The Green LED light flashing indicates a bad Power Supply. Weak voltage power rail.
http://h10025.www1.hp.com/ewfrf/wc/product?product=404643&lc=en&cc=us&dlc=en&submit.y=7&submit.x=7&lang=en&cc=usTroubleshooting Power Supply Issues HP Pavilion A520n,
http://h10025.www1.hp.com/ewfrf/wc/document?docname=bph06788&tmp_task=solveCategory&lc=en&dlc=en&cc=us&site=null&lang=en&key=null&product=404643#N207
Click on > Power Supply light is on or flashes
"If the LED is still flashing (it should not flash with all connectors
removed), plug the power cable into a different power outlet that is
known to be good. If the LED still flashes, the power supply should be
replaced."
Cause?
The leading cause is the computer is dirty inside, to include the Power Supply
The next cause is that Electrolytic Capacitors have failed inside the Power Supply.
Solution is to replace the Power Supply.
The Power Supply used is the ATX form factor. The type of Power Supply used in over 80 percent of PC computers out there, and very readily available.
Size of the power supply case is approximately 6 inches Wide, 5-1/2 inches long (5.5), and 3-1/2 inches Tall. (3.5)
It is a 250, or 300 Watt generic unit, probably made by Bestec, Delta, or HiPro.
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00072236&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&product=404643Scroll down to see the illustration and photo of the motherboard.
Power cables needed are;
A) 20-pin ATX main power cable,
http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20{ ATX power supply's today come with a 20 + 4-pin ATX main power cable.
The power cable can be used as a 20-pin, or a 24-pin.
Add the 4-pin connector to make it a 24-pin, or do not use it, and have a 20-pin }
B) Enough standard 4-pin Peripheral power cables for the IDE (PATA) harddive, {1}, and the optical drive/s.
http://www.playtool.com/pages/psuconnectors/connectors.html#peripheralToday's power supply's come with either at least 4, or 6 of these power cables.
C) One Floppy Drive power cable.
Won't be used for a Floppy Disk Drive on your computer. It's power for the Memory Card Reader.
http://www.playtool.com/pages/psuconnectors/connectors.html#floppyAll of the above power cables are typical when purchasing an ATX power supply today.
Need guidance in choosing a Power Supply, please state in a Comment.
Regards,
joecoolvette
Usually, the only green light in the back is where the Ethernet cable connects to the network-adapter. So, unplug the cable, and see if that light goes out. Take a close look at _all_ the capacitors on the motherboard. Capacitors are those small circular "towers", with a silver top, with a cross-etched in the top. Capacitors exist to provide "smooth" voltage to components. Any voltage-fluctuation is just plain bad. The top of each capacitor should be dead-flat. If the top is at all domed, the capacitor is leaking. If there is any red-brown "rust" on top of a capacitor, it is leaking. If it does not power-up far-enough to give one (or more) "beeps", then the likely suspects are: 1. power-supply (you've tried various ones) 2. motherboard 3. CPU (can you swap-in a different one?)
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