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How to resolve diagnostic LED 3 not lit on A1058 iMac G5

I have moved the question here on the suggestion of the moderator:

I spent most of the day yesterday replacing the 25 1800 uF and 1000 uF capacitors on my mother's old A1058 iMac G5, trying to resurrect it. 6 of the capacitors had obviously bulging capacitors.

Before the replacement, LED 1 would light up, then LED 2 would briefly light and go out. Now LED 2 lights up solid, but LED 3 doesn't light up. I reset the SMU, still can't get LED 3 to light up. LED 3 is supposed to indicate that the screen and the CPU are in communication, if I recall.

I did manage to get a hold of an external display adapter and the iMac is perfectly happy connecting to it, that display works fine! In fact, I have confirmed the iMac G5 does now boot, it just can't seem to talk to the internal screen.

Any suggestions on what could be interfering in the communication between the iMac motherboard and the internal screen and how to test for it? I have been told the LCD displays DON'T generally fail. I imagine it could be the inverter, but don't know how I would test for that.

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I did reset the SMU, didn't fix it.

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I also re-seated all the inverter and video cable connections, didn't fix it.

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I checked with a flashlight and this is not a case where the display is not backlit, the display is not on, period. So this is unlikely to be the inverter.

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I am also in the same exact situation as you except I have another iMac g5 to test it on. It's definitely not the screen nor the inverter. I switched a working setup with the problem board and it still did not light up led #3. It's something on the board. I'm going to reflow the GPU but possibly might not fix it.

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Please start your own question as it will (did) get lost tagged onto another question

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Hi, I am including a link to Apples troubleshooting guide for your unit. Please pay special attention to steps 10 and 11. Good luck.

http://support.apple.com/kb/TS2094

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I did reset the SMU after reading that guide, something I didn't mention. Sorry.

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Since it works with an external, your logic board should be fine. I would check the video cable and inverter connections. Make sure the inverter is correctly seated on its pins. Can you see the desktop dimly on the screen? Was it working before you tore it down?

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Would an inverter failure cause LED 3 not to light up? I will also check the video cable connection.

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LED 3 indicates that the computer and the LCD display are communicating. This LED will be ON when the computer is turned on and video signal is being generated.

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I'm not clear if your response answers the question because what does an inverter failure mean for "the computer and the LCD display are communicating"?

My naive guess would be that if LED 3 fails to light up, that would not necessarily be connected to an inverter failure since the inverter can fail (causing the display not to be backlit) without turning _off_ the display.

I have to wait another 30 minutes for this OS update to finish and then I will check the inverter and video cable connections (although, since the video cable is screwed down onto the motherboard, I can't see how it could be disconnected unless it is disconnected from the display side of things).

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I have torn open the iMac again. I re-seated both the inverter and video cable connections (although they looked fine). No luck. I then removed everything to see if the screen might have an obvious flaw in the cable, nothing. Reassembled everything. I also checked, the screen is NOT active, even when looked at with a strong flashlight at an angle. Clearly the video signal doesn't get to the internal display. The external display works fine. Is there any definitive way to check if the internal LCD is damaged somehow?

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Sure, hook it up to a known working LCD & inverter.

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Did you check the power supply?

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Why would he do that? What out of all this would lead you to believe that there's anything wrong with the power supply?

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i changed the gpu and the power supply even and it doesn't work

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