820-3787-A - no backlight - image on external monitor - defective Q7706? pin4 11.41V, pin3 11.32V, p1/2/5/6 at 16.4mV

[stef4n]

Hello,

The MBP is able to boot, load the OS, but the screen stays black.
If I plug an external monitor, I get an image. The keyboard and trackpad work properly.

Measurements on J8300 are:

• 1.978V on pins 28-29-30.
  As I get 5.00V on pin 1 of L8300, and 1.978V on pin 2 of L8300 & pins 28-29-30 of J8300, I assume that 1.978V is normal
• 0.7mV on pin 1 (PPVOUT_S0_LCDBKLT) which is not good at all

I did a few more checks on:
C7719 pin 1: 0.0mV,
C7716 pin1: 0.0mV,
C7717 pin1: 0.1mV,
C7715 pin1: 0.1mV,
C7718 pin1: 0mV
D7701 pin1: 14.5mV, pin2: 0mV
C7710 pin1: 12mV
C7712 pin1: 13mV

Regarding the fuse F7700, I do have 10.7V on both side, the schematics shows 12.6V. What is correct voltage of PPVIN_SO_LCD_BKLT ?

I checked Q7706;
On pin 6,5,2,1 I read 19mV. It's too low.
After the L7710, the PPBUS_SO_LCDBKLT_PWR_SW should be at 45V referring to the schematics;
I would therefore expect something around 11V on pin1 of L7710 and pins 6/5/2/1 of Q7706

On the source of Q7706, pin 4, it reads 10.67V, and on the gate (pin 3) it reads 10.57V.
Q7706 being a type P, power should go through, (10.57 < 10.67), and I should have 10.67V on the pins that have 19mV

I also checked LCD_BKLT_EN on pin 1 of R7742, it shows 1.4mV, and BKLT_SD is at 10.47V


Above measurements taken while the MBP runs on battery.

If plugged, then:
PPBUS_G3H is at 11.67V,
L7130 pin1: 12.05V, pin2 11.8V
J8300: pins 28-29-30 unchanged at 1.978V, pin 1 at 3mV (should be ~45-50V)
F7700: both side at 11.4V
Q7706: On pin 6,5,2,1: 16.4mV, pin4 at 11.41V, pin3 at 11.32V. Q7706 is open
R7742 pin1/2: 2.5mV
BKLT_SD on pin11 of U7701 shows 11.32V

Anyone to confirm that the Q7706 should be replaced ?

PP5V_S0_BKLTCTRL is at 5.00V

 

[2informaticos]

"Anyone to confirm that the Q7706 should be replaced ?"
Yeah, I confirm; do NOT replace it!

"Q7706 being a type P, power should go through, (10.57 < 10.67), and I should have 10.67V on the pins that have 19mV"
STOP making wrong assumptions; 0.1V less on Gate than Source, doesn't help.

"I also checked LCD_BKLT_EN on pin 1 of R7742, it shows 1.4mV"
Why do you expect backlight, if is not even enabled?

"As I get 5.00V on pin 1 of L8300, and 1.978V on pin 2 of L8300 & pins 28-29-30 of J8300, I assume that 1.978V is normal"
Didn't think that L8300 is bad???
Start there...

 

[stef4n]

Hi 2informaticos, thanks for your swift reply.

"Q7706 being a type P, power should go through, (10.57 < 10.67), and I should have 10.67V on the pins that have 19mV"
STOP making wrong assumptions; 0.1V less on Gate than Source, doesn't help.

Do you mean that the difference of 0.1V is not significant and that 10.57V should be considered as equivalent to 10.67V, ie difference is 0 ?
Otherwise, 10.57V is smaller than 10.67, and I do not get what is wrong in the assumption

Regarding the L8300, you're right, there should be 5V on both sides, I missed it
I'm going to order a bunch of these inductors. Unless I can find a donor board.

About the LCD_BKLT_EN, which is at 1.4mV, this signal comes from the U9600, whose datasheet is 1059 pages long

 

[stef4n]

Re L8300, I read this post: https://boards.rossmanngroup.com/th...662-no-image-after-screen-repair.19968/page-2
Does the current rating max matter ?

 

[2informaticos]

You can bypass L8300 with a wire; you will not be the first doing that.

"About the LCD_BKLT_EN, which is at 1.4mV, this signal comes from the U9600, whose datasheet is 1059 pages long"
Why would you bother with such datasheet?
Nobody on the MLB knows the LCD is connected, untill LCD_HPD goes high (coming from LCD).
So LCD requires some previous power; 5V on pins 28-30.

"10.57V is smaller than 10.67, and I do not get what is wrong in the assumption"
If you really like to check datasheets, that's a good point to start; checking MOSFETs specifications.
Small current MOSFETs accept 1.5-3V difference (G-S); even 100 ohm for internal resistance is good where they are used.
High current MOSFETs must have very low internal resistance in working state, to avoid overheating; that requires higher difference (4.5-10V).

 

[stef4n]

I suddenly realized that I had ordered 12x MI0805M221R-10 a few month ago, to reach the minimum cart value with a previous order. That's quite fortunate because I was not that motivated by the "wire option". If an engineer put an inductor there, maybe he had a valid reason to do so. I'm not questionning your suggestion, but I prefer to remain careful.

Regarding LCD_HPD, I had missed this one too (!), I was watching too closely on pin 1 and pins 28-29-30. Obviously, I lacked the global picture. Kind of obvious now that this LCD HPD goes to the U9600 as an input for this chip, and that without a valid 5V on pins 28-29-30 there will be nothing on LCD_HPD, so, nothing on LCD_BKLT_EN, and nothing on pin 1....

Thanks for the enlightenment!

I replaced the bad L8300 by a good one; the macbook now works like a charm.

Regarding the MOSFETS specs, I'will have a closer look on the datasheets and on all these curves and differentials tomorrow. Time to get some sleep.

Thanks again for your help, very much appreciated.

KR,

S.