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Old 12-11-2008, 05:40 PM   #1
pvone
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Default Asus P5P800 - odd issue

Hi,

I have this P5P800 (LGA775) board which I recapped a while ago. The thing is it seems to work only sometimes. I can boot it up and it will POST, install windows, pass prime, etc. and be rock stable with no issues at all, and keep working no matter how many times I turn it off and on... then as soon as I stop paying attention, poof, it won't POST anymore. At this point I'll try everything I can think of, plugging and unplugging things, clearing the CMOS, leaving the memory or the video card out to see if it beeps (nope), looking for a short somewhere, can't find anything, nothing works. Then when I least expect it, it will just start working consistently again, and I can never figure out what (if anything) I did.

When it's not working, the CPU fan will spin at full speed (normally it's temp controlled) and everything on the board appears to be at least getting power, but no POST and no beeps, even if the video card or RAM is missing (when it's working, it WILL beep in those cases).

I did figure out that when it's on one of its working streaks, the only way I can make it behave as above is to leave the ATX12V 4pin cable unplugged. This almost makes me think it's something to do with that, but I can re-plug and wiggle around that cord all I want and it won't change its working status.

Also I've tested 3 different known-good power supplies - same deal with every one. Sometimes works, sometimes doesn't.

When I got the board, it had a bunch of low-profile Elite caps around the CPU - they were rated and sized like polys (680uF/4v I think) but had bulging/leaking vents like lytics
I replaced them with Nichicon LF 560uF/4v as those were the only ones I could find with the required dimensions and I'm 99% sure those low profile caps should have been polys in the first place...
Also, there are five 1000uF/16v KZG caps which I left in (they look fine). Could it be the caps?
The last thing I can think of is, well, I've only got the one 775 CPU, so maybe the CPU is crapping out? I really don't think so as I've never seen nor heard of a CPU that was half-working, but it was pulled from another board that was toasted by a Deer PSU, so... who knows.

Any ideas would be appreciated Thanks!
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Old 12-11-2008, 06:15 PM   #2
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Default Re: Asus P5P800 - odd issue

Eck! My MSI 845 E Max (socket 478) is like that, too!

I doubt it's cap-related, because Prime95 passes.
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Old 12-12-2008, 08:43 AM   #3
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Default Re: Asus P5P800 - odd issue

Usually I would blame the power supply, but in your case I can't because you've tried 3 power supplies. My ECS 661FX-M does the same thing sometimes... a real PITA. Maybe theres an intermittent short somewhere on the board that's causing this. It could even be a solder joint somewhere which intermittently comes loose or something like that?
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Old 12-12-2008, 03:53 PM   #4
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Default Re: Asus P5P800 - odd issue

Sometime my biostar will do that when i move around my computer...then i remember to plug the video cable back in
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Old 12-12-2008, 05:30 PM   #5
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Default Re: Asus P5P800 - odd issue

^

I'm cautiously optimistic that I may have fixed this. You know how the stock pushpin heatsinks make the board flex a bit, well I finally noticed if I un-flexed the board just a little (to mimic it being screwed into a case) then tried it again, it often made it start/stop working. So I decided to try reflowing the solder joints, and when I finished and went to put the CPU back in I saw there was one pin in the LGA socket that was slightly bent (I'm always extra careful with LGAs, I swear! ). I carefully pushed it back into position, put the whole thing together again and it booted right up. So far I've reseated the heatsink a couple times, taken the board in and out of a case, lots of plugging and moving... hasn't failed to post even once. So, I'm hoping the pin having intermittent contact was the problem. Time will tell

Last edited by pvone; 12-12-2008 at 05:32 PM.. Reason: grammar
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Old 12-13-2008, 12:38 PM   #6
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Default Re: Asus P5P800 - odd issue

To fix my symptoms, I would have to discharge the PSU, unplug all the power connectors and then plug all the power connectors back in, especially the molexes!
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