iMac 820-2733-A won't power on

[protocold]

Used known good PSU, Diagnostic LED601 will intermittently blink after a few try but won't stay on...

where should I start?

 

[2informaticos]

Check diode mode to ground on all secondary power supplies (big coils).

 

[protocold]

i can only find a few big coils but they dont seems to have short..

Here are some measure of voltages:

PP12V_G3H = 12v
PP3V42_G3H = 3.4v
PP12V_S5 = 12v
PP5V_S5 = 5v
PP3V3_S5 = 0v. (NO short to ground)

 

[2informaticos]

Try changing U7700.

 

[protocold]

ok U7700 replaced, now LED601 is on. I now have PP3V3_S5.

However, it wont respond to power on if I short J5010.

I saw SMC_ONOFF_L goes low from 3.4v to 0.3v when i short J5010. PM_SLP_S3_L is 3.3v.

 

[2informaticos]

If CMOS cell is removed, the board should start automatically; no need to press power button.
PM_SLP_S3_L should stay high in such case.

 

[protocold]

removed CMOS cell, still only have LED601. PM_SLP_S3_L = 3.3v

whats wrong now?

 

[protocold]

U6900:
PGOOD_SYSPWROK_R = 0v
PM_MXM_PGOOD = 3.3v
ALL_SYS_PWRGD = 0v
PGOOD_P12V_S0 = 3.3v
PM_SLP_S3_BUF_L = 3.3v
PCHCORE_PGOOD_R = 3.3v
PM_SLP_S3_B_R = 3.3v
S4_ENABLES = 3.3v
PM_SLP_S4_1_L = 3.3v

 

[protocold]

u8000:
PP12V_S5 pin 1: 11.92v
PP5V_S0_EN pin 2: 3.3v
PP5V_S3 pin 5: 0.4v
PP5V_S0 pin 6: 0.4v
P5V_S0_EN_G pin 7: 11.87v
PGOOD_P5V_S0 pin 8: 1.25v

 

[2informaticos]

Missing 5V_S3?
First check diode mode there (L7750).
If no short, check U7700 solders.
P5VS3_EN comes high?

 

[protocold]

you are correct once again, U7700 bad solder..removed and resolder again fixed this. Thanks!