I've read through a number of other forums posts that reference this issue but none of them have solved the problem for me. Sorry about the length of the post. I figured it would be best if I explained what I've tried up to this point.
Because I know someone will ask the system specs here they are:
CPU: Intel Pentium Dual Core E5800 3.20GHZ LGA775 2M Cache (new)
MOBO: Asus P5B
RAM: OCZ Platinum 4GB (4 x 1GB)
HEATSINK: Arctic Cooling Alpine 7 GT Intel CPU Heatsink Cooler LGA775 With 80MM PWM Fan (new)
Corsair Professional Series Gold AX750 CMPSU-750AX 750W ATX Fully Modular 80PLUS Gold (new)
GPU: Sapphire RADEON HD 3870 Graphics adapter - 512 MB - GDDR4 SDRAM
Nothing fancy but it does the job.
I woke up Saturday to my monitor telling me that there was no signal coming through. I checked the computer and the power light was on but on closer inspection it looked more like it was pulsing. I tried rebooting it and then it started to pulse a little slower, still not getting to any kind of boot.
I took the computer out and started inspecting it to see if anything looked wonky inside. When I hooked it up again it started to power up even worse than before. Sometimes it would restart 3 times before it would completely die, sometimes twice, sometimes only once. Never really having those changes correlate with any of the pieces I was taking out of the computer. The restarts never take longer than 1 second. After the boots stopped I would have to hit the power switch on the back of the PSU and wait until the power light on the mobo died. Then I could turn the PSU back on and I could try the boot again.
I've tried moving RAM between bays, removing all hardware, resetting the bios, checking power cables, etc. Initially I thought it might be the PSU so I replaced that. Turns out that wasn't it at all. I have taken all of the hardware out of my computer and nothing made any difference until I got down to the CPU. Once I took the CPU out it stopped restarting itself.
I thought that it might be the CPU then so I went and bought a new CPU. That hasn't changed anything either. It is still barebones on my floor and the only thing that I can control is the following:
I have the 24pin in the mobo but there is the separate 4-pin plug. If I have the 4-pin plugged in the computer will keep trying to boot indefinitely. If I remove it I have to hit the power button to get it to do its 1-second boot.
Does anyone have any ideas on this? Could it be the mobo?
Breadboard with just motherboard, CPU & HSF, case speaker, and PSU.
Make sure you plug the CPU power cable in. The system will not boot without it.
I always breadboard a new build. It takes only a few minutes, and you know you are putting good parts in the case once you are finished.
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. Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.
If no beeps: Running fans and drives and motherboard LED's do not necessarily indicate a good PSU. In the absence of a single short beep, they also do not indicate that the system is booting.
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.
A way that might be easier is to use the main power plug. Working from the back of the plug where the wires come out, use a bare paperclip to short between the green wire and one of the neighboring black wires. That will do the same thing with an installed PSU. It is also an easy way to bypass a questionable case power switch.
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, long single beeps, or series of short beeps indicate a problem with the memory. If you get short beeps verify that the memory is in the appropriate motherboard slots.
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. At this point, if the system doesn't work, it's either the video card or an inadequate PSU. Or rarely - the motherboard's PCIe interface.
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.