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Randy
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Why does my iMac G5 not power up after logic board exchange?

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Hello

I exchanged my Logic Board in this computer now it will not start up.

The old board had a couple bad capacitors and the computer would sometimes start and run for awhile before freezing up.

I have checked the LED's and the # 1 light is on when plugged in.

When I hit the power button the # 2 light comes on, the chime sounds and then the # 4 light comes on, red color, and the computer powers down. The # 2 light goes out and the # 4 light stays on.

I can get the # 4 light to go out by bumping the power button but when I try to start up again the same thing happens.

Checked my power supply and it has a small burnt spot in the upper left part of the lower right quadrant and a white foaming looking substance on the board. Not sure if this is why it is not powering up because of the # 4 light coming on.

In all the research I have done I have not come across this issue so not sure what is going on.

So my questions are;

Why is the # 4 light coming on while the computer is starting up?

Is this why it is shutting down?

What is causing this issue and how do I fix it?

Or is it all tied to the power supply?

Any advice would be very much appreciated and thank you in advance.

Randy

Edited by: Sam Lionheart ( ) , Randy ( ) , pollytintop ( )

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thelectronicnub
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LED 1 indicates that trickle voltage from the power supply has been detected. This LED will be ON when the computer is turned off and your power supply is working correctly.

LED 2 indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.

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.

LED 4 lights only if the computer detects an over-temperature condition. This LED will be OFF when the computer is turned on and running at the correct temperature.

More at http://support.apple.com/kb/HT2831]

Edited by: thelectronicnub ( )

Try reapplying the thermal paste.

thelectronicnub,

Hello Thank you for the info. Re-applied thermal paste and changed the DC to DC board as that is what the diagnostics of the number 2 LED suggested to do. Also replaced the Power Supply while I was into it that deep, it shower two burnt spots on it. Not sure why the number 4 LED showed red at startup, it was not running long enough to heat up. Put everything back together and I am loading the OS back on the Hard Drive, everything seems to be running ok now. Randy

Randy,

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DaveyJake
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The only reason that I can think of as to why a Mac (of any kind) wouldn't power up is due to a wrong part--or piece of that part--being the wrong size or having the wrong connector. A replacement piece for a Mac must be IDENTICAL in every way, shape, form, and physical build to the old piece.

I'll give you an example: the sound card in PowerBook G4. While they are all identical in physical size, the physical differences are the placement of Torx screw holes that hold the card in place AND the size of the connector that that the TrackPad cable connects to. If the TrackPad cable itself cannot properly connect to the head found on the sound card, the PowerBook will not boot up.

When you installed the new logic board, did you find ANYTHING different when compared to the old logic board? Were any of the connectors different?

Let me know when you get a chance. Good luck!

Daveyjake Thank you for taking the time to send me the info. I checked the replacement Logic Board and it is identical in every way, shape, form, and physical build to the old one. I reapplied the thermal paste and changed the DC to DC board as that is what the diagnostics of the number 2 LED suggested to do. Also replaced the Power Supply while I was into it that deep, it shower two burnt spots on it. Not sure why the number 4 LED showed red at startup, it was not running long enough to heat up. Put everything back together and I am loading the OS back on the Hard Drive, everything seems to be running ok now. Randy

Randy,

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