[CentOS] Server hangs on CentOS 5.5

Lamar Owen lowen at pari.edu
Wed Mar 9 15:22:15 UTC 2011

On Wednesday, March 09, 2011 03:24:48 am Leen de Braal wrote:
> While you open the case, check for the bulging capacitor problem.
> Will have the effect you describe, freezing up the system so that even
> bios routines don't work (your fans).
> If that's the case, replace mainboard.

I've seen capacitor problems in the past, and they can be rather interesting.

What the caps do is open up (electrically speaking) meaning they no longer can smooth out the ripple in the output of the switching regulator; this ripple is very high frequency due to the switching regulator's design.  As the CPU draws more current (which happens when it's loaded, of course, since MOS gates by design consume the most power during the switching period (capacitor charging time constants on the gates of the transistors themselves)), the switching regulator has to supply more current, and if the caps are open they can't smooth out the deeper ripple.

I actually had one motherboard blow two caps; one of the cases of one of the blown capacitors was violently ejected off of the 'guts' of the cap, hard enough that it dented the PC's case from the inside.

The PC kept running, until it was put under load, then it would lock up.  When the second cap blew, about an hour later, the PC hung; it would power up and run POST, and even run the BIOS setup's memory check and health check, but as soon as the CPU was shifted into protect mode as the OS booted it would hard hang due to the CPU's increased current draw overwhelming the ripple absorbing capacity of the remaining good capacitors on the CPU's switching regulator.

There's really only one way to determine this, and that's by putting an oscilloscope on the CPU's power supply output rails and looking for ripple while running a CPU burnin program.  The hard part of that is actually finding a good place to measure the output, thanks to the typical motherboard's multilayer design.  

And while with the proper desoldering equipment and training/experience one can re-cap a motherboard, I would not recommend doing so for a critical server, unless you want and can assume personal liability for that server's operation.  Better to get a new motherboard with a warranty.  For a personal server that if it breaks isn't going to open you up to personal liability, sure, you can re-cap if you'd like and have the patience, time, equipment, and experience necessary to work on 6 to 8 layer PC boards, with may be soldered with RoHS lead-free solder, which requires special techniques.  Otherwise, as you said, you can damage the 'vias' (that is, the plated through holes the capacitor leads solder to, which may be used to connect to internal layers that you can't resolder) very easily.

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