Posted 11 July 2012 - 07:11 PM
Two main problems on those, power supplies and motherboards.
I this was me, I would throw another cheapie used ATX PSU at it to absolutely verify a motherboard concern. Examine the motherboard. Any swollen/bulged/blown caps? Look around the CPU. Any blown/melted mosfets/vregs? If yes to either, you can assume the mobo is faulty. Caps can be replaced cheaply if you have rudimentary solder skills. Remanufactured/surplus mobos usually run~ $35-45 shipped from Hong Kong.
Don't forget something silly. Some screw or extra/miscellaneous piece of metal might have come loose, and shorted something. Turn the case upside down, anything fall out? You might loosen all of the motherboard screws, move the board away from the backplane and see if it will post. Or better yet, remove the board from the case, hook up the power supply(or a replacement supply) to see if it will post.
Wish I had better diag advice. HP/Compaq desktops run darn near 50/50 on power supplies vs motherboards as being faulty.
I am a retired Ford tech. Next to Fords, any computer is a piece of cake. (The cake, its not a lie)
3770K @4.5, Corsair H100, GTX780, 16gig Samsung, Obsidian 700 (yes there is a 700)