Motherboard fails to switch on psu

[cjcj1949]

My desktop computer does not switch on when the mains is first switched on. After a few hours it is ok. I suspect the motherboard is not providing the correct signal to the psu.

 

[o1die]

Could be a loose or worn case switch in the front plastic bezel. You can gently pry it off if you're careful and check the wiring. But first check the motherboard case pin cluster for a loose or worn wire. Then check all the power supply leads to the motherboard and drives. Also give the fans, including the power supply, a good cleaning if it's dusty. I use a canned air solvent that evaporates quickly. They sell for less than $5 at computer stores such as frys.

 

[cjcj1949]

There's no problem with the switch. I've removed all disks, memory and graphics card from the motherboard and the same problem exists. The computer still boots after a while. I can tell it's booted from the power consumption and the fact that it will switch off from the front panel switch. The PSU can be switched on from cold when not connected to the motherboard. All rails are correct. It runs happily at 5amps on the 12V rail. I haven't checked higher powers from cold as I know that higher power is ok once the psu has started. I did think the input capacitors in the psu were faulty, but I've checked them with my meter and the capacitance is ok, if a bit low. Ripple is ok as well. If the psu is left switched on for a while when disconnected from the motherboard and then briefly switched off and connected to the motherboard it will then start the computer, so the fault seems to lie in the psu. I do not have a scope or know how the motherboard could stop the psu starting. I can only think that the rails are not coming up together due to a fault on the secondary side.

The motherboard is a Gigabyte 7N400 Pro Ver 2 and the PSU an Antec truepower 380W.

It is a bit dated now and can not be improved economically. I would like to buy a scope and investigate further, but there are really better things to do and no sensible support, in the way of documentation is available from GigaByte or Antec.

I am also amazed that the wattage of a power supply can be described as the sum of the individual rail powers, when each rail power is measured individually. Do we still have Trading Standards?

 

[marbski]

try to test the PSU connector if all pass the voltage requirements....

 

[cjcj1949]

It's not the connector. It works reliably so long as the psu has been on for a while. With or without the psu connector plugged in.