How to tell difference between full logic board and power supply failure?

Question

How to tell difference between full logic board and power supply failure?

Okay so I'm considering buying my friend's power mac G5, which will not turn on. ABSOLUTELY nothing happens when you press the power button. Is this just a power supply failure, like this particular symptom often indicates, or is it possible that it is a fullllll mother board/logic board failure? I'm willing to replace the power supply because it doesn't sound overly hard to me, and the replacement unit itself would only cost 145 bucks... a new motherboard on the other hand would be much harder to get ahold of and would cost well over 200 dollars, which I'm not exactly willing to do... If anybody can give me some insight as to how likely it is that its a power supply issue or if its a motherboard issue, please do so. I'm already taking a chance on a unit that won't power on, but if theres a possible that repair costs will be through the roof, I might just blow it off.

powermac g5, Mac OS X (10.5.6)

Posted on Jun 7, 2009 2:49 PM

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Answer 1

Jun 7, 2009 8:00 PM in response to michaelwiggins

You need to check the voltage across the power supply with a multimeter, while the computer is attached to mains power. Remove the front fans, and there's a black cap over the PSU connector (below the RAM). Take this off.

The connector has 24 pin spaces, four of which are blank spaces (no pin). Before you unlatch the connector, make a note of the location of two pins, as you see them with the connector attached. Pin 1 is the rightmost pin on the bottom row; pin 23 is second from the left on the top row.

Remove the connector and flip it over. With the red lead touching pin 1, and the black lead touching pin 23, you should get +5V across the pins. If not, your power supply is likely dead.

Matt

Message was edited by: Matt Clifton

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Answer 2

Jun 8, 2009 11:46 AM in response to michaelwiggins

That sounds easy, I can borrow my physic's teacher's volt meter thing... but is there anyway to tell /before/ taking it apart? If the motherboard failed completely, is there there any responsiveness when you try to turn it on? Flashing lights, beeping, fans starting? Because I know without a power supply absolutely nothing happens.

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Answer 3

Jun 8, 2009 12:08 PM in response to michaelwiggins

If there was a failure in your motherboard, but the power supply was good, I'll still expect some sign of life. You'd at least get an LED on the logic board and possibly flashing power LED depending on the boot configuration (whether or not you have RAM installed, etc). Seeing nothing at all does point strongly to a failed PSU. It's not a big deal to check the connector - you really only need to pull out the front fan unit.

Matt

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Answer 4

Jun 9, 2009 8:22 AM in response to Matt Clifton

Oh that sounds easy, thanks. And I see you have this early 2004 model as well. All of the searches I do for broken power supply queries send me to pages about 2005 models, I understand they had problems with burning out power supplies as well as leaking liquid cooling units; are problems like that solely associated with the 2005s or have 2004s been known to have that problem eventually as well?

Thanks

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Answer 5

Jun 9, 2009 8:32 AM in response to michaelwiggins

Well, the problem is fresh for me because my PSU just failed ... two days before we received its planned Mac Pro replacement (it knew its time was up). I do have the liquid cooled CPU (and there were certainly problems reported with that, although mine always ran fine).

I know there was an official Apple warranty extension for failed supplies for late 05/06 models, so there was certainly a known issue for those G5s. I haven't heard how common 2004 failures are.

Fortunately, perhaps, the test is very simple - unfortunately, replacing the PSU is a true PITA! You basically have to remove everything but the logic board. Luckily you can test the new PSU right out of the box without having to install it first - plug it into mains and run the same test. You should also hear a click when you plug in the power, which is another sign of life. Note that the 5V is approximate - you might get a little more, but it'll be obvious if it's working.

Good luck!

Matt

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