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Motherboard Power Connector Testing Question

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Smack2k

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On a motherboard connector, what is the best way to test the PS_ON pin for any shorts?

I know testing the 5V and 12V pins against ground for continuity can confirm shorts on those, but what about the PS_ON pin?
 
If you mean on an ATX PSU: If you plug in the PSU, the motherboard should start immediately without the use of the power switch if the PS_ON is shorted to ground. The power switch should be unable to switch the motherboard off properly.

I don't know what the ohms should be between PS_ON and GND with the motherboard unplugged if that's what you mean. You could measure it on a working board.
 
That is exactly what is happening except I think its the board that has the short, its powering on immediately once PSU is plugged in. But it happens with other PSU's I try as well....

Is there any way to fix this? The keyboard lights are blinking off and on as well, unless you short the reset pins, then the lights stop blinking but once removed, they start blinking again....

should I look at the traces where the connector is? Or could the issue be anywhere? If I test a ground pin on the
 
I'm afraid i don't know the power on circuitry very well. When trying to fix a motherboard with the opposite problem I did find a simplified schematic of the circuit. Don't know where that is now though. All I recall is the 5VSB feeding to a switch that set off the latching of the PS_ON to ground.

The first most obvious thing to check is if there is a jumper on the power switch header. Some other motherboards have a jumper to force a motherboard into this mode. Failing this you may be able to find that schematic and work it out. I think if 5VSB and 5V were shorted together you would get similar issues to this. Its easy to check if there is 0ohms between 5VSB and 5V to rule that out.

You could just install a switch on the PS_on line. I have a 2007ish supermicro board with the opposite problem, wont power up without a paper clip between PS_ON and ground. I accidentally removed a card without turning it off and shorted 5VSB to ground blowing a fuse somewhere on the motherboard. It's too small to find so I just power it with a paper-clip, really should come up with a more permanent solution.
 
How do I check the ohms between the 5VSB and 5V on the motherboard power connector? When its powered off? I set my multimeter to 200 ohms and the meter had a reading of 1 on it. I checked between the 5VSB and one of the 5V pins and it stayed at 1....not sure Im checking it properly though......sorry, not great with the ohm checking.....continuity I can do, LOL!

Any help on proper way to do this is greatly appreciated!
 
0 ohms, between two points is the same as finding there is continuity between two points. So if you have a multi meter, and it beeps when you test two points, if you move it to the ohms range you should get 0 ohms. Yes i meant when it was powered off. Curious you get 1ohm, I would have though it should be more than that, have you got another motherboard you can test for reference? this may be a wild goose chase, the problem is likely the circuit in that schematic i mentioned.
 
Some meters display a 1 for "Out of range" and / or "Open circuit" Check the manual for your meter.
 
OK,

So I used a different multimeter and found that there is no continuity between the 5VSB and any of the 5V pins, and they all have ohm readings of various values, all pretty close.....

So back to square one, no idea why once the power supply is applied the keyboard starts blinking and the fans all come on.....
 
Try isolating the PSU pin for a test. If power is applied and the system stays off, at least you know to look at the motherboard.

With the PSU disconnected, what resistance reading to get between PS_ON on the motherboard and ground?
 
Smack2k said:
....no idea why once the power supply is applied the keyboard starts blinking and the fans all come on.....
Click to expand...
On some systems when power is first applied, ie: from the wall socket, It will run through a short self test of some internals, Hence the fans starting, Led's blinking, And then go into standby mode, Some will display a fault code by LED on the front panel, What system / motherboard is this ?.
 
I don't recall having one of those boards, It might be normal when power is first applied for a self test to run then drop into standby, One thought comes too mind have you removed the battery and with a cleared cmos tried to power on normally.
 
Yeah, with a battery, without a battery, shorting the CMOS jumper, does the same thing everytime.....as soon as power supply is turned on, keyboard lights blink on and off and fans come on.....tried multiple PSUs to.
 
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