820-3126 Just the first led

[raainxx]

Hi,
i have this imac motherboard don't start, the first led is green when i press power (when the imac i complete), the led1 shutdonw.
I try to change the power supply but same problem.
I plug just the power supply and button ON, the led2 don't light when i push the on button, but the led1 stay on.
i measure
PP12V_G3H_ACDC=12,05V
PP12V_S0_PS=0V normal?

 

[aprendiz]

PP12V_S0_PS=0V is ok because this will come high when computer is fully on (S0 stage)

I bet you have USB ports burned.......

 

[dukefawks]

There is a short somewhere. As soon as you power it on it pulls down the power supply. Measure all rails for shorts to GND.

 

[raainxx]

Hi,
i don't find any short on all rails
PP1V8_S0 = 0,35Kohm
PP5V_S0 = 0,75Kohm
PP12V_S0 = 8,9Kohm
PP1V05_S0 = 18ohm ???
PP1V5_S0 = 80ohm ???
PP3V3_S0 = 4,4Kohm
PP5V_S3 > 4Kohm
PP3V3_S3 = 95ohm
PP5V_S5 = 2,2Kohm
PP3V42_G3H = 2,4Kohm
PP12V_G3H 9,1Kohm
PP3V3_S0_T29 12Kohm
PP1V05_S0_T29 16ohm
PP0V75_S0 4kohm
PPVTT_S0_DDR 2,8K
PPVCORE_S0_CPU 9ohm
PPVAXG_S0 19,5ohm
PP1V05_S0_PCH 18ohm
Do you have any idea?

 

[dukefawks]

There must be a short somewhere. Video card, USB port? Only thing I can tell you that there must be a short somewhere pulling the PSU down. You could inject 12V on 12V_S0 and monitor current draw.