Macbook Air A1466 that fancied a milkshake.

[RESQ-MYTECH]

Hi all again, back to pick your brains. Now as I said before, I am new to the world of mac related repairs and there is a really good chance I may have got this wrong but here goes.
Customer has brought in an A1466 with a 820-3437-b logic board. As the title suggests the owner droped milkshake on the machine. Came to me with no power. Magsafe when connected went from green to orange but no fan spin. So, here is what I have done so far:-
Q8150 had corrosion around it as did R8151/52/54 so replaced theres with known good components as another 820-3437-b board. I restored power as when connected to the magsafe the fan spins. Connected the screen ( after checking the condition of the cable and J8300 and the screen illuminates but no image or chime.
After watching Louis video Macbook Retina no backlight NEW things to check ( https://www.youtube.com/watch?v=nH-mWAt0CDI&t=403s) I started following his line of trouble shooting.
I have checked for PPVCC_S0_CPU and I get 1.752v. The CPU is getting warm.
Checked pins 3&4 on J8300 and I am getting 3.301v. I have checked pins 18 & 19 PP3V3S0SW_LCD_UF and I am getting no voltage. Tracing back I have no voltage at L8304 and C8315 appears to be working showing 998 pf with no short. Working back again along PP3V3_S0SW_LCD_R at pins 4&5 there is 0.003v, pins 2&3 show 3.301v pin 6 is grounding OK but pin 1 is showing 0.003v. working my way back even more along EDP_PANEL_PWR to I get to R1431 which is 100 Kohm resistor. It shows 31.30 Kohms when on the board and 100 Kohms when off. Run a continuity check from pin 2 of R1431 to pin 1 of u8300 and it shows 0.001 Kohms. Pin 1 of R1431 shows no voltage going to it when powered on ( 0.003v) but is going to ground. next step back is C6 on the CPU.

So long story short, should I be suspecting the CPU/ solder to be the culprit? Should I be looking elsewhere? I do not have the facilities to remove and replace the CPU or more to the point the confidence to do it.

As usual all advice is greatfully received.

One last thing, I can obtain a replacement logic board abeit 2nd user for £179 ( approx 200$USD), would I be better to go for that?

Thanks in advance.
Marc

 

[2informaticos]

"Connected the screen ( after checking the condition of the cable and J8300 and the screen illuminates but no image or chime.
----------------------------
Checked pins 3&4 on J8300 and I am getting 3.301v"
How do you get backlight with 3V3 at pins 3/4???
Are you sure it is illuminated?

LCD is eDP, so you must have 3V3 at pins 18/19 at least few seconds when the board starts.
If DP_INT_HPD will not go high, pins 18/19 drop to 0V; check R8360.

BTW, do you get USB activity?

 

[RESQ-MYTECH]

"Connected the screen ( after checking the condition of the cable and J8300 and the screen illuminates but no image or chime.
----------------------------
Checked pins 3&4 on J8300 and I am getting 3.301v"
How do you get backlight with 3V3 at pins 3/4???
Are you sure it is illuminated?

LCD is eDP, so you must have 3V3 at pins 18/19 at least few seconds when the board starts.
If DP_INT_HPD will not go high, pins 18/19 drop to 0V; check R8360.

BTW, do you get USB activity?

You are correct, I had so much I wanted to put in the description i forgot that... pins 3&4 do provide 3.301v and the screen is illuminating ( can see the difference as the system starts and if I disconnect the magsafe it goes dark black) but no display ( image or apple logo) and the reason I was looking at pins 18/19 was that it did/ does give a short burst of 3.301v and then goes to zero, hence I was back tracking along that line.( said I was a noob). Is it meant to do that? sorry for the simplton question. as for usb activity, i have not checked for that, but back in the workshop on monday so will have a look then.
Thanks for the points to check.
Marc

 

[2informaticos]

Illuminating means backlight, in my opinion.
You just got a reaction of the TFT matrix, not the backlight.
If DP_INT_HPD doesn't go high when pins 18/19 got 3V3, then try another LCD assembly.

If you don't get USB activity, possibly your board is stuck, doesn't really boot.

Check the board for corrosion on morea areas.
Pay atention to U1950 and SPI chip U6100...

 

[RESQ-MYTECH]

Illuminating means backlight, in my opinion.
You just got a reaction of the TFT matrix, not the backlight.
If DP_INT_HPD doesn't go high when pins 18/19 got 3V3, then try another LCD assembly.

If you don't get USB activity, possibly your board is stuck, doesn't really boot.

Check the board for corrosion on morea areas.
Pay atention to U1950 and SPI chip U6100...

Thank you, I will try that on Monday. Should ppins 18&19 stay at 3.3v?
Marc

 

[2informaticos]

Please, don't quote an entire message which is posted 5cm above.
Use quotes when you reffer to some lines from a previous message...

 

[RESQ-MYTECH]

2informaticos sorry, will avoid this in future...
Marc

 

[2informaticos]

"Should pins 18&19 stay at 3.3v?"
If DP_INT_HPD stays high, pins 18/19 should stay high too...

 

[RESQ-MYTECH]

right got you.... thanks for the pointer

 

[RESQ-MYTECH]

Hi all. Just an update, the customer has requested i replace the logic board as ( in his words) he is not confident that other components will not fail in the future, and seeing as the logic board is not that dear he wants to replace. on top of that he found an i7 board with 8gb RAM so he opted for the upgrade. I got as far as full illumination but still not display. however, it has been put on the to do later pile as other work is piling up. thanks for all the advice but I will be revisiting this thread as and when i get time to go back to the board.