Solved

Is it the PSU or the Mobo?

Posted on 2011-09-19
8
394 Views
Last Modified: 2012-05-12
A customer brought me a Compaq Presario SR1300NX that would not turn on.  The PSU tested OK with my tester (not a load tester).  Shorting the power switch pins on the Mobo had no effect.  I back probed the Mobo connector and shorted pin 14 (PS-ON) to ground; the PSU turned on and the computer booted.  Thereafter, the computer would start with the front panel switch.  However, when he got home the computer would not turn on once again.  He brought it back and, same thing; jumpered the Mobo connector and the computer boots and thereafter boots with the front panel switch.  Currently, I am shutting it off, waiting awhile and trying to turn it on again, attempting to reproduce the circumstances that cause it to not turn on.  Any idea what is causing this?
0
Comment
Question by:wgmichaelis
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 3
  • 2
8 Comments
 
LVL 88

Expert Comment

by:rindi
ID: 36561932
I would look at bad electrolytic capacitors on the mainboard and PSU. Strange effects like that are very often caused by bad caps.
0
 

Author Comment

by:wgmichaelis
ID: 36562032
I did notice three caps close to the processor socket that are ever so slightly swelled; that is, the tops are not completely flat.  I have seen some systems where the caps are actually leaking a bit or swelled significantly more than these and the computer still works so I was looking for another possible cause.
0
 
LVL 88

Assisted Solution

by:rindi
rindi earned 200 total points
ID: 36562159
That's the problem with bad cap's, how they act with the system isn't predictable.
0
Industry Leaders: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

 
LVL 70

Accepted Solution

by:
garycase earned 300 total points
ID: 36563034
What's happening is the soft power-on function isn't working -- most likely due to the momentary short from the front-panel switch not generating the correct interrupt to the chipset CPU (NOT the main CPU).     This is almost certainly due to a bad capacitor, as rindi noted.

Once you force the PSU on directly (by doing what the chipset CPU WOULD have done if it had "seen" the interrupt), the system works fine, since the standby voltage sustains the charge on the capacitors at a high enough level that the soft power-on signal can then work (since the capacitor is only degraded -- not completely failed).      You can almost certainly replicate the problem by physically unplugging the system for a reasonably long time (perhaps an hour).

There are a variety of ways to fix this ...

(a)  Replace the motherboard;
(b)  Have the capacitors replaced [http://www.badcaps.net/pages.php?vid=2 ]  or replace them yourself if you're comfortable doing that;
(c)  Install a momentary contact switch between PS-On and ground that can be used to turn on the system when needed [Clearly this is a "kludge" ... but if the system is otherwise working fine and cost is a major factor it's certainly the cheapest].
0
 

Author Closing Comment

by:wgmichaelis
ID: 36589011
As I always do in situations like this, I replaced the PSU unit with a known good unit before assuming the MoBo is bad--and, in this case, that solved the problem. The MoBo caps perhaps aren't even swollen it may be that the scores on the cap tops just make the top look like it's swollen. I have replaced the caps on the MoBos of computers with the same symptoms but the caps were obviously swelling or even leaking and fixed the problem that way. Because the caps on this MoBo did not appear to be swollen I submitted the question to the experts on this site. Both rindi and garycase were on the same track but garycase's explanation was much more thorough and I like that thoroughness.
0
 
LVL 70

Expert Comment

by:garycase
ID: 36589265
Very interesting result.   I'd guess that the issue was still bad caps -- but in this case it was bad caps in the power supply => the fundamental issue was almost certainly exactly as I detailed earlier ... once they had some charge, as long as you didn't lose a/c power they maintained enough charge for a soft power to work; but if you lost power and they discharged they were sufficiently deteriorated that they wouldn't work correctly.
0
 

Author Comment

by:wgmichaelis
ID: 36589351
And, that's exactly what was happening...if I unplugged the power cord and left it for about an hour (I didn't try different periods of time) the computer would not start.  But, once the PSU was activated by grounding PS-ON, it would start thereafter.  At the owner's residence, two siblings were fighting over using the computer and one of them pulled the power cord from the wall socket.  That's what got the whole thing started.  You all are quite knowledgable and were quick to reply to my initial submission; thanks a bunch!  Why, though, wouldn't the caps eventually charge back up after the PSU was plugged in and restore the soft-power-on function?
0
 
LVL 70

Expert Comment

by:garycase
ID: 36590107
They don't charge until the PSU is on.    So the initial power-on required forcing it with PS-ON.    Afterwards, they were fine unless/until the system lost A/C for long enough for them to discharge.
0

Featured Post

What does it mean to be "Always On"?

Is your cloud always on? With an Always On cloud you won't have to worry about downtime for maintenance or software application code updates, ensuring that your bottom line isn't affected.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Windows 7 does not have the best desktop search built in. This is something Windows 7 users have struggled with. You type something in, and your search results don’t always match what you are looking for, or it doesn’t actually work at all. There ar…
Skype is a P2P (Peer to Peer) instant messaging and VOIP (Voice over IP) service – as well as a whole lot more.
Finding and deleting duplicate (picture) files can be a time consuming task. My wife and I, our three kids and their families all share one dilemma: Managing our pictures. Between desktops, laptops, phones, tablets, and cameras; over the last decade…
Monitoring a network: how to monitor network services and why? Michael Kulchisky, MCSE, MCSA, MCP, VTSP, VSP, CCSP outlines the philosophy behind service monitoring and why a handshake validation is critical in network monitoring. Software utilized …

636 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question