Burned motherboard power connector - what to replace?

June 8, 2009 7:27 PM

The power connect for my motherboard is partially burned! Should I replace the plug, the whole PSU or the entire motherboard?

So a week or so ago the computer wouldn't boot. After wiggling all the connectors I noticed the 4-pin power plug from the PSU to the mobo was a little blackened. Odd, but reseating it worked. Now it's very blackened and I'm unsure of the extent of the damage to the mobo & connector there. This is a HP desktop system with a 400W PSU and an ASUS mobo. The easiest solution is simply to pull the components and build a new system but I'd rather find a cheap, cheerful & reliable solution, emphasis on the cheap. Opinions on whether to replace just the connector, the whole PSU and whether the mobo is likely damaged as well (I probably won't find out until I replace the PSU)

posted by GuyZero to computers & internet (12 comments total)

This isn't the 20-pin connector but the 12V 4-pin connector.
posted by GuyZero at 7:29 PM on June 8

I think before you do anything you try to figure out how it happened.
posted by Chocolate Pickle at 7:32 PM on June 8 [1 favorite]

There's been no freak power surges although I don't have a properly grounded outlet anywhere in the house... that will obviously take some more work to fix although I'm working on it. Most of my electric appliances are not bursting into flame or becoming mysteriously blackened so I'll restrict it to being a PC-specific issue for the moment.
posted by GuyZero at 7:35 PM on June 8

I'd just recondition the blackened connectors with emery cloth or somesuch and spend the seventy bucks or so on a line conditioning UPS.
posted by torquemaniac at 7:39 PM on June 8

I'd just recondition the blackened connectors with emery cloth or somesuch and spend the seventy bucks or so on a line conditioning UPS.

Pink pencil erasers work well for jobs like this.
posted by The Light Fantastic at 8:22 PM on June 8

Connectors don't burn from 'dirty' power or poor grounding, at least not on the fully-isolated DC side of a switchmode PSU. They burn from heat due to excessive current or bad contact.

Which means that either a) something's drawing too much current through that 12v connector, b) you've got a loose connector on the lead from the PSU, or c) you've got a loose connector on the m'board.

a) is unlikely without other symptoms (like fire & smoke ;-), so that leaves b) & c). The big problem with those sorts of connectors is that the springy sleeve in the plug loses tension in cheap connectors, causing high contact resistance that burns both the plug and the motherboard connector.

Determining whether either or both sets of contacts are burnt badly enough to warrant replacement takes a bit of electrical knowledge & experience to gauge (more than I'd trust the average PC tech to have, to be honest). If the pins on the motherboard look OK, or can be cleaned up to look OK (i.e. not burnt/pitted), then I'd just replace/fix the PSU & see how that goes. If not, replace the mobo as well.
posted by Pinback at 11:47 PM on June 8

I agree with Pinback; that 4-pin connector isn't the only 12v lead to the motherboard; if it was a problem with the supply or demand for 12v, you would expect the (yellow) 12v leads on the 24-pin ATX power connector to also be blackened.

So I would suggest that the problem was in the 4-pin connector on the lead from the PSU.

IMHO trying to source a replacement 4 pin connector and fit it would be a PITA compared to just getting a complete new PSU. So I'd just get a cheap replacement PSU.

If you're the type of person who buys expensive PSUs it might be worth borrowing a PSU from a friend to make sure the motherboard isn't fried.
posted by Mike1024 at 12:42 AM on June 9

I agree with Pinback; that 4-pin connector isn't the only 12v lead to the motherboard; if it was a problem with the supply or demand for 12v, you would expect the (yellow) 12v leads on the 24-pin ATX power connector to also be blackened.

It is, however, a different buss. The 4 pin +12V connector is +12V2DC, the two lines in the main connector are +12V1DC. So, it's possible that a failing +12V block in the power supply would overcurrent just that one set of wires, since the other block is independent.

Easiest test there is to measure the voltage -- if it's having to send enough current to melt that connector, the voltage will almost certainly sag.
posted by eriko at 7:35 AM on June 9

a) I got a PSU tester a fry's but it was defective
b) they also had IDE power connectors that convert the 12V line there to the 4-pin mobo connector - who knew such things existed?

so for the moment I have a functional PC. Believe it or not I haven't yet found the 20/24/26 pin connector yet as it buried somewhere behind other components (or this mobo doesn't have one?) but I'm going to check it to ensure it's not burned and replace the PSU tester with a working one and then see if the voltage levels are indeed correct or whether I need a new PSU.
posted by GuyZero at 8:00 AM on June 9

eriko: So, it's possible that a failing +12V block in the power supply would overcurrent just that one set of wires, since the other block is independent.

How? Current just doesn't work like that - just 'cos a supply is rated at 12v 20A doesn't mean it's going to shove 12v @ 20A into whatever poor little thing is hung on the end of it. As I've pointed out here before, current ratings are an "up to, and including" thing, not an "always" thing, and current is dependent on the characteristics of the load, not the supply.

Outside of the poor contact scenario I mentioned*, the only thing that'd cause burning like that is arcing between contacts due to overvoltage - in which case, the m'board would be toast.

(* Or, possibly a short on the m'board leading to excess current drain on that rail [up to 20A in the case of the ATX12v2.xx supply spec, IIRC], or low voltage from the PSU leading it to ramp up current to maintain the same power o/p - but, given that those connectors [at least the real certified versions] are rated @ something like 40A, that still shouldn't cause burning.)
posted by Pinback at 4:49 PM on June 9

Pinback, I think you are right on the money, but.. You could get a partial short on the motherboard somewhere causing an over current. Bad capacitors, for example, put too much load on the power supply...

eriko, not all PIV style ATX supplies have split 12V rails. My guess would be that most don't, though that is changing fast.

GuyZero, I thought everybody knew about that part, but good job answering your own question :)
posted by Chuckles at 9:21 PM on June 10

Current just doesn't work like that - just 'cos a supply is rated at 12v 20A doesn't mean it's going to shove 12v @ 20A into whatever poor little thing is hung on the end of it.

Sloppy writing on my part -- current is drawn, as you said. What I was trying to say is that on many power supplies (and by spec, though Chuckles notes that many of them cheat) the 4-pin +12VDC connector is on a different buss, thus, too much current there may not manifest on other +12VDC connections to the motherboard.

You're correct that corrosion (thus, resistance) could cause this. However, a short could also cause a big current draw. It probably wasn't a dead short, usually the PS will shut down on that, but if the component drawing is show a tenth of the resistance it should, well, I=V/R is a harsh mistress.

Guy Zero -- a cheap PS tester isn't as useful as a decent multimeter, if you have one. Here's the specs, see Figure 11 on Page 36. Obviously, don't try to meter out the SATA power connectors, but the rest are easy -- just slide the positive probe into the connector along the wire. Use any wire labeled "COM" for the ground probe. +5VDC and +12V1DC are the easiest to check -- just find a disk drive power connector (Not so easy if all you have are SATA power connectors.)

In general -- yellow wires are +12V1DC, Blue is -12VDC, Orange are +3.3VDC, Red are +5VDC. Black are COM (ground, but not really) By spec, +12V2DC is supposed to be Yellow with a black stripe, but many cheat. Though, given what Chuckles said, if it isn't a split rail supply, not using Yellow/Black there is actually correct.
posted by eriko at 7:03 AM on June 12