Back Power test button only way to turn on computer

[melbrett68]

So here's something weird, the only way I can turn on my friend's Dell Optiplex tower computer is by keeping the back black Power test depressed and then I can power up the computer as normal. I tried to jump the switch via the motherboard and that didn't work, thinking just the power button was defective. Any thoughts?

 

[NeoPTLD]

Unplug and replug the power cable in the computer. Clear CMOS with the jumper.

test. If no go, try a new CMOS battery. Clear CMOS jumper and test.

If that's a no go, try a PSU from another PC. It could be that 5V standby that is always on is dead which is needed for soft switch to work.