LED power supply and logic board tests

"Plug the power supply back in and check the diagnostics lights on the logic board.

For the G5 not for Intel

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."

"Um.. doesn't LED 4 on this model denote the LCD comms status?

2007/8 models

LED #3

Indicates that the computer and the video card are communicating. This LED will be ON • when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.

LED #4

Indicates that the computer and the LCD display panel are communicating. This LED will be • ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement."

"FYI - From the 2009 21.5/27 inch service manual

LED Startup Sequence

LED #1 = Power available.

If no LED is visible:

• Verify AC source

• Verify known-good AC cable is connected

• Verify cable connection between AC inlet and power supply

• Verify cable connection between power supply and logic board

• Verify power supply

LED #1 + LED #2 = Power available, and system is powered on.

If second LED is not visible when power button is pressed:

• Verify power button connection to logic board

• Verify power button functionality

• Verify cable connection between power supply and logic board

• Verify power supply

• Verify logic board

LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found.

If third LED is not visible after power on:

• Verify that MXM video card is seated properly

• Verify logic board

LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card found, and internal LCD found.

If fourth LED is not visible after power on:

• Verify cable connections between LCD panel and logic board

• Inspect LCD display cables for cable damage

• Verify external video functionality, and according to result check the following items:

-If external display works then verify/replace LED backlight board

-If external display works then verify/replace LCD panel

-If external display does not work verify/replace logic board"

"Hi,

When none of the LED's are lit, this means there is no power at all, and one would suspect the power supply.

Don't jump to conclusions too fast, however, and check the AC input connector, mains filter and AC wiring to the PS board. If you know how to handle a multimeter, check the mains voltage on the PS board. You can also unplug the connector to the logic board, and see if you have the tricle (stand-by) voltage (about 2,5 V DC).

Be aware that a power supply has protective circuits on the main output voltage line (12VDC). When there is a short circuit in the logic board (or audio board, video card etc), you won't be able to start the computer (LEDS 2/3 remain dark) but you can have the trickle voltage. I was fooled once by this situation, thinking of 'only' a defective PS, but when I pulled out the video card my iMac came back to life !

I don't know if the trickle voltage line is protected (guess not)."

"----

If no LED is visible:

• Verify AC source

• Verify known-good AC cable is connected

• Verify cable connection between AC inlet and power supply

• Verify cable connection between power supply and logic board

• Verify power supply

-----

All above verified.

May I be shure I have to replace the power supply, or could it be something else ?

Update

Question: how can I have 10 results while first solder is the reference?

Here are my 9 results :

1: n/a

2: 1.1

3: 0

4: 2.5

5: 2.6

6: 0

7: 46

8: 37

9: 12.3

10: 0"

Hi all. I have an 20" imac 2133 and yesterday after a short circuit in my house imac dropped dead. I choosed to open my imac since i got no money (see economic crisis - Greece). Found 2 burned part on PSU a T4A fuse and a 420V 150uF Capacitor. I will change them and if i got good news I will post...