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

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LCD backlight blackouts and start only after SMC reset possible

Hi everyone,

I'd like to revive my old iMac, but encountered some troubles.

So first of all, the iMac only starts after it has been cut from the grid for some time or after a SMC reset including the removal of the button cell. Otherwise the Mac is stuck on startup: You only hear DVD and HDD spinning, no starting sound or anything on the display. Thanks to one of iFixit's great repair guides, I opened the iMac and saw that during these failing startups, only diagnostic LEDs 1 and 2 are one.

After a SMC reset the Mac starts normally. But during runtime, the LCD display sometimes flickers or has some blackouts (just for a short time). The image on the secondary monitor works without problems. During the blackouts, all 4 LEDs are on.

My question is now, what's the cause for that and how can I fix it?

Thanks :)

Answer this question I have this problem too

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For quick reference:

LED 1: ON if computer is plugged to AC. Turns OFF if disconnected or PSU is dead.

LED 2: ON when logic board gets a proper amount of power from PSU. OFF if iMac is not running.

LED 3: ON when logic board is communicating with the GPU. OFF if iMac is not running, or GPU is faulty.

LED 4: ON when logic board is communicating with the LCD. OFF if iMac is not running, or LCD is faulty. If ON while the screen is black, LCD panel is not properly installed, or inverter requires replacement

(Source: Here)

My money's on the inverter being faulty. That's what controls the backlight on the LCD.

If you look really closely on the screen during a blackout, you may still see your desktop, but super dark. That means the inverter is malfunctioning. I've dealt with a similar case on an A1186 MacBook.

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Thanks for your answer. But could the inverter also be the cause for the starting problems of the Mac? I assume that the power on self test fails somehow. If I can't fix these starting problems, for me it wont be worth replacing the inverter at all.

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