Question Force board to boot

Jan 14, 2021
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I have an older board that will not turn on the power supply (PS). (Yes, I have done extensive troubleshooting and can say the board is the issue.) I can short pins 15 & 16 and the PS will start. So I shorted the wires for pins 15 & 16. This works to turn on the PS. I turned off the PS at the main switch and reconnected the cable to the board and turned the main power back on. To my surprise, the PS did not start. Without turning off the PS, I disconnected the cable and the PS started. I pugged the cable back in and the board started. From an electronics point of view, this does not add up. So what is keeping the board from starting when the cable is connected vs being connected after the PS is on?

I run 24x7 so no need to turn off the computer. I can do a Windows restart without issue, so for now, I just don't turn off the PS. :)
 
Reasons could be many but the most important one is a short on the motherboard that the PSU sees and so won't start. It's also very likely since the motherboard is 'older', so a VRM capacitor could be shorted.

Remove everything from the board, one by one: CPU, GPU, memory, etc, at try to turn it on after each removal. If it turns on at some point, then the just removed part is a likely culprit.
 
Jan 14, 2021
4
0
10
0
Reasons could be many but the most important one is a short on the motherboard that the PSU sees and so won't start. It's also very likely since the motherboard is 'older', so a VRM capacitor could be shorted.

Remove everything from the board, one by one: CPU, GPU, memory, etc, at try to turn it on after each removal. If it turns on at some point, then the just removed part is a likely culprit.
Thanks for the troubleshooting tips. I have done the troubleshooting on the board and I am 99.99% sure the board is bad in some way. I'm OK with this as long as I can get it to boot somehow. The question is more related to the 24pin connector and why shorting pins 15 & 16 will start the PS when the connector is not in place on the board, but will not start the PS when the cable is connected to the board. Right now my guess is it has something to do with Pin 8 (PWR_OK) but I cannot find any references to what this pin does or how I could override that function.
 
..(PWR_OK) but I cannot find any references to what this pin does or how I could override that function.
The way I understand PWR_OK... it's a signal furnished by the PSU (a +5V logic "high") to the motherboard that says "power is good and stable now". That's when the motherboard actually turns 'on'. You could put a peak-reading multimeter on that to see if it ever enables. But a PSU with decent enough OCP might never even turn on if it sees a short circuit on the +12V output so it would never enable.

To see if your PSU uses it, with the PSU disconnected from the motherboard put the multimeter on PWR_OK of the 24 pin and see if it goes HIGH after shorting 15-16 to turn it on.

From WikiPedia:
"The ATX specification defines the Power-Good signal as a +5-volt (V) signal generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes between 0.1 and 0.5 seconds after the power supply is switched on. The signal is then sent to the motherboard, where it is received by the processor timer chip that controls the reset line to the processor.​
The ATX specification requires that the power-good signal ("PWR_OK") go high no sooner than 100 ms after the power rails have stabilized, and remain high for 16 ms after loss of AC power, and fall (to less than 0.4 V) at least 1 ms before the power rails fall out of specification (to 95% of their nominal value).​
Cheaper and/or lower quality power supplies do not follow the ATX specification of a separate monitoring circuit; they instead wire the power good output to one of the 5 V lines. This means the processor will never reset given bad power unless the 5 V line drops low enough to turn off the trigger, which could be too low for proper operation."​

So, if you have a cheap PSU you might need a more elaborate setup than a peak-reading MM to determine proper functioning of the PWR_OK line, but you'll at least see if the PSU is providing a +5V output there to turn on the MB.
 
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