820-00840 no brain/post

[Narek]

I have an 820-00840 on bench which came with water damage. The only area which had slight corrosion was U8210 and R1355. After cleaning and refreshing this area board starts, takes 20V, trackpad clicks but board is stuck at 0,26A. There is no chime, no image, no post.

The voltages I have measured:

PPBUS_G3H - 13,05V
PP3V3_S5 - 3,3V
PP5V_S4 - 5,1V
PP1V2_S3 - 1,196V
PP1V_S5G_REG - 0,992V
PPVCCPCH_S5G - 0,995V
PP1V8_S5G - 1,8V
PPVCCCPU_S0G - 0V (CPU ressistance is around 6ohms)
PLT_RST_L - 0V


I have replaced U8210 with the chip from donor board. Inspected every inch of the board and haven't found anu other corrosion except mentioned. Also have tried known good BIOS from this forum (clean ME) and no difference. Any ideas?

 

[2informaticos]

You've posted some voltages, but not U8210 outputs; where you had corrosion.
Also you've mentioned iGPU missing voltage (which is not expected yet), but nothing about CPU core (L7210/20).

 

[Narek]

I'm sorry, I was writing this post late at night and forgot about some voltages, so here are the missing ones:

PP1V8_S0 - 1,8V
PP1V8_S3 - 1,8V
PP5V_S0 - 5,10V
PP3V3_S0 - 3,3V
PP3V3_G3H - 3,41V
L7210/20 - 0V

U8210 voltages by pins:

1/2 - 3,3V
3 - 3,27V
4 - 5,1V
5 - 3,12V
6/7 - 5,1V
8/9 - 5,10V
10 - 10,72V
11 - GND
12 - 10,67V
13/14 - 3,3V

I forgot to mention, that after connecting charger board starts, gets stuck at 20V 0,26A, then restarts once and after that it is stuck at constant 20V 0,26A

 

[2informaticos]

CPU core not present, so the board is stuck; didn't complete Power On Sequence.

Do you get L7370 voltage?
Check all voltages from pages 61-64.

 

[Narek]

I do, voltage on L7370 is 1,049V

 

[Narek]

The only votlages I'm missing from the listed in schematic are PPVCCGT_S0G and PPVCCCPU_S0G. Sadly I just hav noticed, that PPVCCGT_S0G is reading 1,6ohm and PPVCCGT_S0G is 3ohm... So my guess is CPU is dead.

 

[2informaticos]

"The only votlages I'm missing from the listed in schematic are PPVCCGT_S0G and PPVCCCPU_S0G."
As I already said, iGPU voltage is not expected yet; only appears when high graphics load is detected.

CPU coils have very low resistance to ground as normal; compare with good board.
However, CPU dies easy on A1708 machines.

 

[Narek]

Ok, so if missing voltages are related to iGPU and every other voltage is present, it means that CPU doesn’t turn on anyway, right? PLT_RST_L is 0V, so board is in reset mode. I have no idea what else could keep board in that state..

 

[2informaticos]

"every other voltage is present"
Every other no; you don't get CPU core voltage (L7210/20)!
May have BIOS issue, bad CPU, U7100 problem.
This is not an exact science...

 

[Narek]

I have already tried another BIOS which I can confirm is good (btw. You shared it with me in another thread). I will try new U7100 and inspect board again for corrosion, maybe I have missed something…

 

[Narek]

Unfortunatelly customer couldn't wait longer as he had to flight back to US. Thankfully, he lives in NY, so I advised him to get to Rossmann repair shop. I gave him our visit card and contact, he will give it to you while leaving computer in Rossmann repair shop. If you will need some additional info, photos and and what has been done on the computer you can contact us.

 

[2informaticos]

I'm based on Madrid, Spain.
Rossmann repair shop will solve the problem in US...