cpu - New computer won't boot

I've recently purchased a set of components and set about building my own desktop. It is not booting and I am stuck.

When I hit the power button, the fans spin, but that's all. There is no video. There are no beep codes.

I have tried swapping out the power supply. My backup power supply is 850W. Both power supplies are brand new.

I have tried swapping out the processor. My backup processor is an AMD FX-4100 AM3+. Both processors are brand new.

I have tried swapping out the video card. My backup video card is a Sapphire Radeon HD 4650 PCI Express. It was working in the last computer it was in. This motherboard does not have on-board video.

Because I was able to swap out all components other than the motherboard itself, I thought that the motherboard must be faulty. I RMA'd it and got a new one, of the same model. This did not fix the problem.

The monitor is known to be good; I am using it to ask this question.

I have verified that both the 24-pin motherboard power and 4-pin CPU power cables are in place.

I have 2 pieces of RAM, each 8 GB. I have four RAM slots. I have tried the RAM singly and in pairs, in all fourteen possible combinations of RAM + Slot. I have tried this RAM in another computer, and it was able to boot Windows.

I have other components, but I have not yet begun to connect them. I assume I should be getting some video or beeps or something, even if I don't have a hard drive hooked up.

I don't know what else to try.

Answer

Is the speaker plugged in? It should beep if there is a POST error. Remove the RAM (memory) and video card, leaving in the CPU and heatsink/fan, and you should hear three beeps.

Is the power to the video card plugged in? Does it need extra power?

Is the motherboard properly mounted and not shorting itself out? Pull the motherboard out of the case and try powering it on