When I plug in both the 20-pin and 4-pin connectors to the mobo and hit the power button, the lights flicker briefly and turn back off. When I have ONLY the 20-pin connector attached (ie, once I remove the 4-pin connector), the power supply will stay on (fan running).
I initially thought my power supply was dead but I bought a power supply tester and I have 2 working PSUs. I am nearly certain that this isn't a PSU problem -- I have 2 PSUs and the same thing happens with both. The only thing attached to the mobo are the CPU/HSF and PSU.
Yes, I went through the checklist -- reset CMOS, breadboarding, etc.
Is this a mobo problem? Or is my CPU dead? I've had overheating problems in the past. The last time my computer was on, it suddenly turned off, and the motherboard was very hot to the touch. It hasn't turned on since.
System: (note that the problem occurs even when it's just CPU / HSF / PSU)
HD -- WD Caviar 500GB Serial ATA HD 7200/16MB/SATA-3G
Memory 1 -- Corsair XMS3 2048MB PC10600 DDR3 1333MHz (2x1024)
Memory 2 -- OCZ 2048MB PC10666 DDR3 1333MHz (2x1024MB)
Mobo -- MSI NF750-G55 nForce 750a SLI Socket AM3
Case -- Cooler Master Centurion 534 Blk Case
Video card -- Sparkle GeForce 9500 GT 512MB PCIe w/DVI
CPU -- AMD Phenom II X4 965 Black Edition Deneb 3.4GHz 4000MHz Socket AM3
PSU -- Antec EA-650 Power supply
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This is the basic breadboard. "Basic" is defined as the minimum that you will get a useful results from. Try both PSU's at this point. Keep in mind that finding two bad PSU's is not unusual. That is why you should test the PSU's in another system.
Breadboard with just motherboard, CPU & HSF, case speaker, and PSU. You do have a case speaker installed, right? If not, you really, really need one. If your case or motherboard didn't come with a system speaker, you can buy one here: http://www.cwc-group.com/casp.html
You can turn on the PC by momentarily shorting the two pins that the case power switch goes to. You should hear a series of long, single beeps indicating memory problems. Silence indicates a problem with (in most likely order) the PSU, motherboard, or CPU.
Motherboard LED's mean very little. When on, all they are telling you is that the computer os plugged into a live power socket and the PSU is switched.
Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.
If no beeps:
At this point, you can sort of check the PSU. Try to borrow a known good PSU of around 550 - 600 watts. That will power just about any system with a single GPU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.
The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.
This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.
If the system beeps:
If it looks like the PSU is good, install a memory stick. Boot. Beep pattern should change to one long and several short beeps indicating a missing graphics card. Silence or long single beeps indicate a problem with the memory.
Insert the video card and connect any necessary PCIe power connectors. Boot. At this point, the system should POST successfully (a single short beep). Notice that you do not need keyboard, mouse, monitor, or drives to successfully POST.
Now start connecting the rest of the devices starting with the monitor, then keyboard and mouse, then the rest of the devices, testing after each step. It's possible that you can pass the POST with a defective video card. The POST routines can only check the video interface. It cannot check the internal parts of the video card.