imac 820-3478A have 12v and all led off

[okianeko]

Hello,I have a problem for my imac 820-3478A
power supply have 12v to board ,but board 4 led all off.
U8470
PIN 1 :12.4V
PIN 2 :0.03V
PIN 3 :0
PIN 4 :GND
PIN 5 :12.4V
PIN 6 :0
PIN 7 :0
PIN 8 :0
--------------------
U8450
PIN 1 :12.4V
PIN 2 :0
PIN 3 :1.5V
PIN 4 :GND
PIN 5 :12.4V
PIN 6 :0
PIN 7 :0
PIN 8 :0
Both have a same problem,should i change Q4870/50?
thanks

[mon2]

No, not yet.

If you have NO leds enabled on this model, then you are missing PP3V3_S5 to drive the first LED which should be enabled once you plug in the power supply. 'It is plugged in' LED.

Remove all power. Measure the resistance to ground of PP3V3_S5_REG @ L7610. What is the resistance? If the resistance is low, then you have a shorted part on this rail and this is causing this power rail to turn OFF.

If the resistance is not low, then check if this rail is being enabled or not.

Carefully, measure the voltage to ground of PM_EN_REG_P3V3_S5 at the resistor junction shown below - this voltage is used to enable the PP3V3_S5 missing power rail.

[okianeko]

thanks
resistance to ground of PP3V3_S5_REG is 2.5k ohm.
I plugin power supply i didn't get any voltage by PM_EN_REG_P3V3_S5

[okianeko]

Q7610 PIN 2 don't get a 12v

[mon2]

Check voltage to ground on other side of the same resistor at R8590.

[okianeko]

both are 0V

[mon2]

PP12V_S5_PWRCTL = PP12V_S5_FET = 0 volts.

PP12V_S5_FET is created only if mosfet Q8470 (N-channel) is enabled. The gate voltage of Q8470 pin #7 is 0 volts so this mosfet remains OFF.

Gate is 0 volts because the ENABLE pin # 2 SMC_PM_G2_EN (U8470) is at 0 volts.

SMC_PM_G2_EN is driven by the SMC @ U5000.

Confirm if U5000 is being powered by checking the voltage to ground @ PP3V3_G3H_SMC L5001 (either side should be ok).

This power rail is driven by PP3V42_G3H - confirm this rail is present by checking the voltage to ground of L6961.

Respectively if not enabled, then check the voltage to ground of R6965 - check both sides of this 0 ohm resistor.

[okianeko]

L5001 : 3.3v
L6961 : 3.3v
R6965 : both sides are 12.07v

[mon2]

L6961 should be 3v42. Can you confirm again?

Found a great study of this case:

https://www.xinxunwei.com/chenggonganli/06059635.html

[okianeko]

L6961 3.39~3.4V.
thanks for this web ,but they are different board i'm not sure is work.

[mon2]

L3961 should be stable @ ~3v42. Remove all power. Meter in resistance mode. Measure the resistance to ground of L6961. What is the resistance in ohms?

[okianeko]

resistance to ground of L6961 : 0.18ohm

[mon2]

Can you share the pic of the front face of your meter? What is the scale being used here? Perhaps 2k ohms is the max which means measurement is 180 ohms.

[okianeko]

I found R5187 maybe died ,it should be 100k ohm front the u8470 pin2.
I need to check which died.
thanks

[okianeko]

sorry about that
resistance to ground of L6961 : 180ohm
R5187 only have 100ohm is wired

[mon2]

If you are comfortable, flux and carefully remove R5187 off the board. Do not lose it although if you are in Taiwan - your local 7-Eleven stores probably stock it Use low air pressure and then carefully measure the resistance of this removed resistor. The part can go flying off the table so proceed with patience. Have done this too many times. The parts are dirt cheap and should be easy to source if in Tapei. I recall one of the largest resistor factories during the ride from the airport into the city in Taipei.

If at 100 ohms - that is too low and can impact the CS signal line.

[okianeko]

thanks
i order some smd 0402 on onlineshop,just waiting

[okianeko]

bad news ,when i change new resistance it still 0 ohm , maybe i need to check u8470

[mon2]

Carefully measure the resistance of the replacement resistors while it is on your worktable. Is the resistance ~ 100k ohms when you measure across the tips of this resistor with your meter?

Next, flux and remove the resistor that is onboard @ R5187. After this part is removed, measure the resistance to ground of the PCB pads @ R5187, pin #1. This is to confirm if some other part is showing this low resistance to ground.

Use flux, with low air pressure and enough heat to solder or remove the part.

[okianeko]

在工作臺上是 100k 歐姆，但在板上是 0 歐姆。