Model A1225 / Mid 2007 and Early 2008 / 2.4, 2.8, or 3.06 GHz Core 2 Duo processor

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Does not power up but Firewire 800 port wakes up

My iMac Intel 24" EMC 2134 (Mid 2007) had not been able to power on for a couple of years. I mean completely dead, no chime sound, nothing when the power button is pressed. I recently bought a matching power supply from Amazon and replaced the old one per the instructions on this forum. It still does not power on, but an external 1 TB hard drive connected to the back of the iMac via the FireWire 800 port wakes up if and only if the iMac's power cord is connected to the wall power outlet. If I pull the iMac's power cord from the wall, the external hard drive would shut off after 10 seconds or so. If I plug the iMac's power cord back into the wall power outlet, the external hard drive once again wakes up . (By "wake up" I mean the external hard drive's blue LED turns on and you can hear the drive rev up and remain spinning.)

What can you say about the iMac's power supply given this behavior? Is it "working" but some other component is not? Is the external hard drive supposed to wake up when the iMac's power cord is connected to the wall even if the iMac's power supply itself is completely dead? Any conclusions and suggestions?

Update (08/31/2017)

I checked the LEDs. When the power cord is connected, LED1 remains on. When I then press the power-on button at the back of the iMac, LED2 blinks for a split second while the fan tries to rotate for a bit but then stops. See attached pic and video.

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Do you get any sounds, tones, anything on the screen, does it turn white?

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No, absolutely nothing, except that the external hard drive wakes up. I have to open up the iMac again so I can look at the LEDs. I was confident it would work so I put everything back together. Now I have to open it up again. Ugh.

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So far you've proved the power supply is responding. If the computer it's self is still not making any noise or visual change on the screen when you attempt to start it you have still a deeper problem.

The best thing here is to open the system and see what the diagnostic LED's tell you. Here's the IFIXIT guide you'll need to review: iMac Intel 24" EMC 2134 and 2211 Diagnostic LED's

Let us know what they say.

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I checked the LEDs. When the power cord is connected, LED1 remains on. When I then press the power-on button at the back of the iMac, LED2 blinks for a split second while the fan tries to rotate for a bit but then stops. (Sorry I can only attach a pic or video as an "Answer" not as a "Comment". See my "Answer" below for the pic and video.)

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Could a dead PRAM battery be the cause of why my iMac (bought in 2007) wouldn't turn on?

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OK, I bought a new CR2032 3V battery and replaced the dead one on my iMac. As soon as I plugged the power cord into the wall, the fan started to spin for a second then stopped, spin, stop, spin, stop, and continued to repeat this continuously without my doing anything. Each time the fan spins, LED2 turns on briefly. Pressing the power-on button behind the iMac doesn't make any difference. Any ideas?

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Sorry Tim - The logic board needs repair or replacement. There is a point one has to throw the towel in when the problem needs a better set of skills set that what one has (as well as the needed tools & parts).

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Can you at least tell me how I can salvage the data on the hard drive? Even if I successfully remove the hard drive, can it be fully accessed by another Mac computer? If so, how?

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Tim will be eternally grateful.
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