Problem with 5V rail on Dell Workstation PSU

[taste_tester]

Hi all,

Today after shutting off my Dell T3610 it wouldn't power back on. The power supply passed the self test (it would make all fans spin) but the computer still wasn't powering on at all. (It's a Dell D685EF-00).

I measured the voltages at the connector and then took out the power supply to measure the voltages directly from the pins of the power supply.

From the attached image you can see the pinout of this power supply. When I measure it, the 5V at Pin 13 comes on, and the 5VSB at Pin 18 comes on until I push the self-test button where it goes to 0V. Pins 9 & 10 (red wire) stay at 0V.

The problem is I can't find any schematic of this power supply or any repair logs about it. I took it apart and nothing is obviously wrong from a visual inspection. The +12V outputs are clearly labeled, but the ground and 5V is not labeled. There is a small section over on the bottom side which is separated from the other ground pins and I was thinking this might be the 5V.

The power supply is supposed to be able to put out 4A of 5V on 5VSB, but the single pin of the power supply labeled 5VSB is going through  a 220 uF capacitor - I can't imagine this is for the full 4A if it's going through such a tiny capacitor?

If anyone could help identify if this mystery section of the edge connector is in fact for 5V then i can begin attempting to track down the problem in my power supply.

Thank you if you read this far!

[aliarifat794]

Check the capacitors, especially those in the 5VSB and 5V circuits, using an ESR (Equivalent Series Resistance) meter.
Test diodes and transistors for shorts or open circuits.

[CaptDon]

I can't imagine any power supply for a P.C. style computer being designed to provide 4 amps at 5 volts of 'standby' power? 20 watts isn't exactly standby! Maybe 40ma. or at the most 400ma.? Even 400ma. would seem excessive for standby.

[HwAoRrDk]

I can't imagine any power supply for a P.C. style computer being designed to provide 4 amps at 5 volts of 'standby' power?

The 5V SB is typically also used to power the USB ports, so modern PSUs often have capability to supply several Amps on that rail.

[CaptDon]

You have a point about the USB stuff!! I was thinking more along the lines of the older supplies where the standby circuit sort of evolved from the old 'Keep Alive' term which basically kept the 'soft' power on button alive and then woke up the main power.

[tunk]

Have you tested that the on/off push button works?