Computer help forum: Motherboard Problem (experienced tester here)

by: jjmalove January 19, 2013 4:56 AM PST

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Motherboard Problem (experienced tester here)

by jjmalove - 1/19/13 4:56 AM

Hey all,

I'm an Electronic Technician by trade, and am trying to narrow down just why my nephews computer won't power at this point. The computer won't get any power (no fans or nothing) when you press the power button, and the LED which usually is on 24/7 near the power is off. Here is what I have done:

Computer has no hard on/off switch. It is on 115 V.
Disconnected 24 pin power cable, and shorted Green to Black. This successfully turned on the PSU Fan, and dvd player (and probably hard drive but didnt look). So the PSU is working correctly. Also it is correctly outputting 5V DC to the motherboard cable and 12V DC to the hard drive and dvd cables.

Confirmed the Power Button is working correctly. With power disconnected, a meter shows open across the switch's pins. When the switch is pressed, it shows short. Therefore, the switch is working. Confirmed that the mechanism which toggles the switch is also working correctly (the plastic piece which your physically press which then actually presses the switch inside the computer).

Removed the cable which connects the power button and 2 LEDs. A meter across the power button terminals shows 2.5V DC, is this correct? A meter across the Blue LED terminals shows open, and across Yellow LED shows open, this strikes me as odd?

With the cable still removed, I hard shorted the Power Pins (2.5V) with a jumper. I tried a couple things and at one point I actually booted the computer! The computer fan kicked on, as well as the other stuff which tested the PSU had kicked on previously, and I heard the usual beeps. It was hard on, so to shut it off I had to remove the power cable from the back of the computer. I can't replicate this again, but I have confirmed again the PSU is working correctly and all the voltages I had seen are still the same.

If anyone has any knowledge on what to do next, let me know!

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