Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.

jtreml60

macrumors newbie
Original poster
May 13, 2013
5
0
Hi, really need some help as I've run out of ideas! Friend asked me to have a look at his iMac G5 20" with iSight as he couldn't power it on. I agreed and after a fair bit of dismantling and troubleshooting, decided it was the PSU and replaced the PSU with one off eBay. I installed the new one tonight but still no joy. I've followed the instructions in the service manual to help troubleshoot and find that none of the LEDs on the logic board light up. When LED1 doesn't even light up, manual suggests the PSU is at fault but it's been replaced and the new PSU is making a faint hum so I know there's at least some juice getting to it. I've also checked the power outlet, power cord, and tried resetting the SMU.

If the logic board was faulty would I at least get that LED1 lighting up? Is it likely to be the logic board that is at fault now? Any way I can verify that in any other way?

All thoughts and suggestions welcome. Jim
 

jtreml60

macrumors newbie
Original poster
May 13, 2013
5
0
thanks for responding. What do I need to look for on the logic board to see if it has bad caps? if it was this would that explain why the power supply seems OK but the first LED on the board is dead still?


Have you check Motherboard for bad caps.
 

rabidz7

macrumors 65816
Jun 24, 2012
1,205
3
Ohio
thanks for responding. What do I need to look for on the logic board to see if it has bad caps? if it was this would that explain why the power supply seems OK but the first LED on the board is dead still?

Capacitors that have exploded or are leaking.
 

TUSL2C

macrumors newbie
May 15, 2013
7
0
LED 1: Indicates that trickle voltage from the AC power inlet to 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 main logic board has established communication with the LCD display.

LED 4: Indicates a processor over-temperature condition. The system will shut down when this LED turns on.

As the manual suggests, if there is trickle power but no boot then the MLB is likely faulty
 

Attachments

  • Picture 1.png
    Picture 1.png
    540.8 KB · Views: 127
Last edited:

jtreml60

macrumors newbie
Original poster
May 13, 2013
5
0
so, had the logic board out this morning and the caps look absolutely fine - no leaking or bloating. Put it back in and reconnected all the wires but still no LED 1 comes on, though can still hear PSU humming.

Any other ideas or do you think logic board is gone?

Thanks for all advice thus far!


LED 1: Indicates that trickle voltage from the AC power inlet to 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 main logic board has established communication with the LCD display.

LED 4: Indicates a processor over-temperature condition. The system will shut down when this LED turns on.

As the manual suggests, if there is trickle power but no boot then the MLB is likely faulty
 

TUSL2C

macrumors newbie
May 15, 2013
7
0
Is the PSU known to be good and working, If all was well there should be at least trickle power shown by LED 1. You state that the PSU is making a humming noise. That may not be an indicator that it is working. All you can really do at this time is ensure a good connection for both the power supply connector, try and reseat the PSU cables or verify that the PSU is definitely working.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.