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I am building a computer. First attempt there was no video signal. To cut a long story short, I swapped the motherboard (which tested ok at the shop) but when I installed it in the case, there was no video signal again. What could possibly be wrong?

Could it be the case contributing to it somehow (case purchased with PSU installed)?

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What motherboard do you have? Is there any sign of error message? What graphics card (or did you plug it directly into the motherboard?) I don't think the case has something to do with it(or the PSU) – RnBandCrunk Dec 17 '12 at 8:09
It's an Asus P8H61-M LX3. I'm not using a graphics card. – Sandra Dec 17 '12 at 8:16
Although there is no video, does the machine boot up as expected and (and I admit hard to be sure with no video) appear to be working as expected? Please remove all devices other than the hard drive, CPU and 1 stick of RAM. Does it still fail? – Dave Dec 17 '12 at 9:14
I tried booting up with just the CPU and RAM. Didn't work :( – Sandra Dec 17 '12 at 14:34

It sounds like one or more of your parts may be broken or incompatible with your system. It could be anything, but I'll try to help narrow it down.

I don't believe the problem lies with your case. However, if you can't turn the system on at all, you may want to try shorting the power pins on the motherboard. If this works, then that means that the power switch is dead.

However, more than likely, the issue may lie in your PSU, RAM, or CPU. Double and triple check that they are working correctly (I once had a broken PSU that tested fine) and that all of your parts are confirmed to be compatible with your motherboard. It is likely you can google your motherboard and get a list of compatible hardware for it.

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It's an Asus P8H61-M LX3. The machine is powering up fine. Fans are all working. The PSU came built into the case - Codegen 3330-CA. They connected this new motherboard to their monitor in the shop and it worked, but now it doesn't. They tested the first one I had, which didn't work for them either. – Sandra Dec 17 '12 at 8:19
So the only difference between the test setup and your setup is the PSU? If thats the case, then it's likely that you either have a bad PSU or one that doesn't supply enough power to the motherboard. (Also, check the monitor cord on both ends. Sometimes it's the simple stuff.) – Kosyne Dec 17 '12 at 8:27
I've tried 2 monitors. The monitor comes on when the computer is switched on, but then goes to sleep because there is no signal. At the shop, they just tested it out of the case, with their monitor and PSU. At home, I put it in the case. I wonder if the shop will now refund me another motherboard and case? This is a pain :( – Sandra Dec 17 '12 at 8:31
The motherboard isn't the problem I don't think. However, you may need to return that PSU/Case combo. The PSU was probably broken/too weak for your hardware. A Codegen 3330-CA only comes with a 350W PSU. – Kosyne Dec 17 '12 at 8:37
Ok. So assuming that both motherboards were working fine, but became 'faulty' after being installed in this case, would a faulty PSU cause an issue like this? I'm expecting that this motherboard won't work now if they test it. I checked the estimated power supply on the thermaltake website. It said I only needed 195W, so think the issue may be damage to the PSU. Thanks for your help too :) – Sandra Dec 17 '12 at 8:44

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