I can't tell if my mobo or PSU is bad

My desktop is out of commission but I can't tell whether its the motherboard or PSU at fault. Here are some notes I took as I was trying to figure out the problem.

Mobo lights working
CPU fan working
One video card aftermarket fan works (the fan power hooks direcctly into motherboard)
One video card fan does not work (the power for this fan comes from the PSU power connectors to video card)
No power to either card? No video picture from either card. No post screen, nothing at all. Switched cards around, one at a time, etc..
Both video cards are good, tested in another system.
PSU fan and led working
All case fans working
All components including memory reseated, etc..

This problem occurred as I was replacing a CPU fan with a larger fan and heatsink. I may not have been careful enough, and damaged something myself.

I don't really have an extra PSU for testing. My other PSU is an antec 500 watt, an older PSU with no sata and a single power connector for video card. I can use an adaptor for making a second connector but I'm afraid to try and run the broken system on 500w.

MSI 790fx GD70
2x Radeon HD4870 1gb (one with aftermarket fan)
2x 2gb OCZ
Rosewill 750w PSU

I'm guessing it's either the PSU or MOBO but I don't have the money to spend on both, and can't afford to buy one and it not be it. Can the PSU be tested with a voltmeter or something? Can the MOBO be tested the same way? I would be willing to get a volt meter and learn how to test things if the problem can't be figured out.
6 answers Last reply
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  1. Try it with your Antec one GPU installed, if it works then get rid of the Rosewill (POS) and get a quality PSU from Antec, Corsair, Seasonic or some other quality brand.
  2. Looks like it is definitely my motherboard. I tried the antec and still no power to the video card fan, no post screen etc.. I put the video card in another computer with the antec and it powered right up and worked perfectly fine. I will test the rosewill PSU from my fried system in the other system to make sure that is working.
  3. The rosewill psu works fine. I tested it on my older system. It seems apparent that it is the motherboard, but one more question.

    A bad cpu wouldn't cause the video card fan to not work would it? Might be a stupid question but since I narrowed things down to my PSU being good, video card being good and my motherboard most likely being the problem, that was the last question I could think of.
  4. When the questioned comes down to CPU and motherboard it is more likely the motherboard.
  5. Ok I just bought a new motherboard. MSI 890fx gd65. Still no post screen... i'm pretty furious.

    Bad cpu? Bad ram? Wrong settings for ram voltage?
  6. The typical reasons for no display are (by logical order):

    1. RAM - Bad, incompatible, not installed properly
    2. GPU - Bad, under powered, not installed properly
    3. BIOS - Some mobos default to on board; some to PCI. Check manual to verify.
    4. PSU - Most of the time people focus on the max output (Watts), instead of the max load (Amps). Using discrete GPU(s) requires dedicated 12 V rail(s). Make sure you're not overloading the PSU, by only connecting the bare essentials (One RAM stick, one HDD, no ODD/FDD, No external devices (except mouse/keyboard), and only one GPU.
    5. Mobo - A lot of times bad mobos aren't the case, but mobos are not immune to defect. A good start to building is breadboarding. This ensures that the mobo is functional before it goes into the case, thus ruling out potential shorts.
    6. CPU - Since CPUs have the ability to shut themselves off when they get too hot, they remain the last to replace.

    You mentioned replacing the CPU cooler.
    Did you remember to reapply thermal paste?
    Did you push down on the CPU too hard?
    Did you check the surface of the CPU for any scoring?
    Did you check to see if the CPU has any bent/broken pins?
    Can you verify your RAM is not damaged? (It stands to reason that if it was working before the HSF upgrade, that it should work now, but checking won't kill you).

    Lastly, if you haven't already, test your PSUs with a multimeter. It is the best way to determine if they are the problem.
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