iMac 2017 21.5 820-00597 Logic Board Will Not Power On

[2informaticos]

"1.0V for PM_PGOOD_FET_P1V0_S3 on R8410 Pin 2"
That's no good; check if R8410 has 10K (measure it out of board).
If really good, try changing U8500.
If still the same, change U8410.

 

[ideachief]

Alright, I checked R8410 off the board and it came in at ~10K Ohm like it should. So I replaced U8500 as recommended and now I get 3.3V for PM_PGOOD_FET_P1V0_S3. However, the board still won't power on. Should I replace U8410? I suppose I can go back and recheck all the other voltages.

 

[2informaticos]

Don't tocuh U8410!
Don't you have correct 1V0_S3 on its pin 8?

Post again voltage on all U8500 above mentioned pins.

 

[ideachief]

On U8410 Pin 8 I have 1.0V. The voltages for U8500 are: Pin 1: 3.325V, Pin 2: 3.322V, Pin 3: 3.325V, Pin 4: 3.325V, Pin 5: 3.325V. Pin 6: 3.325V, Pin 8: 3.271V, Pin 9: 3.325V, Pin 10: 3.319V, Pin 14: 3.326V

 

[2informaticos]

U8500 looks good, so you must have voltage on various coils of the board.
Do you get 12V output now from Q8450?

 

[ideachief]

I get 0V output on Q450 (Pins 1, 2, 3). I also have 0V on FET_EN_P12V_S0 (Pin 4). I do get 12V on PP12V_G3H_P12V_S0 (Pin 5).

Just in case it helps, U8450 voltages are as follows: Pin 1: 11.97V, Pin 2: 1.77V, Pin 5: 11.97V, Pin 6: 0V, PIN 7: 0V, Pin 8: 0.051V. On R8542, Pin 1 has 3.3V (Pin 2 is connected to U8450 Pin 8).

 

[2informaticos]

"U8450
Pin 2: 1.77V"
You've said to have 3.2V on pin 8/U8500; confirm that.
If yes, check the components in between.

Q8450 is classic failure, you can try changing it directly.
May need to change U8450 also.

 

[ideachief]

Alright, looks like the issue was U8450. I replaced Q8450 but it didn't fix the issue until I replaced U8450. Hopefully this is all that's going to be needed. Should I check or replace any other components?

 

[2informaticos]

Don't touch anything else, if machine works good.

 

[ideachief]

Can I ask what programmer would you recommend to flash the BIOS on this iMac?

 

[2informaticos]

Any BIOS programmer should work; even the cheapest CH341A.