iMac G5 shuts off during every startup?

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ub6ib969

Registered
Well my iMAC G5 (1.6ghz,Gen1) was shutting down intermediately so I opened it up and removed the power supply and found many leaking capacitors. All were replaced. The capacitors on the logic board look fine with none leaking nor bulging ( I will take a picture.). Once I plugged in the newly fixed power supply I notice LED1 is on! Meaning that the power supply is fine. Once I try powering up the system I hear the cpu fan start spinning, LED2 turns on, hard drive fan, case fan and the dvdrom and then the system just shuts down. Any help would be great.
-Thanks
 
Have you tried resetting the PRAM? (Restart, holding Opt-Command- P & R. Hold those 4 keys until you hear the boot chime at least 2 times. It may not boot far enough to do that , but you should try it. Hold for up to 2 minutes before you give up. If the power simply shuts off, don't waste your time then :)
Have you ever pressed the SMU reset button? It's the second button, underneath the internal power button.
LED #1 will always be on when the power cord is plugged in, unless the power supply is not working properly.
LED 2 should come on after the power is turned on. It just means that the computer is powered on, and the power supply (again) is working OK
LED 3 comes on when the logic board is providing a video signal. If #3 comes on, the LCD screen should be bright.
So, your iMac is not turning the screen on, and is just shutting off at that point.
Try different RAM, or just one RAM chip at a time.
If there's no change after trying RAM chips, try removing the optical drive (the CD/DVD drive). If you still get no video, and continues to just shut off, then most likely failure is the video chip, which is part of the logic board.
Although replacing the capacitors will often be a good fix, that doesn't always fix every issue. The video chip may have failed during the other problems that you had.
 
ub6ib969 said:
Once I plugged in the newly fixed power supply I notice LED1 is on! Meaning that the power supply is fine.
Click to expand...

Just about every bad power supply I've changed in a G5 iMac (and there have been several hundred) had LED 1 on, so its still broke. One clue to tell you that PSU is still bad is you will see LED 1 slightly flickering, as opposed to on completely solid and steady. What do you mean the caps were replaced in the power supply? You mean the caps were replaced on the logic board, right?
 
I replaced the caps in the power supply and the LED 1 light is on soild as long that it is plugged. It now boots up to LED 3, I see video then shuts down. Can this be video problem or a overheating?
 
Yes, LED 1 is always on when your Mac is plugged into power.
Not likely overheating if it shuts down even if completely cold, but previous overheating may have taken out the video chip. That's one scenario. The other (more likely) is that your power supply won't take the additional load of the video. The power supply is marginal in the older iMac G5s, and it doesn't take much to give problems. I think you have more wrong than just the caps in the power supply. The easiest thing to try is swapping in a known-good power supply. If your iMac still shuts off, then your logic board still has problems.
 
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