G4 MDD PSU or other problem ...help?

W

Windsong

Registered
Hi all,
I've had a MDD for 4 years. Dec 2007 the PSU died. I had it replaced under my renter's policy. Brand new PSU.

Now, after 1.5 years, I was having disk corruption issues which led to numerous forced shutdowns (there's no "reset" button on the MDD) finally, after about 5 of these in one day, the "power on" light lit when pressed, and I heard a crackle & smelled a bit of smoke. I pulled the power cord immediately.
btw pressing the power button after that did nothing at all, no "fan attempts" no crackle.. nothing.
Same deal, I called the ins co, took MDD to the authorized techs, who checked & said the PSU was dead... AGAIN.
The Ins Co. "totaled" the MDD & sent me a check. I'm getting used G5, I think.

BUT! I'd love to get the MDD working again. Just to have it as a second machine, and run 5 volt only PCI music cards I need to use.

I found instructions for testing the PSU.
I found a diagram of the pins and voltages.
Connected the PSU to "the juice" and started testing the connector, where it plugs into the motherboard.
Here's what I found (connected to power but obviously still not starting the computer):

Pin 1 (says +5volts sb), reads 5 Volts.
No other pins showed ANY voltage EXCEPT pin 14 (which says +25Vsb). That read +24.8V when tested.

What I DON'T know is...
If disconnected from the MDD, should all the pins read voltage as the pinout diagram says? I'm brand new to this level of diagnosis. Instructions say connector MUST be snapped in to test.

Also, In testing, I discovered the power button was broken.
I just got a new one, installed it, and NOW pressing it starts all the fans like crazy, and seems to send power to the hard drives & optical drives.
The red light on the mother board lights.
I've reset the CUDA chip, I've tested the battery (it's fine)
But the sceen never lights up, and no startup chime. The power button does glow white as expected.

I've tried booting from a Hard drive, a CD, & tried target disk (T).... drives spin but the machine doesn't boot.

As above, I do have correct "trickle power" from the PSU and obviously power to all the fans, and seemingly to the drives, since they spinup.

HOWEVER, testing instructions tell me to continue and test voltage (with the PSU connector snapped into the motherboard). pin 12 to pin 24 should read 5 volts. I get nothing.
btw it's nearly impossible to jam the test lead into the connector WHILE it's snapped into the motherboard, but that what the instructions say...:mad:

Is it possible I get all this power, and disk spinup and the PSU is still bad?
Is it possible power to the logic board etc isn't there?
The red light on the motherboard IS on.

I'm worried the logic or mother board might be damaged.
Before I get ANOTHER PSU for $180, does it sound like the psu IS really the culprit (as the techs said), or could the logic board chips be fried? When it came back from the techs it was still acting dead UNTIL I replaced the power switch, now it "tries to startup".

How important is the "no startup chime".
What does that symptom mean?

I really need some help here.
Thank to any and all in advance.
Tom
 
I'm concerned about the "crackle & a bit of smoke"! That usually means that you have lost enough that you won't get this work without more expense.
I usually try to find a smaller 'probe' to get into those power supply connectors. I will unfold a paper clip to stick into the relevant pin on the connector, and create a make-shift extension to measure voltages. You _must_ be careful if you try that. I do that if I don't have anything else at hand, but it's very easy to short the contacts together.

You don't see all the voltages from the power supply, because the logic board/motherboard turns on the power supply. That's why it needs to be connected. A PC repair shop may have a power supply tester, which provides the load or connections needed to turn on the power supply.

Your current symptom (power, fans, drives, but no boot chime or video) could mean that the processor or the logic board has failed.
You could try reseating _everything_ that you can disconnect/reseat. PCI cards, RAM memory cards, modem, Airport card, AGP video card. Also, and most important, I think, is removing the processor, and reseating that in its connector.
You should be looking for any obvious burn/flash marks on any of those cards, etc.

If none of this changes anything, I suspect that your processor is bad, or the logic board.
It will likely be simpler to get a complete replacement MDD on eBay, I bet...
 
Hey Delta,
Thanks. The crackle & puff came from the psu. My face was right there.
But you pegged my worry, that somehow something else got fried.
Still, I've already done all the things you suggested. I've seen no burn marks anywhere around the heat sink processor or any cards. I took a pic of the processor.
Attached below. The gray stuff if thermal compound. All I see is slight gray-ish marks on the two chips above the processors.. hard for me to tell.
Certainly nothing obvious. But everything's been pulled reseated, looked at etc. as you suggested.
But I'm fairly certain about no voltage between pins 12 & 24. I stopped testing there, because the instructions said if there was no voltage there, the PSU is bad, so I'll retest, but assume it is bad.

You're saying that the mother board & or the processor might have gone as well?
That's always been my concern. Let's assume the PSU failed. Could it have "taken" the processor with it, or is that an unlikely scenario?

pics of the processor are attached.
Thanks
Tom
 

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Yes, your power supply should be bad, if you don't measure that 5 volts. You're getting something (fans and etc.), but there's also a 3.3 volt feed for the processor. I bet you're not getting that, either. That won't matter if the main 5 volts is not available.

The only way to find out if something else has failed is to replace the power supply. You already know that's bad... You also probably know it's not cheap to replace...
 
I bought a replacement PSU, tested it, installed, and reseated the processor card. The fans now spin full on (madly) on start, lights on mother board come on, but no chime, no startup.
Finally re-examining the CPU wit a magnifying glass I found a tiny pencil point black burn "dot" on one of the two CPU's.

Answer: fried CPU. Good motherboard.
NOW I have to find a new processor.
Much thanks all.
Tom
 
You may end up replacing more than your hoping, it could be the processor, it really could, but theres many many more things that could have burn't up.
 
I've bought a new motherboard and processor so other than a PSU I don't think there are ANY things left to have "burnt up.
The fans all work fine.

Doesn't basic "computer" equal PSU, motherboard, CPU?
I'm not talking about drives, RAM etc.
I know the PSU is bad so I can't check ALL the RAM but I have 2 Gig, and I just highly doubt ALL of it fried... all at once

I'm told I can pull the video card and powerup the system, so other than the video card, I'm left with a dead PSU... which is where this all started.
The power button unit is also new.

Am I missing something else in the "possibly fried" checklist?
 
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