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Iphone 6 no display or backlight

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  • Iphone 6 no display or backlight

    This iphone 6 was mailed into me after the customer had tried to change the screen. They told me it worked before this, but now doesnt work.They told me they possibly had plugged the front camera flex into the lcd connector.
    The phone boots and is detected by itunes but nothing is displayed on the screen. The screen has been tested and is fully functioning using another motherboard.
    I noticed when doing a visual on the board that FL2024 had been changed and the screw hole next to it removed.

    It also had long screw damage BS2502 (screw hole near the lcd driver). I investigated this and all lines are intact apart from two camera related ones, I have left these for now until I solve this issue.

    I also noticed that FL2027 on PP1V8_LCM_CONN was burnt so replaced this.

    I made a diode map of the lcd connector using a working board and compared it to this board and all readings are pretty much identical.

    I replaced chestnut but this did not resolve the problem.

    Next I took the following readings from chestnut :

    PP_VCC_MAIN @ C1547 = 3.9v
    CHESTNUT_LXP @ L1519 = 3.9v
    PP_CHESTNUT_CP @ C1554 = 5.76v
    PP6V0_LCM_BOOST @ C1502 = 6.05v
    PN5V7_SAGE_AVDDN @ C1504 = 5.66v
    PP5V7_SAGE_AVDDH @ C1577 = 5.73v
    PP5V7_LCM_AVDDH @ C1569 = 5.71v
    PP5V1_GRAPE_VDD @ C1541 = 5.11v
    LCM_TO_CHESTNUT_PWR_EN @ FL2035 = 1.787v

    Diode Readings
    AP_TO_I2C0_SCL = same as good board
    AP_BI_I2C0_SDA = same as good board
    RESET_1V8_L = same as good board
    CHESTNUT_TO_PMU_ADCIN7 @ C1323 = same as good board
    R0206 resistance = same as good board

    Looking at those readings I notice that CHESTNUT_LXP I presume is supposed to be the boosted voltage which should be 6 volts,but it is 3.7v which is the battery voltage, so chestnut is not boosting the voltage up to 6 volts. Why im not sure as LCM_TO_CHESTNUT_PWR_E is correct, so it is being told to turn on, but is ignoring this.

    Im not sure if I should pull chestnut again and measure between pads d1 and b2 to make sure that line is intact ?. Its strange that all other voltages are present and correct, but this line isnt being boosted. The coil itself looks fine, no bulging or burning.

  • #2
    If I check the line that comes out from under chestnut for pad c3 on LCM_TO_CHESTNUT_PWR_EN I get 1.786v so the enable signal is getting to chestnut ok.


    • #3
      Not sure where you are going with chestnut---you have all chestnut outputs, no? (assuming that your pn line is actually negative 5v7)
      First differentiate no backlight from no image. Which problem is this? Split a screen and see if you have image when shining a light from behind.


      • #4
        Which one is the pn line ?. No we dont have all the chestnut outputs, as CHESTNUT_LXP should be 6 volts but its only 3.9 volts. Iv tried the light on it and dont see any apple logo. I had a 6 in just last night with a blown backlight ferrite and could see the apple logo just fine under my microscope light once the screen was illuminated. This 6 has zero display output what so ever, no apple logo or anything using the same light as the one last night which I could see the apple logo under just fine.
        There is also no backlight too, but I didnt think the backlight circuit needed to be intact for chestnut to display anything ?. I have tested the backlight ferrites and all are good, iv not tested backlight output though or changed the backlight driver.


        • #5
          Ok so no image. I’m not
          sure what you mean about chestnut LXP, I don’t believe that is a chestnut output. You listed the chestnut outputs and they are all present. Chestnut is working—as long as pn 5v7 is negative look at the names—pp5v7, pn5v7
          lxp sounds like a line that would go to a coil, not an output. But I don’t have chestnut in front of me

          dont do anything with backlight until you have image.

          this really has to be something you can pick up in diode mode at connector or it is the connector itself—tiny changes in pin position can take out image by not making contact.

          if it were my board I’d be looking hard at the 1v8 line that goes through middle screw since it had lsd elsewhere and is missing a screw bracket there.


          • #6
            Ok thanks that would make sense that that line was blown. There is no damage to the pcb coating at all on that middle screw. Il try soldering a wire to that line and test it whilst plugged in as I don't think it's accessible otherwise. Il diode it first but the first time I did the connector it all checked out fine. I tried squeezing the connector last night to make it tighter,il try taking a look at the pins themselves tonight.


            • #7
              By the way on the schemetic it lists chestnut lxp as 6volts that's why I mentioned it. I don't think it is ah output but it looks like some kind of boost circuit that's not boosting.

              that line goes as follows....
              Vcc main comes in from the left, it then splits, one branch goes to chestnut directly with a cap on the line, the other line goes through the coil and becomes ixp, then enters chestnut on a different pad.
              Last edited by crea2k; 12-13-2017, 04:55 PM.


              • #8
                Oh my god lol what a complete tool I am, turns out it was in fact a bad pin that was flush with the connector !. After pulling that out it works now !. So chestnut may have been a step too far, but I only measured voltages after changing it.
                just have to fix the long screw mess and then we'll be good to go !.
                thanks for the help!, that's one less rabbit hole done. Just janky Ipads a many now....
                Last edited by crea2k; 12-13-2017, 05:19 PM.


                • #9
                  Originally posted by crea2k View Post
                  the other line goes through the coil and becomes ixp, then enters chestnut on a different pad.
                  Right--it's not an output. Think about what chestnut does---it uses the coil to create boost voltages which are the outputs. If the outputs are present, chestnut is working. Measuring the coil traces doesn't tell you anything---it IS boosting, as evidenced by the fact that you had chestnut output. This phone could really ONLY be connector if you were sure about diode mode in the connector. If not the connector, then one of the diode mode readings had to be wrong, and with LSD I'd guess 1v8 in the middle screw, but it would be full OL so that is tough to miss in diode mode. Sounds like you got it solved.


                  • #10
                    I did the diode mode after I had done the visual, seen the blown 1.8 line ferrite and replaced it. Il always do a visual first and fix the obvious stuff before I go to the effort of building a voltage and resistance map from scratch .

                    i did fix the camera and flash lines for them when I investigated the long screw damage so at least they have a 100% working phone now so all good . Just messaged them so ask about posting it back and they were really happy.

                    The reason I was confused about that coil was just that it didn't match the voltage in the schematics so I couldn't figure out what it was supposed to do as like you say the outputs were all correct so I wasn't sure why the voltage on it wasn't to spec either. Would be interesting to see what voltage these line is with a screen plugged in and working as it's supposed to be 6 volts.

                    At at least I have a IPhone 6 lcd connector map now, slowly building them up for different devices.


                    • #11
                      What do you mean ”it is supposed to be 6 volts?” It’s not a power line. It is part of the boost voltage system where the chip switches the voltage on and off to generate the boost voltage for the outputs. It might as well be internal to the chip.


                      • #12
                        On the schematic it says on the voltage properties page pp_chestnut_lxp , voltage = 6.0v . Between the coil and the chip is marked as that line on the schematic that's why I probed the coil. It probably is 6 volts with a working screen plugged in. We just know now that when all pins arnt connected it isn't 6 volts. Iv got an iPhone 6 with water damage in today I'm re assembling after fixing it so il probe it when on and see what it is.


                        • #13
                          No, that means “the maximum voltage this line will ever see is 6v”. Not “you should expect this line to always be 6v and if it’s not, you have a problem”. Think about how dc boost circuits work. This line is just a wire between chip and coil. It must be working if the chip is able to produce *any* boost voltage. Once you see a chestnut output is good you know this line is good.