Newly built system won't power on (CPU fan twitches)

[ggab]

I just put together a new computer with the following components:
4 GB OCZ Reaper 1066 RAM
AMD Phenom II 940
Asus M3A79-T Deluxe
PC Power & Cooling S75CF Silencer 750W Quad Power Supply
Re-using a 7900GT with a molex connector for power, not a PCI-E connector

When I turn it on the CPU fan twitches and then nothing else. It doesn't do a full spin, just a twitch. The lights on the motherboard light up fine. If I turn off the main power switch on the power supply and then turn it back on, the CPU fan will twitch again after pressing the power button, but then subsequent presses there won't be any twitch until I turn off the main power switch off and on again.

I suspected the PSU so I jumpstarted it by jumping the green and black wires with a fan on it for a load. It started just fine with that. I made sure that the 8-pin EATX cable was plugged in as well as the other 24-pin connector.

Do you think this is the motherboard or the PSU? Maybe the PSU isn't able to handle the load or maybe the motherboard isn't telling the system to stay power on correctly? I checked the power switch connector to the CPU and it seems fine. This case worked fine for my previous build. Thanks for any advice.

 

[aford10]

First guess, I would guess a weak psu, but that psu is plenty to push your system. Double check the 4/8 pin cpu power. Make sure the cpu fan is plugged into the sys fan port.

 

[ggab]

Thanks for your reply.

I checked them several times. I tried the 4 pin connector first and it did the twitch thing, then I noticed there was a black plug over the other 4 connectors on the motherboard making the 8-pin EATX connector when removed. I found that connector on the PSU and it did the same thing, the CPU fan just twitched. I've never built a computer before that had the 8-pin EATX connector. I checked and the CPU fan is plugged into the correct plug.

 

[Proximon]

Pretty odd. What happens if you do not plug in the 4/8 pin? You should get a full fan spin... if that doesn't happen I would suspect the PSU as well.

If you seem to have full power without the CPU power connected, then it's probably something else, like the MB.

 

[aford10]

You might want to get your hands on a known working psu, to test the board.

 

[aford10]

If your psu doesn't have the 8 pin cpu power, the 4 pin will still work. They are shaped so it can only fit in one way. If you have the 4 pin cpu power plugged in, you should have power.

 

[ribs15183]

Pretty odd. What happens if you do not plug in the 4/8 pin? You should get a full fan spin... if that doesn't happen I would suspect the PSU as well.

If you seem to have full power without the CPU power connected, then it's probably something else, like the MB.

Hey.

I'm having a problem similar to what you're describing here. My motherboard appears to power-up if I don't have the 4-pin CPU connector plugged in. If it's plugged in I get nothing but a fan twitch.

You're thinking that's probably a mobo failure?