820-00134: Not powering up after HDD replacement

[macRepairAmb]

Got an iMac 27" 2015 that stopped powering up after HDD replacement. I've confirmed the psu works, i have all the PP12V_ACDC and 3.3v on the PWR_BTN_R, which drops to .5 when power button is pressed.

PP12V_S5_FET_P3V3_S5_P5V_S4 - 12.1
PP5V_S5 from U7600 is 0. Is this controlled by an enable signal?

Any suggestions on this one. Also, none of the LED lights are on

 

[2informaticos]

"PP12V_S5_FET_P3V3_S5_P5V_S4 - 12.1"
What this means???

 

[macRepairAmb]

That is vin power for U7600 pin 17. This is the same power signal PP12V_S5_SSD coming from Q8490.

 

[2informaticos]

Oh my god, crazy engineer choosed such a name .
Didn't look at schematic at first moment, sorry.

Check pin 12 voltage.

 

[macRepairAmb]

yeah thinking it must have being an intern trying to impress the big guys. Anyway,

pin 12 - 0.0

 

[2informaticos]

Check R8489 voltage.
Do you get PP12V_S5_SSD_FET?

Is a craziness with signal names in this schematic; did someone kill the project designer?

 

[macRepairAmb]

yeah i believe that would be the most reasonable assumption as these naming are so out of the norm from all other schematics.

Anyway, some how i missed this when i was digging around, the board was missing R8489 and R8496. I found a direct replacement for R8489 but had to use a different rated resistor for R8496 (i used a 68K 1% 1/20w and MF 201 package resistor as i currently didn't have the 5% MF402 1/16W)

Readings after installing resistors:

R8489 - 0.7
PM_EN_FET_P12V_S5_SSD - 3.4
PP12V_S5_SSD_FET = PP12V_S5_SSD - 12.1

Diode reading @ R8489 - 0.6

PP12V_S5_FET_PWRCTL to R8496 is 12.1

So my guess would be U8490? Unfortunately i don't have any donor board that has this part and no one sells them locally. Soonest i can get one is 11/08.

 

[2informaticos]

Make stock of scrap boards, if you plan to stay in business with Apple repairs.
Many components can only be found in donor boards.
Aliexpress is good place to get such boards...

 

[macRepairAmb]

this is my first iMac repair, but yeah i do try to purchase donor boards when i'm able to get good deals on a batch.

I've ordered a 2012 donor board as 2015 are being sold in the $100's. The 2012 does have the U8490 chip. I'll update you if this fixes the issue once i get the board.

 

[macRepairAmb]

Replaced U8490 but still not booting up. Then removed D7600 and PM_PGOOD_FET_P12V_S5_SSD was restored. So replaced U7600 and the board now has a new problem. The first led keeps blinking and blinks faster when i hold down the power button. PP3V3_S5_REG_SSD keeps bouncing from 3.3 to 1.09. When i hold the power button, it bounces from 3.3 to 2.4.