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Old 06-13-2018, 09:22 AM   #1
Flippers McCoy
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Talking 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

Hello, everyone!

I have an 820-00138-A motherboard that doesn't seem to be outputting PPVOUT_S0_LCDBKLT properly. I've done some poking around, and I suspect that Q7706 isn't working properly. It's a P-Channel MOSFET, so it should let PPBUS_SW_LCDBKLT_PWR (which reads 12.56V) if LCDBKLT_EN_L (which reads 12.46V) is lower, but PPBUS_SW_BKL reads a solid 0V.

I just wanted to confirm my suspicions before I went ahead and tried replacing this chip. Is assuming that Q7706 is not working a proper assumption?

Thanks!

(I've attached schematics and boardview for reference, please note that the boardview is for a slightly different model, as the boardview for 820-00138-A is not available at this time)
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File Type: zip 820-00138 MacBook Pro Retina 15 A1398 (Mid 2015).zip (2.10 MB, 50 views)
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Old 06-13-2018, 10:55 AM   #2
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

Hi
I think the differnt is too low to switch Q7706
i Woud messure the voltage on u7701
IC u7701 if its works pin11 goes to GND and then R7701 and R7702 works as voltagedivider
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Old 06-13-2018, 11:36 AM   #3
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

LCDBKLT_EN_L is high so that won't work. U7701 probably doesn't pull BKLT_SD to ground.
Check BKLT_EN_R. Check lid sensor. Only a no backlight issue?
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Old 06-13-2018, 11:41 AM   #4
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

I get 12.38V on pin 2 of R7702 and 12.45V on pin 1. According to a calculator I found online, if BKLT_SD (AKA pin 11 of U7701) is properly connected to ground, then LCDBKLT_EN_L should be 5.55V, which would then allow Q7706 to pass along 12.6V, right?
Is there a way to diagnose U7701 without replacing it?
I checked and BKLT_EN_R reads 0V
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Old 06-13-2018, 11:58 AM   #5
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

Yes, the only issue is there's not backlight. I can see an image on the screen if I use a flashlight.
Sorry if this shows my ignorance, but how can I check the lid sensor?

Turns out I get slightly different, more accurate readings when I have the battery plugged in as opposed to just on the adapter. In this state, I can see that there's an image, but the computer goes into self protect mode with the fans blasting and taking forever to load the OS.
I get 3.33V on BKLT_EN_R, 12.2V on LCDBKLT_EN_L, and 13V on PPBUS_SW_LCDBKLT_PWR
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Old 06-13-2018, 02:39 PM   #6
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

BKLT_EN_R should be high for the backlight to turn on. If BKLT_EN_R is 3.3V but LCDBKLT_EN_L doesn't go low, chances are U7701 has an issue.
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Old 06-13-2018, 04:03 PM   #7
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

I swapped out U7701, still the same exact symptoms. Is there anything else it could be?
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Old 06-13-2018, 04:45 PM   #8
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

What about the PP5V_S0 rail. Can you Check R7743, R7744 and R7740 .
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Old 06-13-2018, 05:03 PM   #9
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

PP5V_S0 reads at ~5.04V. Is it good enough to check on the respective pins on U7701? Because the boardview for this particular board isn't available I've been having to deduce which components are which based on a similar boardview, but I don't think the one I have is similar enough for me to determine which resistor is which in this area :/
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Old 06-14-2018, 03:40 AM   #10
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

Hook up the lcd. check voltage on U7701 pins 5,18 and 19.
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Old 06-15-2018, 08:49 AM   #11
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

Pin 5: 5.05V
Pin 18: 5.03V
Pin 19: 0V

Does this mean that the chip just isn't telling the backlight to come on at all, for some reason?
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Old 06-18-2018, 08:39 AM   #12
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

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Old 06-20-2018, 10:30 AM   #13
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

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Old 06-20-2018, 03:52 PM   #14
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

Quote:
Originally Posted by Flippers McCoy View Post
Pin 5: 5.05V
Pin 18: 5.03V
Pin 19: 0V

Does this mean that the chip just isn't telling the backlight to come on at all, for some reason?
I`m not sure myself. Can you check R7740 , 1MOhm
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Old 06-20-2018, 06:02 PM   #15
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

When your area of interest surrounds an IC with a relatively small number of pins like U7701, just measure all the pins and post it. Otherwise you get a request for you to measure this pin and then that pin and then that pin. And the chip might not even be acting the same between measurements.

Indeed, this area has not been acting the same. But in post #5 you are indicating one situation where BKLT_EN_R went high and LCDBKLT_EN_L was still not being pulled low. U7701 has a data connection through which parameters can be set, so I can't totally rule out another cause (checked for corrosion and such around the SMC and elsewhere?). However, let's assume there may be a bad component in the vicinity of U7701 for the moment, just to rule it out if nothing else.

Though you may have done so, I don't see it mentioned anywhere that you have checked the backlight power rail for shorts to ground. It should be checked before and after D7701. And D7701 itself should be checked that it hasn't failed shorted or open circuit. And Q7701 should be checked to be sure it has no shorts between source, drain, and gate. And the feedback pin (pin 21) should be checked for continuity to the resistors and to PPVOUT_S0_LCDBKLT. And the current sensing circuit (pins 9 and 10) should be checked for continuity to the sense resistor.

A failure in any one of these places might cause U7701 to clamp the power rail off as a protective measure... either because the backlight power rail is shorted, or because U7701 is fooled into thinking it is.

Having said all that.. you have not always been getting a high BKLT_EN_R signal. I don't know if that could happen as a reaction to U7701 reporting a fault or not. But, just be aware that the problem may ultimately lie somewhere else.
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Old 06-27-2018, 08:31 AM   #16
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

Thanks for the tip on the IC, I'll remember that for next time

D7701 doesn't get continuity to ground on either side.
Q7701 has no shorts between source, drain, or gate.
BKL_FB doesn't get continuity to PPVOUT_S0_LCDBKLT, and pin 21 gets continuity to both resistors and obviously not to PPVOUT_S0_LCDBKLT.
Pins 9 and 10 both get continuity to their respective sides of R7700.
R7740 seems fine, no continuity to ground and is not creating an open circuit.

I just learned that there was a small nick in the LCD connector. The customer had cracked the LCD, so we had a third party repair it, and when it returned we had, foolishly, assumed it was fine. I think it's entirely possible that the damaged LCD connector caused damage the board, but I'm unsure of which components to check that may have been damaged by a faulty LCD connector. Unfortunately we have already sent the lid back to said third party for them to fix their mistake, and it's also possible that they will repair the board for us for free.
I've tried a known good test LCD and it still has the same behavior
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Old 06-28-2018, 05:33 AM   #17
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Default Re: 820-00138-A Not outputting PPVOUT_S0_LCDBKLT

Quote:
Originally Posted by Flippers McCoy View Post
BKL_FB doesn't get continuity to PPVOUT_S0_LCDBKLT [...]
Assuming that's not a typo... that would be a problem. That would indicate a break in a trace/via somewhere between R7708 and PPVOUT_S0_LCDBKLT.
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