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iphone 8 plus no touch and half backlight

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  • iphone 8 plus no touch and half backlight

    iphone 8 plus purchased off ebay.
    Came with a screen that was smashed to oblivion. What the seller didn't tell me but was clearly evident was that they had attempted to replace the broken screen no doubt with the battery still connected. Missing battery and sensor plates, no touch and half backlight. All chestnut voltages were absent so I replaced chestnut and the touch worked for a while. Now the
    touch has disappeared? Readings as below

    pin 19 5.1v missing(PP5V1_TOUCH_VDDH_CONN)
    pin 21 1.8v missing(PP1V8_TOUCH_CONN)
    pin 23 -5.7v present
    pin 25 +5.7v present
    pin 18 1.8v present
    pin 20 5.7v present
    pin 45 0.19v (PP_DISPLAY_BL34_ANODE_CONN)0.19v regardless whether screen is on or off
    pin 39 PP_DISPLAY_BL12_ANODE_CONN 15.92v when the screen is on,
    4.0v when the screen is off.

  • #2
    This doesn't make sense--if you could see half backlight, then chestnut is working right? Chestnut function is required to produce image. Chestnut would not make sense as a failure point in this scenario---why would chestnut fail if you plugged in a screen without disconnecting the battery? It wouldn't.

    So why did you think chestnut votlages were missing? Most likely because chestnut voltages go away when the screen sleeps or you were testing voltages with no screen present. Chestnut replacement probably only introduced more variables.

    On the next case, check in here BEFORE you introduce new variables that can complicate the process to make sure you're on the right track and not missing any signature failures.

    No touch on iPhone 8 signature failure = someone plugged in the wrong screen once and now the connector is a bit stretched out. Usually you can squish the connector a little bit so that the pins contacts the screen. Another thing to try is use the iBridge---does the screen have touch in the iBridge?

    Also--pry damage knocking something off near the connector.

    Use diode mode in the connector to check for short or open on all lines--iBridge makes this very easy.

    Second problem--backlight side 34 is out. Why? Diode mode in the connector to determine between short/open/chip not doing its job.


    • #3
      Touch was definitely not working before I replaced chestnut. (the phone booted to a passcode lock screen and did not respond) neither was the home button(another issue). Given that I couldn't even press a home button to keep the screen on for a while yes it makes sense that first time round the screen had gone to sleep by the time I took the voltage readings off the display connector.(I did this thru iBridge).

      BUT replacing chestnut DID restore the touch for a time(the phone was passcode locked). Long enough(With the board connected back in KNOWN good housing) for me to factory reset, complete setup, setup an onscreen home button with accessability assistant, go to the Appstore, install phone diagnostics software and test the phone. But sometime after that, still inframe, touch came and went intermittently until it failed again completely

      The next time round, after taking the board out of frame and connecting with known good charge flex etc, I kept the screen alive for a time whilst taking voltage measurements(thru iBridge) by pressing a hard iphone home button that connects through the lightning connector.

      1. The touch voltages are definitely missing
      2. ALL the seated resistance measurements test normal except for pin 45 - testing OL. (this must be the half of the backlight circuit that failed)


      • #4
        No touch on an iPhone 8 with a history of DIY repair and blown backlight filter is most likely “someone plugged in a screen a bunch of times and stretched out the connector”
        look at the pins of the connector under the microscope and see if they look stretched. Rework with tiny tweezers back to normal. Try not to replace the connector unless you really have to—because that will introduce more variables. Usually you do not have to replace connectors.

        If you don’t have any diode mode short or open at the screen for touch, and you are missing touch voltages then that is even more likely that you have a stretched out connector problem. See if the touch voltages are present on the board itself on a component before the connector —check the schematic.

        be sure the screen is awake and present when measuring touch voltages.

        at the same time, find the location of your open on the backlight line. Find the schematic. Walk back through the circuit measuring diode mode resistance to find the point that the circuit is no longer open.


        • #5
          Measurements 22-May-21
          Screen awake and present the entire time

          No Display issue (it's not the connector)
          1. C5721 PP5V1_TOUCH_VDDH missing
          2. C5601 PP1V8_TOUCH missing

          Seated resistances
          1. C5721 = 0.585 ( KNOWN GOOD board 0.577)
          2. C5601 = 0.667 ( KNOWN GOOD board 0.675)

          On one odd occasion after connecting up the screen, battery and charge port flex for a retest touch returned(half backlight persisted). Touch remained active the entire time(set the screen timeout to Never). Next reconnect for a test - touch was gone! I see the PP1V8_TOUCH voltage is produced by PMIC whilst the 5V1 voltage is produced by Chestnut. I don't think it's a chestnut problem or a PMIC problem? Not sure how to resolve this one? Could it be a faulty Touch IC?

          Half backlight issue - measured voltages
          1. C5733 0.18v (should be >10v)*
          2. C5663 3.86v (should be > 10v)*

          *10v - measurement taken off known good board at half brightness. Noticed this half of the backlight circuit(they look like two independent circuits?) has a lower voltage output 9.99v - 11.86v, the other half 11.86 - 16.92v (display settings 0 brightness to full brightness). Is this normal?

          Half backlight issue - measured seated resistances
          C5733 = 0.638 (KNOWN GOOD board 0.568)
          C5663 = 0.570 (KNOWN GOOD board 0.566)

          Seated resistances look normal to me - no opens/shorts/partial shorts? I think this is a power issue - a straight out failure of the Backlight IC to produce the voltage. My solution would be to replace the backlight IC. Is this correct?
          Attached Files
          Friendly Contributor
          Last edited by lots2learn; 05-22-2021, 01:47 AM.


          • #6
            Re: backlight. I would expect this to either be a connector issue or loose/damaged backlight filter on the backlight anode line that goes to pin 45. Do not replace backlight ic.

            diode mode pin 45 in the connector. If normal—replace connector. This is just such a common failure in so many devices—the chance that yours is an unheard of failure (backlight ic stops working) that just doesn’t happen is super low. I’ve never seen that.

            Re: no touch. You don’t have 1v8 touch voltage measured on the back of the board with a screen connected and alive?


            • #7
              Backlight issue. There are no backlight light filters coming off PP_DISPLAY_BL34_ANODE_CONN so I decided to cut to the chase and just do a simple continuity test between R5733 and the rest of the backlight circuit. It's testing open. I'm going to assume R5733, usually a 0 Ohm resistor, has gone faulty. Unfortunately it's snuggled right under the corner knee of an EMI shield. Gaining access right now is my biggest issue.

              Regards the display issue. Yes the display was on the entire time I was measuring voltages. I turned the board over at regular intervals to check and be sure that the screen was still on!
              I measured for PP1v8_TOUCH at C5601, which is the first component off PMIC. Touch voltage is not there. The same for 5v1 C5701 is the first component coming off chestnut - I tested for PP5V1_TOUCH_VDDH_CONN there. I took the measurements a number of times - they are consistent.


              • #8
                Step one on all of these problems is always the same “diode mode the connector”

                Diode mode in the connector will tell you “OL” on pin 45 and then you’d look for why—and quickly find it —-missing/damaged backlight filter.

                Re: touch—diode mode the connector will tell you if ANY line is short or open. Only if not short or open do you switch to voltage testing to find out why.

                Once you fix backlight then you’ll be left with no touch—diode mode the connector to see if it reveals a problem such as the screen touch data line short or open. If so—then that problem could spark a cascade of events that result in touch voltages not being produced because the device doesn’t have data communication with the screen.

                If you start with voltage testing and then trying to figure out why some voltages aren’t there you may be missing a broader view—voltages missing because they are not told to be there.

                Always diode mode the connector.


                • #9
                  I did diode mode the connector.(It also prompted me to raise a post sometime ago, Trolled by iBridge iphone 8 plus, in the 'Tools of the trade' forum).I got side tracked by the iBridge issue and forgot to attach it to the post.Sorry attaching now. (My readings are in black) Am also attaching a photo of the display header. . PS I would take seated resistance readings in ZXW 'with a grain of salt'. Sometimes there is a big difference when I compare them against a known good board.
                  Attached Files
                  Friendly Contributor
                  Last edited by lots2learn; 05-25-2021, 12:43 AM.


                  • #10
                    This board has a lot going on. You can see the missing mesa components right? It's no mystery why home button isn't working. With this amount of pry damage on the board, beat up connectors, and signs of either flux or water and a general "overworked" appearance, we need to see the back side of the board.

                    I would use the iBridge for testing--this will overcome any connector problem for now. I would press on the CPU to see if 1v8 touch comes back.


                    • #11
                      Thanks Jessa

                      I remeasured all touch line seated resistances(see attachment J5700 Header Touch Lines) comparing them against the seated resistances of a known good board(as rectangled in red) NOT ZXW!
                      I am satisfied that they are normal

                      "Press on CPU to see if 1V8 comes back"……. never got the chance!

                      Desoldered EMI shield to facilitate pressing down on CPU. Connected up with known good charge flex, battery and screen. When the phone booted touch had returned.
                      1. C5721 PP5V1_TOUCH_VDDH = 5.68v (triple checked to make sure I wasn't measuring display line voltage!)
                      2. C5601 PP1V8_TOUCH = 2.1v
                      Shutdown and reconnected two more times - touch remains.
                      Gave the board an ultrasonic clean. Reconnected and booted - touch is still there.

                      "find the location of your open on the backlight line. Find the schematic. Walk back through the circuit measuring diode mode resistance to find the point that the circuit is no longer open. "
                      Please see attachment 'Diode Moding' ..... I think the 0 ohm R5733 resistor has gone open
                      Attached Files
                      Friendly Contributor
                      Last edited by lots2learn; 05-27-2021, 07:53 AM.


                      • #12
                        Great—fix the backlight filter to solve and fix the mesa missing components.

                        Although your touch problem could have been connector, it can also be CPU in iPhone 8. They are very susceptible to flexion damage problems where they will just lose connection either at the cpu or one of the coils along the edge.

                        If Touch goes out again then press on cpu to prove.


                        • #13
                          Touch is still working. I'm curious if the heat applied to remove the EMI shields had some effect on CPU and resolved the touch issue. I replaced the missing components off the home button header - a filter and a resistor. It hasn't restored home button functionality. I could see no physical damage to the home button when I transferred to new screen. Am wondering should I remove the surrounding overfill near the home button header and check for further dislodged components
                          Attached Files
                          Friendly Contributor
                          Last edited by lots2learn; 06-06-2021, 10:59 AM.


                          • #14
                            I think that your no touch may well have been a CPU problem that was temporarily resolved with heat to remove the shields. It may come back over time.

                            If you want to troubleshoot home button, then same strategy. Diode mode the connector.


                            • #15

                              Desoldered corner EMI shield(to facilitate backlight repair) - still had touch
                              Replaced open 0 ohm resistor.
                              Half backlight issue resolved
                              No touch
                              Pressed down on CPU - touch not restored

                              Resoldered EMI shield
                              Booted image with light vertical lines
                              After a short time image faded to black(not on - off but a transition)
                              Could not be turned back on.
                              This is the current behaviour - phone briefly boots with image and quickly fades to black.
                              Attached Files