Here is a quick reference troubleshooting guide for ATX power supplies

To test an ATX SMPS PSU: **
Use the following to find the correct supply wires (20 pin connector).
+5v, red wires, pins 4,6,19, and 20
+5v standby, purple wire, pin 9
-5v, white, pin 8 (may not be present)
+12v, yellow, pin 10
-12v, blue, pin 12
+3.3v, orange, pins 1,2, and 11
Common or signal ground, black, pins 3,5,7,13,15,16, and 17

Important Note! On version 2 of ATX Power Supplies  there are 4 additional pins
The 2 additions pins above pin 10 are +12v yellow, and +3.3v orange
The 2 additions pins above (previously numbered) pin 20 are +5v red, and ground black
Where pin numbers are used in this article they refer to the 20 pin connector.

Disconnect everything from the PSU (including MB). Plug AC cord into the rear of the PSU. IF the PSU has its own
rocker on/off switch, turn it ON. There should be +5v on pin 9 of the 20 pin connector (+5vsb, usually a violet wire.)
There should be >0.8v on pin 14 (PS-ON, usually a green wire.) This is from an internal pull-up to the +5v StandBy.
If the +5v standby is missing the power supply is bad.
Turn off and unplug the AC power cord from the power supply.

Connect PSU power leads to 1 or 2 IDE HDs (for a load). *Jumper pin 14 (PS-ON) to pin 13 (ground, black wire.)
Now plug the AC power cord back in and turn on the PSU's rocker switch (If it has one).
At this point, fan in PSU should start spinning, drives should spin, and + 5/12v, -5/12v, +3.3v, and +5v StandBy(+5vsb) should
be present at 20 pin connector.

Disconnecting the pin 14-13 jumper should turn the power supply back off.

If your system failed to start the post and boot process, and the voltage on pin 9 is less than 4v, either;
a) the logic on your motherboard is shorting. (Bad mobo)
b) your +5v StandBy is under powered, and you power supply (PSU) is bad.
Nine times out of ten in this case the issue is with the ATX power supply.

If your +5vsb is OK, measure pin 14 of the PS/MB connector. If it is <0.8v (and the +5vsb is OK), the PSU should
be on. If pin 14 is <0.8v, the +5vsb is OK, and the PSU is OFF, the PSU is bad. If pin 14 is >2.0v, the PSU is
being told to stay OFF by the MB. If depressing the front panel ON/OFF switch does NOT lower pin 14 to <0.8v (and
the +5vsb is OK), then the problem is NOT the PSU, but rather Motherboard related.

* Note: With some PSU's, you may not get the +3.3v at the correct voltage since the +3.3v sense on pin 11 is not
* connected on the motherboard or pin one may be missing altogether. 
As a workaround you may try jumpering pins 11 and 2 together before jumpering the PS-ON signal to ground. 
This will provide the +3.3 from pin 2 to the sense circuit for feedback. Do this only if you suspect a +3.3v issue.
Also, In this procedure the Power Good\OK signal on pin 8 is not considered.

** I can't be held responsible for any personal or property damage. I do consider this particular procedure aggressive, a little risky,
and only to be used as a reminder to those trained and experienced in working on PC hardware. Use at your own risk and be careful.

Back to the main ATX PSU troubleshooting article

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