Bad power supply or motherboard?
April 5, 2007 10:57 AM   Subscribe

PC troubleshooting help: Is it the motherboard or the power supply?

A few days ago I connected a working SATA hard disk to a new gizmo purchased from Newegg that would allow me to connect the drive to USB to pull some files off. When I hooked the gizmo up it started smoking, so as quickly as possible I disconnected it. I took the SATA drive over to a working PC and plugged it in as an auxiliary drive. The now-fried disk appears to have damaged the second PC I tried to test it in.

The symptoms of the second PC are: When you switch the power on using the pushbutton on the front of the case, the keyboard and case lights, case and cpu fan come on momentarily and then just die. No POST. Is this symptomatic of a bad motherboard or power supply? I "triggered" the power supply while disconnected from the motherboard as suggested here and the power supply fan stayed on.

This is a homebrew PC with a Microstar K8MGM2 mobo, if it matters.
posted by SteveInMaine to Computers & Internet (6 answers total)
Motherboard. Fried drive controllers will typically smoke a motherboard.
posted by SpecialK at 11:01 AM on April 5, 2007

Yep...motherboard...when the power supply dies, it normally dies completely and you get no lights, no beeps, no fans.
posted by cyclopz at 11:34 AM on April 5, 2007

Response by poster: So then, is it safe to assume that the CPU and RAM are still okay?
posted by SteveInMaine at 11:36 AM on April 5, 2007

While it's probably safe to assume the CPU and RAM are okay, you won't know until you get a new motherboard and try them out. It's possible they fried. It seems unlikely, though.
posted by notnamed at 12:31 PM on April 5, 2007

I'm going to have to go against the prevailing wisdom in the thread and suggest trying a power supply first. ATX power supplies are designed to be fairly smart and shut themselves down when they get too far out of tolerances. The motherboard has to be somewhat there to be able to realize the power button is being pressed and signal the power supply to power up completely. This isn't the olden days where the switch controlled the power supply directly. Also, power supplies are usually cheaper than motherboards, so when I can't decide, I go with the power supply first, and it's usually right.

These days with the disk controller being integrated into the southbridge, it's harder to know for sure, though.

Of course, I'm a person who saw his computer smoking once and turned it off, removed the PC speaker (which had the insulation burned off its leads, causing the smoke) and turned it right back on, presuming it was the speaker's fault ;)

And before you buy anything, try taking anything you can out of the computer (cards, disks, RAM) and see if it will either get to a POST screen in the first two cases or give you a nice beep code in the latter case. Sometimes things just get jostled and removing and reseating everything in sight will solve a problem. Start with the power connector to the motherboard. (removing it and reseating it, not trying to boot with it unplugged! :o)
posted by wierdo at 7:16 PM on April 5, 2007

I suspect a fried motherboard too, but..

ATX power supplies are designed to be fairly smart and shut themselves down when they get too far out of tolerances.

Old AT supplies, as well as ATX supplies, have short circuit protection. If the motherboard is drawing too much current, the power supply tries to start, but as the motherboard begins to power up it draws too much current and the supply quits. It is very likely that this is what you are seeing. However, since it worked before, and doesn't now, it is also very likely that you are seeing it because of a catastrophic fault on the motherboard.
Technically, silicone tends to fail short, but everything in nature fails open in the end. Some faults in non-critical parts of the board could theoretically be "fixed" by burning those non-critical parts out completely. This is not recommended :P

Anecdotally, it seems as though power supply short circuit protection isn't very ideal, and some motherboards just don't get along with some power supplies, even though they are both perfectly fine. Perhaps this is because a short is detected if a lot of current is drawn at one voltage, but not much at another, when the power supply expects the opposite load condition.
So, there is a chance that another power supply, with a differently tuned short circuit protection, might "fix" the motherboard :P
posted by Chuckles at 8:40 PM on April 5, 2007

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