Model A1224 / Mid 2007 and Early 2008 / 2, 2.4, or 2.66 GHz Core 2 Duo processor

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Why does my Intel iMac reboot after 5 minutes of normal use?

iMac 20/2.4 GHz/ 1 GB RAM, Mod A1224 EMC 2210

I inherited this iMac after a lightning strike disabled the wired LAN connectivity 1 1/2 years ago. I discovered that the wireless connectivity still worked and it has functioned normally otherwise until recently it has begun rebooting after a few minutes. After research on iFixit and reseating RAM chip, resetting PRAM and SMC, I pulled it apart to observe the LEDs. The first is on when the Mac has power. All four light up normally from a cold boot. Then when it reboots three light up but not the fourth and the LCD stays black. Observations: there is a black smoky mark on the inside of the front bezel directly on top of the small squarish plug-in immediately below the LEDs on the logic board. No other obvious damage. If I turn off wireless sometimes the machine will run for an hour or more before rebooting. It has always, even when new, had a pronounced buzz or hum after turning down the brightness of the display which goes away after a few seconds. I have pulled the PSU and there is no obvious damage from a visual inspection and I'm not experienced enough to attempt voltage testing on it. If a bad PSU seems the likely explanation I would replace it but if it is the logic board, probably not. Where do I go from here? Many thanks in advance...

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It sounds like the logic board has an intermittent fault due to the previous damage, probably aggravated by heat. Does any part of the system get noticeably warm during use?

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Thanks Jerry. When running a temp monitor program I did notice that the highest readings were from the PSU sensor and the wireless module, with the wireless module temp. climbing very quickly after turning on wireless from the desktop. The PSU eventually climbs to 120 or so before the reboot happens on the ocassions when I can get it to run for more than a few minutes. The upper left corner of the machine gets warm to the touch. That would be the location of the PSU.

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You never want to see 4 lights on an iMac. One of the first 3 say there is correct voltage to the motherboard. iMacs are notorious for having problems with their capacitors on the logic board and power supply. More than likely the 1800mf and 1000mf capacitors on your got hurt with the power surge. Look for bulging or exploded capacitors on your motherboard.

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Good advice, ABCellars.

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Thanks to you both but according to this thread you DO need to see all four LEDs lit to indicate normal operation:

LED power supply and logic board tests

also I thought it was only the G5s that had the bad caps. The logic board would have to be pulled to check caps and there is no guide for me to follow. :>(

Jerry, the wireless card is internal and its temperature can only be monitored with software. Given that this computer took a lightning hit w/o surge protection at the previous owners house I'm inclined to think that there is an intermittent error associated with the logic board and LAN circuitry else why would the computer quit so quickly when wireless is turned on but remain on for awhile when wireless is turned off? When wireless is turned off the temp. of the wireless module stays relatively low according to monitoring software but goes high as soon as wireless is turned back on. Then the computer reboots and the LCD never comes back.

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Replacing the PSU sounds like the first order of business.

Wireless devices do tend to get warm, so unless it's literally hot to the touch or starts having issues, I wouldn't worry too much about that.

Let us know if this fixes the problem or if there's more. I'd like to believe that it's JUST the PSU, but you never know. Power-spike-related damage is unpredictable. I've seen surges take out modems and the motherboards they were attached to, or just the modem. Luckily, with most mid-grade or better surge protectors and UPS (battery backup) units having built-in cable, phone, and ethernet jacks, that's not as much of an issue these days. (Which, of course, you should be using.)

I hope this helps!

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