ATX wont power up via mobo ' power up ' pins

[polyal]

The ATX will power up shorting the green and black ( main motherboard connector )
but wont power on from the power sw
It works ok with a KGU ( ATX )
Is there a secondary circuit in the atx that senses when the power sw is pressed ??
any info would ge gratefully appreciated

[Bruce]

the power switch on the case will be cabled to the front panel header on the motherboard, if you short those two pins, labelled PWR SW, does it start then?

[polyal]

with any ’ good ’ ATX psu
but not the problematic ATX

[Bruce]

i was trying to eliminate the power switch as maybe the potential issue.
so if the PSU is installed in the rig and cabled accordingly, and unplug the power switch from its mobo header and short out those two pins, does it start then?

[polyal]

the ATX with the ’ issue ’ will not start shorting the appropriate header pins
…any other known ’ good ’ psu will

• when the ATX has the green shorted to 0v ( green to black ) all the voltages
  on the 24 ( 20 ) pin mobo connector are present although obviously not loaded

[Bruce]

the ATX with the ’ issue ’ will not start shorting the appropriate header pins
The ATX will power up shorting the green and black ( main motherboard connector )

so it only starts by shorting the pins on it’s own mobo connection cable, not via the front panel header pins when it is connected to the mobo?

if so, you’ve either got a dead PSU or dead mobo.
any chance you can get your hands on another PSU to test that theory?

[polyal]

so it only starts by shorting the pins on it’s own mobo connection cable, not via the front panel header pins when it is connected to the mobo?

if so, you’ve either got a dead PSU or dead mobo.
any chance you can get your hands on another PSU to test that theory?

the mobo works with any another ’ good ’ psu
the ATX with the ’ issue ’ works normally without the mobo ( ie shorting green to black )
with all votages present ie +3.3v / +5v /+12 / -12 etc @ the mobo connector

why is this ATX not triggered from the header pins ? this is the problem

[Bruce]

you have a faulty power supply unit.

can’t be the mobo since other PSU’s work when connected.
even though its fan spins when shorted, it must still be buggered.
a quick test will prove that but you need one of those PSU testers or just take it in to a PC repair shop.

or - get your hands on another PSU from somewhere to test that theory.

[polyal]

the computer has already been fixed with a new atx

the atx that wont start via shorting the header pins is still
a good psu if used stand alone
it seems pin 14 on the mobo connector ( green wire ) needs to be taken to <0.8v
before the psu will kick in
for some reason this is not happening when the header pins are shorted ( pwr sw pressed )

[Bruce]

i’m confused when you say that the computer has already been fixed with a new ATX.
if so and this new power supply is good, then that would suggest that the motherboard is faulty.
if the PC won’t start, how do you know the new ATX power supply is good?

also, how do you know the power supply is working at all, simply because you short it out when it is disconnected from the mobo is no guarantee.
the PSU needs a proper test, and the best test is to find a PC that already is working fine and replace its PSU with yours - if it starts, your PSU is good, if it doesn’t start your PSU is not good.

[polyal]

i’m confused when you say that the computer has already been fixed with a new ATX.
if so and this new power supply is good, then that would suggest that the motherboard is faulty.
if the PC won’t start, how do you know the new ATX power supply is good?

also, how do you know the power supply is working at all, simply because you short it out when it is disconnected from the mobo is no guarantee.
the PSU needs a proper test, and the best test is to find a PC that already is working fine and replace its PSU with yours - if it starts, your PSU is good, if it doesn’t start your PSU is not good.

i dont know why you are confused ?
the computer is working correctly with a new psu . . . no probs
there has never been anything wrong with the motherboard

• the original atx is not working correctly because it wont allow the mobo to apply a negative pulse
  to pin 14 via the pwr sw on the computer case ( this is a psu problem )
  this is why i posted this issue . . . the ATX works normally if pin 14 is taken to < 0.8 v ( short to ground )
  but pin 14 is not shorted to ground via the pwr sw
  ( pin 14 is taken to negative via electonic switching on the mobo )

for some reason the psu is not allowing pin 14 to go below the 0.8v

[Bruce]

so to recap…

you replaced the PSU and the PC is now working fine.
the old PSU would not start the PC when installed, connected and the cases’ power button pressed - correct?
if so, than the old PSU is faulty. what I’m saying is just because you can short it out and get it to spin its fan and the voltages on the rails are correct does not mean it is ‘working’, as proven by it not starting the why it was intended.

I’ll step back and let others chime in, maybe I’m missing something here or they can explain things differently.

@PeterOz @veeg @Rustys @Bastet

[Rustys]

the ATX with the ’ issue ’ works normally without the mobo ( ie shorting green to black )

Bad PSUs can pass the paperclip test. The true test that eliminates a PSU from the equation is a replacement or trying that same PSU in another known working machine. Only these 2 can truly rule out a PSU.

[Rustys]

Do you need any further assistance

[Rustys]

Thread closed lack of response