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820-00138 no backlight though has 51,2V on output

N

Narek

Member
So I got two A1398 2015" with 820-00138 boards from different customers, but with exactly same issue - no backlight (image is present). One of them is untouched, the other one has been worked on, but the other technician couldn't find the solution for backlight and gave up - he replaced LVDS connector, probably tried another U7701 because it's missing, tried another screen (the original screen was working with another board).

What I have done so far:

- tried another known good display assembly (100% 2015 model)
- tried another U7701
- tried another Q7706
- checked LVDS connectors on both boards - they look completely fine

U7701 measurements:

pin 5 = 5,07V
pin 18 = 5,06V
pin 11 = 5,84V
pin 17 = 3,33V
pin 15 = 5,01V
pin16 = 5,00V

PPVOUT_S0_LCDBKLT - 51,2V

D7701 is not shorted

F7700 has continuity

Q7706 measurements

pins 1-6 - 12,58V
pin 3 - 8,78V
pin 4 - 12,58V

I believe boards are fine, as they produce 51,2V output voltage which is present on LVDS connector but for some reason backlight is not turning on...
 
Last edited:
2informaticos

2informaticos

Administrator
Staff member
"PPVOUT_S0_LCDBKLT - 51,2V"
So backlight circuit works.
There is no continuity for this power rail to the LED strip, or back to the ground.
Do you have the high tension present on pin 1 of J8300?

"pin 11 = 5,84V"
This is strange, should be 0V.
"pin 3 - 8,78V"
5.84V should be correct here (5.5V in theory).

Be sure all resistors around U7701 are good; pay atention to R7708/09.

Just in case, do PRAM reset 2-3 times.
 
N

Narek

Member
Yes, I have 51,2V present on pin 1 of J8300 and even on LCD LVDS connector side. All resistors around U7701 are measuring OK except two - R7723/4 by schematic should be 10,2 ohm, but on my two (and one donor) boards they are 0,005 ohm. Anyway, they are not related to LCD but keyboard backlight. PRAM resets gives nothing...
 
N

Narek

Member
@piernov, I've seen this article, but it hasn't changed anything in my situation, or maybe I dont understand something? Actually my I2C_BKLT_SCL and I2C_BKLT_SDA are 5V, they have continuity from J8300 to U7701. I have already replaced connector just in case. I have tested 3 different displays (2015 model). According to article on LogiWiki there is no U7750 on the board, whis is source of I2C_BKLT_SCL and I2C_BKLT_SDA. So it means it's present on LCD side?
 
2informaticos

2informaticos

Administrator
Staff member
Check diode mode to ground on I2C_BKLT_SCL/SDA lines, w/ and w/o LCD connected.
Also check solders and traces continuity for pins 6, 7 & 8 of J8300.
Inspect Q8032 and its related resistors and traces.

Do you still have image present on internal LCD?
 
N

Narek

Member
Without LCD:

I2C_BKLT_SCL - 0,488v
I2C_BKLT_SDA - 0,488v

With LCD:

I2C_BKLT_SCL - 0,463v
I2C_BKLT_SDA - 0,462v

Are you sure you meant Q8032? I can't find it on BV (820-00163/820-00426) nor schematic for 820-00138. Maybe Q8302?

Yes, I'm still getting image on the screen.
 
N

Narek

Member
Resistors and traces around U8302 are OK. These are voltages on U8302 (LCD connected):

pin 1 - 3,3V
pin 2 / 5 - 3,3V
pin 3 - 3,3V
pin 4 - 3,3V
pin 6 - 3,3V
 
N

Narek

Member
Ok, I have repaired one of the boards (the one which has been worked on). It turns out I made a mistake measuring J8300 and missed pin 8 LCD_BKLT_PWM_R. There was R7780 missing on the board. Now this one works. The second one is still on process...
 
N

Narek

Member
Both boards have 51,2V on PPVOUT_S0_LCDBKLT, so that's not the case ;) I feel, that there is something small I'm missing again..
 
N

Narek

Member
Second one fixed too - though J8300 looked perfect I decided to replace it and that was the trick, backlight is back again.
 
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