I apologize for the long explanation, but I would like to be as thorough as possible. I have had a gaming rig for about 4.5 years with the following specs:
-Asus L1N64-SLI WS motherboard
-3.0 GHz AMD Athlon 64 FX-74 CPU
-750W Rosewill RX750-D-B power supply
-Geforce 8800GTX video card
-8GB RAM
I was not there when the problem began a few months ago, so I cannot give the circumstances of when the problem first started, but as far as I can tell, the problem does not seem to have been caused by anything in particular on the part of the users. The problem is this: as soon as I switch the power supply on, the computer immediately begins to start up, then, a few seconds later, shuts down. If I hit the power button after it has turned off (I had never hit the power button to begin with), it does not turn on again until I switch the power supply off then on again, at which point the process repeats, with roughly the same interval in how long it takes before it shuts down. I did notice, however, that after the computer had shut down, the indicator light on the motherboard remained lit, indicating that the power supply still seemed to be turned on.
I checked the power switch with a multi-meter and it seems to be working fine. I tried plugging in the computer with the power switch disconnected entirely and the computer still started by itself then immediately shut down.
I took out the PSU and tested it without a load, and it seemed to run fine. After perhaps an hour or so of being left on without being connected to anything, it was still ice cold, probably eliminating the possibility of an internal short in the PSU. However, I checked the voltages with a multi-meter, and found the +12V rail to be a tad high at 12.38V and the +3.3V rail to be 3.48V, slightly over the +-5% tolerance. Are these voltages acceptable? Could this be tripping some safety mechanism in the motherboard? Unfortunately, I do not have a spare power supply with the 8-pin motherboard connector that I would need to test it with a different power supply.
Finally, I disconnected everything except the motherboard, a single stick of RAM, the CPU, and the power supply, and left the power button disconnected, and the problem persisted when I plugged it in. I tried taking out the CPU and putting new grease on it, but this also had no effect. The computer has an unnecessarily large number of fans, so I doubt that the issue is heat-related, and there is not much dust inside the case. The CPU uses liquid cooling, which makes it difficult for me to remove the motherboard from the case, so I have not yet tried running the motherboard outside the case in order to see if there are any shorts from the motherboard to the case. If anyone thinks this would be a good idea, I will try this.
My concerns are this: if you think the power supply voltage may be causing the motherboard to shut off, is it worth buying a new power supply with the risk that the motherboard might already be damaged? Alternatively, if there is a problem with the motherboard, would it make sense to replace the motherboard without changing out the power supply even though the voltages are slightly high? Do problems with power supply voltages tend to become worse with time? I would be willing to replace either the power supply or the motherboard, but probably not both.
Thank you in advance.