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820-00840 with backlight problem and low PP5V_S0 resistance to ground

sekidata

sekidata

Member
This 2017 MacBook Pro board came with a dark screen and liquid damage in the backlight circuity area. I found a shorted cap (C4811) which I replaced. The backlight came on for a few days, then died again. I've replaced U8400 and checked all resistors and traces in that area. Machine boots and backlight enable signal is present. Still no backlight.

Then I found that PP5V_S0, which supplies U8400, has a low resistance to ground (95 ohms; 0.09 V in diode mode); a good board shows 5+ kohms on this rail. I'm having a hard time figuring out where this partial short is located, since injecting 5 V only gets 0.1 A flowing and nothing heats up. Any way to pinpoint the culprit?
 
2informaticos

2informaticos

Administrator
Staff member
If machine boots and have external video and USB activity, then must be a problem with internal video.
If gets image on internal LCD, then go straight to backlight circuit.
Check anything backlight related connected to 5V_S0 rail; remove one by one, until find the culprit.
 
Last edited:
sekidata

sekidata

Member
Ok, I resolved the short on PP5V_S0 by replacing U8400 a second time (it now measures 0.43 V in diode mode. However, the backlight is still nonfunctional.

I verified that the machine boots and gets an image on internal and external display. BLKT_EN is 3.3 V but there is no output from U8400 at all. The only other observation is that the SCL and SDA data line voltages drop from 5.1 to 4.5 V as soon as BKLT_EN turns on.
 
sekidata

sekidata

Member
Yes, I'm using a known good display. There were burnt vias around U8400 pins 9 and 10 and I had to run wires from those pins to R8400.
Below are the Q8400 and U8400 voltages.

Q8400:
pin 3: 12.6V
pin 4: 13.1V
pin 1,2,5,6: <2V

U8400:
pin 1,2: 0.3V
pin 4: 0.0V
pin 5: 5.1V
pin 9,10: 13.1V
pin 11: 12.4V
pin 12: 0.0V
pin 15,16: 4.5V
pin 17: 3.3V
pin 18: 5.1V
pin 19: 0.2V
pin 21: 0.0V
 
2informaticos

2informaticos

Administrator
Staff member
Backlight adjust is done through I2C bus (BKLT_SCL/SDA lines); is not sufficient to get enable signal.
The information sends by PCH (PWM) passes through LCD T-CON before arriving to U8400 (after conversion to I2C).
You may have other traces, or components damaged around U8400.

Check if EDP_BKLT_PWM is present at least.
 
sekidata

sekidata

Member
EDP_BKLT_PWM is 2.5V.

My understanding of the PWM for these displays is limited. Can you explain the PCH - LCD - U8400 connectivity some more? Is the LCD T-CON the board on the display itself? Does the PCH send info to the LCD via EDP_BKLT_PWM and the LCD then sends info to U8400 on the I2C_BKLT data lines?
 
sekidata

sekidata

Member
How important is the sense resistor R8400? That's where I had burnt traces. Under what R8400 conditions would U8400 refuse to activate? How can I test it?
 
sekidata

sekidata

Member
Yup, it is. I also replaced R8400 to no effect.

I'm giving up on this board - sometime you just have to accept that a deep mystery remains.
 
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