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820-00928 A1707 No Display

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  • 820-00928 A1707 No Display

    Hello,

    I have a touchbar mac that does not output to the internal display. I can get video output via USB C, and the logic board powers on and boots mac os.
    The history is liquid damage, no obvious corrosion except between pins 1 and 43 of J8500. Cleaned up and replaced LCD cable, verified no connection, but the problem still persists.
    I've been troubleshooting this one for a while but have come up pretty much empty handed.

    Originally I thought there was a good chance that U9850 was dead (the short might have sent backlight voltage to the chip), so I did some research and found a replacement for the CBTL06142EEE (I could not find it available for order) with a HD3SS212ZQET, which should be compatible. The system sill powers on but it still has no display.

    I get LCD_MUX_EN ad 3.3V.

    EDP_PANEL_PWR_EN appears at 3.4 volts and disappears a second after power on. Same for DFR_DISP_PWR_EN, DFR_DISP_SMC_RST_L, BUF_EDP_PANEL_PWR_EN, and PANEL_FET_EN_DLY .

    PPVOUT_S0_LCDBKLT is .01 volts (almost nothing)

    PANEL_P5V_EN_D is 0 volts.

    Can anyone with a little more experience with the newer macs assist in troubleshooting? I'd really appreciate it.

    Thanks,

    Stefan

  • #2
    I think 820-00281 is the closest schematic/boardview for your board.
    In this case, GMUX U9850 is fried...

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    • #3
      I have been working off of 820-00281 for lack of the exact schematic.
      I actually did replace U9850 with the TI chip I was able to get my hands on. It is supposedly compatible, and the datasheets seem to agree with that. No change, however.

      Tested with a display that was previously working.

      EDIT - I verified pinout functionality between the two chips.

      Down at the bottom of this page in the section for apple gmux was the only clue I could find about a compatible replacement (had a hard time finding CBTL06142EEE):
      https://people.freedesktop.org/~narm...witcheroo.html
      Last edited by iambehr; 09-20-2018, 04:48 PM.

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      • #4
        GMUX is supposed to have firmware...

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        • #5
          Hmm.. source? I read the datasheet, and my admittedly poor understanding of the chip design makes me think that it is just a high speed capable switch, and it's outside behavior is controlled by the Dx_SEL pin on the chip.
          Or do you mean that the system firmware has to have the proper GMUX chip specified in it?

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          • #6
            So... long shot but does anyone have any ideas? Customer just asked if he can send it to rossmann's in NY, and I am going to have to make it someone elses problem ( out of my control, he asked me)

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            • #7
              Get GMUX from same donor board...

              Comment


              • #8
                Sounds like I should have done that in the first place. As always, thank you for taking time to reply.
                Don't have time to get a donor, had this far too long anyway and I think the customer is done with us.

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                • #9
                  Apple is actually trying to make the repair as more dificil possible.
                  Even charger chips have firmware.
                  So be aware and replace any chips susceptible to have firmware only with chips removed from identical donor board...

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                  • #10
                    Originally posted by iambehr View Post
                    Hmm.. source? I read the datasheet, and my admittedly poor understanding of the chip design makes me think that it is just a high speed capable switch, and it's outside behavior is controlled by the Dx_SEL pin on the chip.
                    Or do you mean that the system firmware has to have the proper GMUX chip specified in it?
                    I have a 00281 with this issue. Tried a to replace u9850 from a 820-3787 with no change in symtoms. Ive now pulled the MUX chip of a scrap 00928 and will get back to you with results

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                    • #11
                      Thanks! It will be interesting to hear what happens.

                      (reason for edit: mixed up customers and said something that was not correct)

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                      • #12
                        I Put a u9850 from another 15 touchbar and still have no internal display. I will do some more digging and swap some more parts out until it works

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                        • #13
                          Don't waste time, if you can't get GMUX from the same board model...

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                          • #14
                            Originally posted by 2informaticos View Post
                            Don't waste time, if you can't get GMUX from the same board model...
                            u9850 is not the commonly known programmed GMUX. I pulled from the same model anyway. That is not the only thing failing on these boards when those lines short.

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