated as causes the power supply and the AC cord. Because sideways pressure on the switch assembly can cause the problem, I assume I need to replace the switch. The faulty power switch is HP Part # 5187-2439 and includes a plunger microswitch, 2 LEDs, 12" cable and motherboard connector. The power switch assembly doesn't seem to be available from HP or third-party sources after 8-9 years -- big surprise!

The FixYa suggested solution with 2x5 motherboard pinouts is the beginning of a solution to my specific problem. Those pinouts do match the pinouts for my switch assembly. If generic power switch assemblies matching this pinout had been available, replacing the assembly would have solved my problem.
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Solution:
SOLVED by replacing a noisy power-on/off pushbutton. Of course, you should observe all typical precautions to avoid electric shock to yourself or static discharge to PC internal circuit elements. Wearing a grounded wrist strap and touching the chassis metal to a grounded lead will help with this.

To start, you must remove the "bezel" (front panel) from the chassis after removing both side panels. Instructions for doing this are embossed on the plastic mounting flanges, and you can also access online instructions at the HP site. There's a small (about 1" by 2") plastic housing for the power switch, power-on LED, and disk-access LED, that you pop out from the chassis, making the switch contents accessible.

For convenience, I unplugged the 2-by-5 plug from the motherboard and worked on the switch assembly outside the chassis. To thread the plug and cabling out of the chassis requires removing the hard drive from its mounting bracket (no screws) and the mounting bracket itself (two screws). You may be able to do the repair without this step.

Then I verified with an ohmmeter that the original pushbutton was a single-pole, single-throw, normally open (SPST, NO) momentary switch, which closes the contact only as long as it is pushed in. It really had to be that type, considering that the switch has just two wires coming off it, and that depressing and holding the pushbutton has a different function from depressing and immediately releasing it. Checking with an ohmmeter also allowed me to verify that the switch was noisy (it intermittently closed when the plunger or housing was laterally stressed). These intermittent closures were causing the spurious hibernate events.

I bought an inexpensive replacement switch of the same type (in mini-pushbutton size, part 275-1547) at the local Radio Shack. As of today (6/16/2012) it is listed online at "http://www.radioshack.com/product/index.jsp?productId=2062539". Although the online site says it is rated at 0.5 A, the package says 3.0 A at 125 V. Any other similar switch would do as well.

In my HP model (HP Pavilion a220n), the pushbutton lives in a square hole in the 1" by 2" switch/LED mounting piece mentioned above. It is easily poked out with a pointed soldering tool or nut pick. I unsoldered the red/black wire pair and soldered them to the new switch. The small size of the mini-pushbutton allowed it to fit partway through the 1" by 2" switch/LED mounting piece. That allowed me to remount the front panel to the chassis (minus the round power-on/off cap with the power logo on it -- which was broken in my case anyway), leaving the pushbutton available for use.