Dead computer,is it MB?

I was given an older desktop computer, an e-machines ET161-03 and decided to soup it up with a new video card. Store clerk told me that I needed a more powerful PS to handle the new card. So i swapped out the original 250 watt PS for a 400 watt PS that I had. All went well after the new install, computer booted up to windows, all seemed fine. Then I saw that I needed to replace the 4-pin molex connector for the optical drive with a sata type power connector. Stupidly, I decided to do this with the computer running, knowing to cut each wire seperatly so they wouldn't short. When I got all of the wires(from the PS) cut and stripped. I should have taped them down, but didn't thinking that I would seperate them using the fingers of my hand. Well, the +5 lead brushed across the +12 lead, there was a spark, the PS crowbarred and the computer shut down hard. Here's the trouble: now when I try to start the computer, the fans spin up but there is no video output. Not with the new card in place. or without it and using the MB video. So I'm thinking that something on the MB got zapped (BIOS, soldered on). But I don't understand how this could have happened, thinking that the PS would not blow up the MB by sending +12 to the +5 lines or something similiar. The PS seems fine, +5.+12,+3.3 all correct. Swapped back to the old PS, same result. So my question is: given no video on start-up does this automaticly point to the MB or could something else be in play? Thanks for reading my post.
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  1. PSU is dead
  2. There is just no telling what you have fried.

    I know that I would no longer trust the PSU.

    You are going to need to do some extended testing.

    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 (standby power supply): 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.
  3. Thanks for the reply. The PSU (power supply unit?) in the computer is known good as it was the original PSU that was in the machine before replacing it with the more powerful PSU. I do believe that the PSU is good, all the voltages measured on the mother board are good. There are no beeps, no video of any kind. I would guess that that means something about the BIOS is honked (not replaceable, soldered on MB. Question: if I removed the CPU (and it was what got toasted) and powered up the thing, would the BIOS put out a POST video signal? I'm still trying to figure out if I can identify the MB, (all I can read is "Made in China"). Tonight I will remove the single HDD, put it into a SATA enclosure and verify that that is good.
  4. MB dead, CPU,HDD,Memory OK. New MB was the answer.
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