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Is it the PSU or the Mobo?

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?
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wgmichaelis
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wgmichaelis
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2 Solutions
 
rindiCommented:
I would look at bad electrolytic capacitors on the mainboard and PSU. Strange effects like that are very often caused by bad caps.
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wgmichaelisAuthor Commented:
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.
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rindiCommented:
That's the problem with bad cap's, how they act with the system isn't predictable.
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garycaseCommented:
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].
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wgmichaelisAuthor Commented:
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.
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garycaseCommented:
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.
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wgmichaelisAuthor Commented:
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?
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garycaseCommented:
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.
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