Login    Forum    Search    FAQ

Board index » Upgrading and Repairing Forum » Desktop PC Hardware




Post new topic Reply to topic  [ 6 posts ] 
Author Message
 Post subject: PSU seems to have lost -5 and +5
 Post Posted: Sat Feb 22, 2014 9:50 pm 
Offline

Joined: Thu May 31, 2007 8:37 pm
Posts: 201
Location: St. Louis, MO
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).


Image

Thanks in advance.


Top 
 Profile  
Reply with quote  
 Post subject: Re: PSU seems to have lost -5 and +5
 Post Posted: Sat Feb 22, 2014 10:28 pm 
Offline
Site Admin

Joined: Sun Feb 04, 2007 11:44 am
Posts: 5855
Quote:
According the my PSU tester, it has lost the -5v line on the main motherboard connector.

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.


Top 
 Profile  
Reply with quote  
 Post subject: Re: PSU seems to have lost -5 and +5
 Post Posted: Sat Feb 22, 2014 11:25 pm 
Offline

Joined: Thu May 31, 2007 8:37 pm
Posts: 201
Location: St. Louis, MO
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>


Top 
 Profile  
Reply with quote  
 Post subject: Re: PSU seems to have lost -5 and +5
 Post Posted: Sun Feb 23, 2014 11:15 am 
Offline
Site Admin

Joined: Sun Feb 04, 2007 11:44 am
Posts: 5855
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.


Top 
 Profile  
Reply with quote  
 Post subject: Re: PSU seems to have lost -5 and +5
 Post Posted: Sun Feb 23, 2014 9:12 pm 
Offline

Joined: Thu May 31, 2007 8:37 pm
Posts: 201
Location: St. Louis, MO
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.


Top 
 Profile  
Reply with quote  
 Post subject: Re: PSU seems to have lost -5 and +5
 Post Posted: Sun Feb 23, 2014 10:24 pm 
Offline
Site Admin

Joined: Sun Feb 04, 2007 11:44 am
Posts: 5855
glen4cindy wrote:
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.

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.


Top 
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
 
Post new topic Reply to topic  [ 6 posts ] 

Board index » Upgrading and Repairing Forum » Desktop PC Hardware


Who is online

Users browsing this forum: No registered users and 1 guest

 
 

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron