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iphone 7 no image inconsistent voltages

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  • iphone 7 no image inconsistent voltages

    This was my 'parts for testing' test board
    One day I connected up to a screen could not get image

    Right now I can't make sense of the behaviour
    seated resistances on the display header have been checked -ok
    The display voltages are there and then no more
    sometimes they are disappear for a time and then appear again briefly
    and disappear

    Last work
    replaced chestnut
    C3707 chestnut's nut
    behaviour unchanged - display voltages come and go

    Multiple applications of heat on the board….
    As per history display header was replaced at least 3 times

    1. Display voltages
    2. Touch voltages
    3. Backlight voltage
    seated resistances✔
    except pin 4 -I2C_DISP_EEPROM_SCL_CONN✘

    Header replaced
    pin 36,38 OL - resoldered L3901✔
    Pin 39 (backlight) OL - FL3903 replaced -restored✔

    Image ✘
    Spongy header

    Header replaced
    pin 36 - pad missing - jumper run
    C3912 lost - resoldered

    Display voltages✔
    abnormal seated resistances

    New header
    Jumper directly soldered pin 36 header


    New header
    jumper lost off pin 36

    Header desoldered
    pin 36 jumper resoldered

    image ✘
    various opens and shorts(checked with multimeter B4 powering on) - resolved by adding/removing solder with hot tweezers
    Power up...………………….

    voltages appearing for brief periods then gone - all the 5V7's and 5V1
    and sometimes PP1V8_TOUCH_CONN
    Backlight voltage ok

    Attached Files
    Friendly Contributor
    Last edited by lots2learn; 10-24-2021, 01:55 AM.

  • #2
    Chestnut voltages will disappear when the screen goes to sleep and reappear when the screen is woken by power button, home button, or charger. That is normal.


    • #3
      Circling back to this one---the connector doesn't look well soldered. Have you looked at the feet from the side angle?
      When troubleshooting chestnut outputs you need to keep waking up the screen and then testing voltages--they only last for a brief time before they disappear when the screen sleeps.


      • #4
        The pulled pad complicated things somewhat. Soldering the jumper directly to the header pin's foot proved problematic as, during the course of soldering on the header, it always came off. Eventually I managed to hot tweezer it on after soldering the header. As I said on completion I checked all the header pin seated resistances which tested normal. On this basis I'm not sure why you think the header may not be well soldered?
        Either way I've attached a side on view of the header each side.
        Attached Files


        • Jessa_the_Professa
          Jessa_the_Professa commented
          Editing a comment
          Look at the pins on a few of the joints--example, the third pin from the left. See how it is just sitting on top of the pad without actually being soldered on? Same thing is going on with the small components that have been lifted and replaced. Some joints have way too much solder while others have none and the component is sitting on a bare(ish) pad.

      • #5
        I completely redid the display/touch header with a new one and jumper wired the missing pad AFTER seating the display header
        1. I continuity tested the 'sitting up' caps you referenced - they test fine.
        2. I have triple checked the display/touch header seated resistances thru iBridge
        3. They check out fine
        4. The seated resistances for the display lines on pins 36, 38 are 0.762 - slightly lower than the other two pairs - 0.800 but that is the only difference.
        1. display voltages pins 1,3,5,7 ✔
        2. touch voltages pin 9✔, pin 11 (PP5V1_TOUCH) missing✘
        Still no image.

        BTW I have confirmed normal boot sequence and that the phone is recognised by the computer/3U Tools
        Attached Files


        • #6
          I’m not sure this problem is amenable to internet troubleshooting. Something is wrong, and it will have to do with the board being “overworked”. It won’t be a classic problem and if you were here at Practical Board Repair School we would look at it and either find the problem immediately because it is something minor like “look under this bracket, there’s a bridge here, or look at chestnut and see how it is a mm high on one side—there’s a bit of goo under it that keeps one ball from making contact, etc. Either way, we would give you a different project. This board doesn’t seem to be doing its job to help you advance your learning.


          • #7
            Thanks for the objective appraisal.
            I am beginning to appreciate the concept of an overworked board.
            The more a board is worked on the less it becomes viable for validating soldering skills
            What I think I have learnt about microsoldering on phone motherboards is that it is important to get it in and out quickly(whilst still doing the job accurately).
            The 10 - 20 second rule(you sighted to me some months back) and using whatever hot air settings and developing the technique required to achieve this is my mantra. It seems to work.
            - this can only be achieved clocking up hundreds of hours of working under a microscope. But, sometimes I have a way of missing obvious things... Did I understand what you said correctly? Just checking in.


            • #8
              Yes except for the “this can only be achieved clocking hundreds of hours”. OR you can get to that point with a couple of days of in person training. We just ended a course where another set of complete beginners that had never tried Microsoldering on Monday left with the ability to do audio ic, connectors, and advanced troubleshooting. They will begin their career about where you are now, except they have a little more experience with recognizing common problems and how to extract and deliver data, but it only took a focused week.