View Full Version : PC Won't Boot - PS Dead?

07-27-2002, 02:20 PM
I have a PC that won't boot. I have no fans, LEDs, etc. Here's what I've done so far:

1. I measured 120V at the input cable to the PS.
2. All connections appear to be good.
3. The power switch on the case is good - I measured the pins at the ATX motherboard while pushing the switch.
4. I read that the system could be started by bypassing the motherboard - I shorted the black and green wires at the MB connector. Still nothing.
5. I removed all power cables from the components. I measured the voltages at the motherboard connector, and I had voltage at two wires only - 5V for a purple wire, and 3V for a green wire.

This is where I'm stuck. Should I see other voltages on other wires right now? Unless I've missed something, it sounds like the PS is bad (although, it does provide 5V and 3V), but I want to be sure before I replace it.

What now? Thanks...

07-27-2002, 02:31 PM
Yes, you should have voltage in more then one place.
What do you mean by not booting up, a black screen?
Take out all your cards, unhook hard drive, CD Rom, floppy, and see if you get a screen. Post back.

07-27-2002, 03:07 PM
Mboard needs to recieve 12V also.

Easy way to test is check for voltage
at the pins on the case fan plug-in. And
this being done with the PSU\Mboard connector
plugged in as normal. Power-on state for the PC.

Evidently the 12V leg of the PSU has failed.


07-27-2002, 03:23 PM
I have no power - no fans, no LEDs, no drives spinning up, no monitor, etc. It's as though the power on switch is doing nothing, but the switch is receiving 5V, and it seems to function properly (I measured the voltage change at the switch pins on the motherboard).

The current state of the system is that all power connections are disconnected. I plug the unit in and turn on the rocker switch in the back (the PS is set for 110V). The power connector that attaches to the MB has 5V on the purple wire and 3V on the green wire (is that how it should be?). If I connect everything and hit the power button on front, nothing happens (except for the voltage change at the MB pins - mentioned above).

Thanks again...

07-27-2002, 04:31 PM
This is an article yoda wrote for my web site, it may help.

There is going to be a time when you need to run the power supply without the motherboard.
Example: when you want to test to see if the power supply is bad, or just testing fans and other gadgets.

Most circuits provide about a watt or less. And then there is the main power such as +12v, -12v, +5v, -5v, +3.3v and ground,zero v. You can mix and match combinations to achieve Voltages of : 1.7, 3.3, 5, 7, 8.7, 8.8, 12, 15.3, 17, and even 24 volts. And only the +12v and +5v Provide meaningful current.

You can fire up the ATX power supply by shorting the Green wire with any Black Ground, Remembering an ATX Pwrs. will not fire up normally without a mother board.This is why it is necessary to jumper it off.

07-27-2002, 04:40 PM
Well, I guess that about settles it. I kept all of the power plugs disconnected, and I shorted the green and black wires on the MB connector. Just to verify that I had a good connection, I measured the green wire again (formerly 3V), and it was pulled down to 0V. I measured all of the pins again, and only the purple wire shows 5V - no voltage anywhere else. I suppose it's time to go shopping...

Thanks for the help.

07-27-2002, 04:50 PM
I would buy at least a 350 watt power supply. I am always adding things to my computer and have a 400 watt power supply. For power supplies, I believe more is better.

07-29-2002, 07:09 PM

HERE'S (http://www.compgeeks.com/details.asp?invtid=LP-6100EA) a "BeastyBoy" that a regular of the forums Ranchdog highly recommends.

They've gone up a little in price...bought mine at $49.00 +s/h a short time ago.

500W ought to keep you energized for a while. Interesting comment on the Computer Geek site regarding being able to run with less heat by using the more powerful supply?