iMac 20" 2008 - shuts down

[pearman]

Hello there,

I have an iMac that intermittently shuts itself down. The power supply has been replaced, but the issue is still there. Usually it takes 5 to 30 minutes before it shuts down. After shutdown it instantly restarts again.

I have not been able to stress the shut down - doesnt seem to matter if CPU is fully stressed or idle - it'll shut down anyway.

I have eliminated heat issues, so I was thinking there's a bad capacitor some where on the logic board. I cant see any bulged capacitors, but at least one of the capacitors has a small crack in the paint (meaning it started to bulge?).

Please see attached images, and let me know what more info is needed to further troubleshoot this issue.

This is the computer in question;
http://www.everymac.com/systems/appl...ryn-specs.html

[ben7]

Re: iMac 20" 2008 - shuts down

They don't look bad to me.

It still could be a heat issue. Typical with a bad solder joint, when it gets warmed up, it expands and breaks the circuit since there is a crack in the solder.

[theokretes]

Re: iMac 20" 2008 - shuts down

The majority of those capacitors are polymer (so they won't bulge and won't fail as easily). You could try replacing the SMT electrolytics, but I honestly doubt that is the cause.
If I had to guess, I'd say it's a BGA chip with a bad joint.

The infamous PowerPC G5 towers suffered from BGA problems causing issues like that.

Easiest option would be to buy a new system board. Unfortunately laptops and all-in-one systems are prone to a lot of issues like that.

Your last resort would be buying a BGA rework station or tossing the board in the oven for some baking, erm, reflowing.

[Maxstang]

Re: iMac 20" 2008 - shuts down

I've successfully brought a macbook pro back by using a heat gun on the video processor. Basically wick some thin no clean flux under the suspect chip, wrap the board in foil playing close attention to covering any plastic parts with multiple layers and cut a hole right around the chip you want to heat up. Then get it hot! Macbook in question has been running fine for 3 years since the repair.

[Maxstang]

Re: iMac 20" 2008 - shuts down

The other thing to check would be bad thermal compound on the processor itself. Apple uses crappy thermal paste that can dry out and crack away from the processor. Try replacing that first.

[whyatt72]

Re: iMac 20" 2008 - shuts down

Change the cpu ! ;-)

[Macman1974]

Re: iMac 20" 2008 - shuts down

Hi everyone, I would replace the cap showing the stress cracks in the paint. After that I would make sure the board is completely clean and dust and debris. Maybe even going as far as ultrasonic cleaning it and then changing the thermal compound to make sure the chip is not overheating.