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iphone 8 plus no touch and half backlight

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  • #16
    Desoldered working chestnut off door board
    display/touch voltages?✘
    • DCPS 3A(full current draw) B4 PTB
    • VDD_MAIN seated resistance measures close to zero
    I spent too much time desoldering/resoldering EMI shield?
    Assumed this caused a short on VDD_MAIN in this area
    Desoldered Trinity
    VDD_MAIN seated resistance now 0.359 (known good = 0.355)
    DCPS 50mA B4 PTB


    • #17
      You desoldered the BRACKET? Oh heavens--I wouldn't do that---just cut it with bracket cutters and remove a few sections and leave off.

      Your problem is that you floated some (many? all?) of the resistors and filters in the area around the LCD connector.
      When you originally had no touch, the next step is always the same.


      You would have found what lines were open due to floating components, you could have dug them out of their black waterproofing and resoldered them and this phone would be working. Whenever you put heat in the area around connectors with waterproofing it is a risk that you will float components.

      Changing chestnut doesn't make sense---chestnut would not have "just gone bad" for no reason. It is not underfilled. Think "how did it come to be this way" as part of your troubleshooting.

      It sounds like you did think this way (good) when you created a vccmain short. It was likely under trinity and it was.

      Trinity also contains the EEPROM chip which must be present for the phone to boot. Put it back on.

      Clear the underfill from around the connectors in this area with 200C and a very very fine tool. You will see that many of them will pop off. Resolder them and check diode mode.

      Show us a clear picture of the board in the area around trinity and the lcd connector.


      • #18
        It also sounds like you still have a current leak---use a thermal camera to see where the heat is and figure out what line is short (backlight anode, speaker boost, battery vcc, etc)


        • #19
          resoldered chestnut and capacitor
          DCPS B4 PTB seated resistance VDD_MAIN tested full short
          desoldered chestnut - short resolved.
          soldered new chestnut DCPS 70mA B4 PTB - the seated resistance on VDD_MAIN is now normal at 0.356 I don't understand this behaviour
          Multiple physical damage across various components noted.
          C3110 & C3114 severed/damaged full short on PP_VDD_BOOST - desoldered short relieved
          AUDIO IC chipped - desoldered
          MOSFET had a chunk taken out - desoldered
          U6110 damaged - desoldered (DCPS 70mA B4 PTB resolved) DCPS is now 0mA B4 PTB
          U_EEPROM_E looks like it has a lesser chunk taken out of it(see picture) - I kept it on - hopefully it is still ok. My understanding is that it can't be replaced.

          Until I have resolved the image/touch problem my strategy is to leave what I have taken off, off but...….
          Is U6110 required for the phone to boot? What about the mosfet?

          Attached Files


          • #20
            Ok right now DCPS 0mA B4 PTB - on PTB current draw increments up to a maximum of 120mA drops back to zero, repeats(no image).
            am assuming a new mosfet at least will need to be soldered on to connect the battery line to VDD_MAIN


            • #21
              VDD_MAIN seated = 0.356 (normal!)
              VBAT seated = 0.450 (normal
              Restore Mosfet
              Desolder Mosfet
              4.0v injection directly into VDD_MAIN
              DCPS 2.8A B4 PTB
              VDD_MAIN seated = 0.00 Dead short
              VBAT seated = 0.439 (normal)

              I cannot make sense of this


              • #22
                You don't need to have the mosfet on there to diagnose. Main will connect to battery through tigris. I would leave it off to eliminate variables. If you would like, you could simply hardwire a jumper to connect battery and main while troubleshooting.

                Regardless---you have a short on vddmain if your multimeter reads 0.00 in diode mode anywhere on main with red probe on ground, black on main. I'm not sure what the word "seated" means in this context, but I am guessing that you are using it to mean "resistance" and you have a a short to ground on main.

                How did you get a short to ground when it did not exist before? By nudging something. Either tigris when placing the mosfet, or trinity on the opposite side when the board was upside down from the weight of the board while placing the mosfet. A thermal camera may quickly tell you which one is the culprit.


                • #23
                  DCPS set to 4v Max current 3A
                  TIGRIS desoldered (it was not nudged)
                  TRINITY has not moved
                  VDD_MAIN hardwired to VBAT
                  now connecting to VDD_MAIN via battery connector
                  behaviour unchanged
                  DCPS B4 PTB 3A 2v
                  No heat
                  VDD_MAIN (and now VBAT) seated resistance remains dead short


                  • #24
                    Thermal camera would really really help you.

                    A short that doesn’t produce heat is because it is produced by main and ground becoming soldered together through a thick connection not a thin one. Usually a blob under a chip, or even a main pad scraped slightly so that it fuses to adjacent ground. I would suspect trinity, but after that you just have to go by process of elimination (thermal camera would find *any* heat at all and save you a ton of work).

                    Before you go crazy, be sure to check backlight anode and vddboost and speaker boost to be sure that main isn’t short because of a connection to those lines and that one of them is the real short.


                    • #25
                      DIODE MODE
                      PP_VDD_BOOST 0.451 (known good 0.419)
                      PP_SPKRAMP_TOP_VBOOST C5006 0.468(known good) 0.490
                      PP_SPKRAMP_BOT_VBOOST C4903 0.480(known good) 0.493
                      BACKLIGHT ANODE Test points pin 39 0.210 (known good 0.551) , pin 45 0.225 (known good 0.545)

                      It looks like there is a partial short on both backlight anode lines.
                      Could this cause a full short on VDD_MAIN?


                      • #26
                        Probably backlight anode is reading low because of the short on main.

                        I would use a thermal cam to find the main short.
                        If no thermal cam Id take off trinity and then continue to remove any chip in the area I’d worked in that has main under it that could be blobbed into ground. It would be a brute force effort. I’d also do a really crisp visual exam to make sure I couldn’t see what chip is slightly crooked etc