iMac A1312 820-2828-A dead

[kingskie]

Hello,

I need some help for iMac A1312 820-2828-A.
The iMac cannot turn on. i already changed the power supply but still cannot turn on.

All the LED indicator is off. can somebody give me a hint where should i start?

[piernov]

Re: iMac A1312 820-2828-A dead

Check voltage on the G3H and S5 rails.

[kingskie]

Re: iMac A1312 820-2828-A dead

Check voltage on the G3H and S5 rails.

i get a little lost here because the schematic i have is different from the board.

here i attach the schematic and boardview that i have

[piernov]

Re: iMac A1312 820-2828-A dead

Looks like it is the correct schematic and boardview for the 820-2828 board.

[kingskie]

Re: iMac A1312 820-2828-A dead

Looks like it is the correct schematic and boardview for the 820-2828 board.

it is different as you can see from the pictures. the position of the components are different.

[kingskie]

Re: iMac A1312 820-2828-A dead

i just check that PP3V42_G3H is only 0.3V does it mean that the i have to change the ISL chip?

[piernov]

Re: iMac A1312 820-2828-A dead

I haven't looked in great details but the board pictures still seem to match the boardview…
Not sure what you mean by "ISL chip". You have to take measurements of all G3H rails so that includes PP12V_G3H .

[kingskie]

Re: iMac A1312 820-2828-A dead

I haven't looked in great details but the board pictures still seem to match the boardview…
Not sure what you mean by "ISL chip". You have to take measurements of all G3H rails so that includes PP12V_G3H .

the position of components are different. PP12V_G3H is present.
all the caps that related to P3V42G3H is short to ground so i think U7900 is short so i take off the U7900 but the short still there...any hints?

ISL chip i mean the U7700 ISL62383

[piernov]

Re: iMac A1312 820-2828-A dead

Please point out which component placement is different between boardview and the actual logic board, I still don't understand.
If there is a short to ground on PP3V42_G3H, you can inject 3V with a lab PSU and try to find a component that heats up (capacitors or SMC for example).

U7700 is unrelated to PP3V42_G3H so for now it's not suspected.

[kingskie]

Re: iMac A1312 820-2828-A dead

Please point out which component placement is different between boardview and the actual logic board, I still don't understand.
If there is a short to ground on PP3V42_G3H, you can inject 3V with a lab PSU and try to find a component that heats up (capacitors or SMC for example).

U7700 is unrelated to PP3V42_G3H so for now it's not suspected.

i followed your suggestion and inject 4.5V and 2A at PP3V42_G3H and found the short cap. it is C7902. after i removed the caps and measure i still got 0.4V on PP3V42_G3H.

here is the picture as you can see there are diferrence between actual and the boardview

[kingskie]

Re: iMac A1312 820-2828-A dead

i followed your suggestion and inject 4.5V and 2A at PP3V42_G3H and found the short cap. it is C7902. after i removed the caps and measure i still got 0.4V on PP3V42_G3H.

here is the picture as you can see there are diferrence between actual and the boardview

here's some update:

i plugged to power up the power supply + i inject 3,5V to PP3V42_G3H and i get 3 led on (led 1,2 and 3)
PP3V42_G3H comes from U7700 right? if yes that means i have to change the U7700 and that is all?

[piernov]

Re: iMac A1312 820-2828-A dead

Press space to show the other side of the board and press M to mirror it.

You didn't follow my suggestion of injecting 3V since you injected 4.5V. Injecting 4.5V into a 3.42V rail will kill components. In any case after removing a component you have to check if the short to ground is still present or not.

[kingskie]

Re: iMac A1312 820-2828-A dead

Press space to show the other side of the board and press M to mirror it.

You didn't follow my suggestion of injecting 3V since you injected 4.5V. Injecting 4.5V into a 3.42V rail will kill components. In any case after removing a component you have to check if the short to ground is still present or not.

first i injected 3V but no heat came up so i raise to 4.5V. there is no short to ground on PP3V42_G3H rail now.

[kingskie]

Re: iMac A1312 820-2828-A dead

update:
i changed U7700 and the LED turn on.. now everything is ok..
thanks to @piernov

thread closed