/ Sign-up
Your question

Bad PSU? Bad Mobo?

  • Homebuilt
  • Systems
Last response: in Systems
March 21, 2011 3:14:29 AM

I just recently built my first pc, and I'm having a strange problem when trying to restart. If I have my pc on for any length of time (2 minutes or more), and I attempt to restart, the pc will only boot up for a split second, before powering back down and attempting to restart itself again, over and over (doesn't even reach post). The same thing happens if I shut down completely and try to power back up. However if I let the machine sit for 5 or so minutes and try to power back up, it has no problems. Nor does it seem to have any problems when I just leave it on all day. Also, my mobo model (evga 655-kr) has a dual boot which can get a little annoying but I think its supposed to be some sort of safety mechanism. No idea if its related to this problem, as it powers up and down much faster when it's acting up. My psu is a corsair 750tx, almost brand new. I'm really hoping its the power supply and not a motherboard problem, any help would be appreciated.

More about : bad psu bad mobo

a c 121 B Homebuilt system
March 21, 2011 4:47:21 PM

Sounds like 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.

You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.

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.