Scott Mueller's Upgrading and Repairing PCs Forum

I'm going to guess we are going to call this a defective PSU.

When you attach the power cord, the green light on the back of the PSU flashes on and off at about 1 second intervals.

The PC it is attached to will not power on via the front panel button.

According the my PSU tester, it has lost the -5v line on the main motherboard connector.

The +5 drive power line is fine.

I know this is very dangerous to the computer, but with the PSU connected to AC Power, I plugged the main motherboard connector into the motherboard and the PC came alive. It ran fine until I powered it off again, at which time it would no longer power on via the front panel.

I have replaced the PSU in said machine and the operation is now normal.

I would think there is no way, nor would it really be feasible to attempt to repair the PSU given the cost, (this is a 250 watt PSU).




Thanks in advance.

That is completely normal for just about any PSU built in the last decade.

PSU testers like that are only useful for confirming that a dead PSU is... well, dead. They are relatively useless for real testing. I recommend you replace the suspect unit with a known-good spare. If the problem goes away then the original PSU was defective. Scott.

Thanks Scott.

It's been quite awhile since I have been thru one of your books, and I had forgotten about the dropping of the -5v.

Clearly since the "known good spare" fixed the issue, it was indeed the PSU.

Now, I also have a good use for my PSU tester. It should make a nice paperweight. <g>

I wanted to add that the missing +5 signal on the left is also normal. The cable connected on that side is the +12V cable that powers the CPU, and that connector has only +12V (yellow) and ground (black) wires.

Now you can see why these testers aren't that useful. In this case it is showing a "passing" score (all the correct voltages present and lights lit) even though the supply was indeed defective. Because they don't test the PSU under a load they will only show a "fail" if the PSU has one or more signals completely dead (or wildly out of spec), in which case the system would most likely not be running at all anyway. Scott.

Thanks for the education, as always, Scott.

I have always been under the now obvious mistaken assumption that since a PSU will not or at least used to not operate without being under a load, that these testers did just that when testing, provide a load that will cause the PSU to operate.

I did realize after I posted my title that I was testing the 12 volt motherboard connector and that I should not expect a 5 volt signal on there. I had edited my post but forgot to edit the title.

Thanks again for showing me just how useless those are. I was about to replace it with an upgraded version, capable of testing more modern supplies. You have saved me cash, which is always good.

They do provide a "load", however it is so miniscule as to be insignficant in the real scheme of things. Using one of those testers is like connecting a dome light bulb to a car battery. A nearly dead or failing battery can easily light the dome light bulb without being able to start the car. If you want a real power supply tester you need one of these (they are about $5K). You can see one in action here. Scott.