[Solved]Can't turn computer on after power outage

Hi everyone

My computer was off when the the power outage occurred. Now when I want to turn it on by power switch in the front, it can't be turned on. When I use the off/on switch on power supply in the back, I saw no beep, black screen, fan is turning, green light on motherboard. However, I still can't use the power switch in the front. Any recommendation will be helpful. Could this be Power supply unit problem, Motherboard problem, or just have to clear cmos battery?

Motherboard:a sus p5b
CPU:Core 2 Duo 6600
9 answers Last reply
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  1. Sounds like a short, from the MOBO sadly your going to have to get a new one :(.
  2. Actually, before giving up on it, try these steps:

    1. Open the case and disconnect the front panel devices (power, reset, usb, etc)
    2. Using a flathead screwdriver, or similar tool, jump the PWR_SW pins.


    If not, see if you can get your hands on digital multimeter so you can test your PSU. To test the PSU, put the black probe against anything bare metal in your case, or behind any black wire on the P1 connector (the 24-pin). Next, take the red probe and stick it behind the following colors and compare their corresponding voltages:

    Red: +5V
    Yellow: +12V
    Blue: -12V
    Green: +5V
    Orange: +3.3V
    Brown: +3.3V
    Gray: should go from 0V to +5V when turning on the power switch

    The acceptable variance is ~5-10%, any voltages within this range would be considered tolerable.

    If PSU checks out ok, check the outlet that the computer is plugged into.
  3. Sounds like you are thinking it's not a psu, and it sounds like it isn't, but for what it's worth, my parents had the same thing happen in a storm. I don't remember if a fan would come on or not, but I was thinking of putting together a new system for them and decided to just try a new psu and it took care of everything. I don't know what you are thinking of spending on a new motherboard if you go that route, but I put a new psu in my parents' system for about $30 I think. Might be worth at least considering depending on your system. I know if I was going to buy a new motherboard, I'd want to get a new psu and new memory at the same time, probably would have to, so I'd just consider whether or not it's the psu.
  4. Hi T_T:
    When I jump PWR_SW pins with flat screw driver, sometimes the cpu fan will spin. However, sometimes it won't. It is kind of strange. Any hint?
  5. Your post suggests that the system has a short somewhere, likely the PSU. Because the system was working fine (I assume) before the outtage, it's safe to assume that there isn't any case shorts, still, if you'd like to rule this out, breadboard your system.

    To breadboard, you should:

    1. Disconnect and remove all devices on the mobo, except for the CPU and heatsink
    2. Remove the mobo from the case, and place it on a non-conductive surface, like a phonebook.
    3. Connect the PSU (both the 20/24-pin and the 4/8-pin) plugs and one stick of RAM
    4. Jump the PWR_SW pins again.

    What are your results?

    Note: consult the mobo manual for DIMM slot population for single stick use.
  6. When I try to test in breadboard by disconnecting everything except CPU and power(20/24 pin and 4/8 pins connected) and shorting two pins for PWR_SW.Nothing happens. Monitor still shows no signal. Does this means the problem was the PSU? I am just wondering.
  7. When I try to test in breadboard by disconnecting everything except CPU and power(20/24 pin and 4/8 pins connected) and shorting two pins for PWR_SW.Nothing happens. Monitor still shows no signal. Does this means the problem was the PSU? I am just wondering.
  8. OK. You have the right idea. You just need a little more troubleshooting help. Your problem could be almost anything - PSU, motherboard, maybe but probaly not the CPU, even a shorted drive that is shutting down the PSU.

    Work systematically through our standard checklist and troubleshooting thread:
    I mean work through, not just read over it. We spent a lot of time on this. It should find most of the problems.

    If not, continue.
    The following is an expansion of my troubleshooting tips in the breadboarding link in the "Cannot boot" thread.

    I have tested the following beep patterns on Gigabyte, eVGA, and ECS motherboards. Other BIOS' may be different, but they all use a single short beep for a successful POST.

    Breadboard - that will help isolate any kind of case problem you might have.

    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.

    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.

    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.
  9. Thank you everyone. The problem is PSU.
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