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6S Shorted backlight cap fixed, but still will not boot.

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  • 6S Shorted backlight cap fixed, but still will not boot.

    Got a 6S in that customer said would not charge. After running my normal diagnostic on it, I found that C4023 was shorted and heating up when running power from my DCPS to the battery connector. I removed the cap and replaced it with a good one which fixed the short, but the phone would not boot still. I removed U4020 (backlight driver) and attempted to boot it again, but still nothing. The DCPS is set to 4.0V and 2A. When request to boot is triggered, the amperage draw consistently goes up from 0.02 to 0.07, then back down to 0.00, then cycles again.

    I checked a few of the caps around tigris and tristar, but everything seemed ok.
    VCC_Main is getting 4V
    PP17V0_Mojave_LDOIN (tested at cap C4041) is 3.6V. Figured that would be low since the phone is not actually booting.
    PP1V8_SDRAM = 1.1V - Low for some reason....

    Voltage is constant on each test point. Any ideas on what may cause the 1.8V line to be 1.1? Open for ideas

  • #2
    How did you remove c4023? Try not to use hot air—it can bother bl driver and/or strobe driver/ Or CPU.
    1v8sdram should be 1v8, if it is 1v1 then that’s not good. 1v8SDRAM goes to backlight driver and you mentioned replacing the driver. Whenever you have a phone that was booting, then you did something, and now it is not booting that means go back and undo what you did—take off bl driver and see if it will boot.

    Comment


    • #3
      The phone did not boot when it came in. The customer was using it one day, then woke up the next morning and the phone was off and would not come back on. When they brought it in, it pulled 0V on the USB ammeter and we tried a different battery to see if it had just given up.

      I remove components around the CPU using 350F hot air and my JBC to prevent CPU damage. This method has worked for me for many repairs without damaging surrounding components.

      The driver is removed and the phone is not booting still. The first post shows that I removed it and the phone does not boot. I figured with the 1V8 rail being such low power, I would need to track this down but that is a BIG section of the board and it could be anywhere... I would hate to inject 1.8V to that rail and permanently fry something crucial, but it may be my only option to track it down.

      Comment


      • #4
        OK, got an update...

        DCPS set to 4.0V 2A

        All these values are 0.00V for 2 seconds, then on for 3 seconds, then back off for another 2 seconds. Repeating....

        PP1V8_SDRAM now has 1.79V.
        PP_FIXED = 0.901V
        PP1V8 = 1.79V
        PP_SOC = 0.823V
        PP3V0_TRISTAR = 2.999V

        Always on / no "pulsing" :
        PP_VCC_MAIN = 4.018V

        Anything else I should check? My customer has been calling me non stop about this dang phone....

        Comment


        • #5
          Let’s go back and clarify order of operations.

          you had a no power phone that would not boot.
          First step is visual exam —no findings—is that true?
          second step is dc power supply—what was result at that point?

          Are you sure vccmain had a normal diode mode reading and was not partial short?

          How did you get from here to backlight anode line? Backlight is not required for boot and a phone with a full short on backlight anode line can boot, so ignore until you have phone booting so as not to add variables and complicate the picture. A short on vccmain can also “appear” to be a partial short on backlight anode line since they are connected through the diode. Similarly, a short on backlight anode typically blows the diode and makes this line an open line, but sometimes it does not blow diode so a short on bl anode with a working diode would pull down vccmain. Tl;dr—verify vccmain diode mode reading is 0.334v or so before doing anything with c4023.

          the phone is now in the “pmic is resetting itself” state. I don’t know that I’ve ever seen a case of that outside of water damage, and these cases are challenging. My guess is that we are missing something on the presentation of this phone—water damage, shield removal nudging something, prior repair attempt. Your best bet is extremely detailed visual exam to see if you can find a physical clue.


          Comment


          • #6
            Also—in the first post you had a constant voltage of 1.1v on 1v8SDRAM and in second post you have the pmic resetting state. (Which is never bad pmic, FYI!) what changed between these two states?

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            • #7
              When the phone came in, the battery voltage was at 0.00V and when connecting it to a USB ammeter, the current was 0.00A. When I opened the device, I checked for water damage and any heat marks around the components by the connectors. Everything visually checked out there. I then set my DCPS to 4V 1A and connected the board only to it. Without triggering it to boot, amperage draw was 0.00A but after triggering it to boot, amperage draw went to 0.76A. At first, I could not find any heat, but after letting the board sit for about 5 minutes, the board was warm above the processor so I removed the shield and used some 91% alcohol to see where the heat was coming from which showed around the backlight circuit. I tested the large cap next to the driver and it was a dead short so I removed it. Short went away and I tried to boot again which is when I was getting the "pulsing" current from 0.02 to 0.07.

              The first voltage test on VCC_MAIN was probably a mistake and I may have read it wrong or not had a good connection. Every time I have tested it afterwards, it tests a 1.79V

              I have done a full visual on this board and it appears in perfect shape. Nothing looks nudged, burnt, water damaged, or knocked off. Right now, the backlight circuit is removed (driver, cap, both diodes) to see if I could get the device to boot.

              Comment


              • Jessa_the_Professa
                Jessa_the_Professa commented
                Editing a comment
                Still not clear---
                You told us that you have 4v on vccmain.
                You told us that the board showed no signs of vccmain short (holds 0A before prompt to boot)
                Now you are telling us that vccmain is regularly testing 1.79v

                My question for you is "What is the diode mode reading of vccmain?" It is either normal (0.3v) or low (anything less). A diode mode test is red probe on ground, black probe anywhere on the line in questions with the multimeter in diode mode. A diode mode test for vccmain is one of your "go to" tools and comes right after visual inspection on a suspected no power board.

                Alcohol is not a good tool for heat detection for iPhones, it is useful for macbooks but not for the relatively low wattage shorts that iPhones have. Alcohol does contain water and can go under chips and cause problems. Alcohol is only to be used for iPhones as a last resort on a short that you know is screaming hot, is not in any common place, and you can't find it.

                Similarly letting a board with a short circuit sit on a power supply for 5 minutes is also not good---we really don't want to dump voltage into a short. It causes damage, for example in the iPhone 7 and initial SDRAM short is curable, but if the phone 'lives with' the SDRAM short, then the heat and current from the short will kill the CPU.

                It sounds like you are using macbook strategies for iPhone repair. Remember---Macbooks are dogs. iPhones are cats!

            • #8
              Ok, I am back at this one again.... Customer is still ok with the time frame, but I would like to get this one off my shelf.... Diode mode test on VCC_MAIN is 0.333. Anything else I need to check for? Do I need to get some freeze spray and give that a shot?

              Comment


              • Jessa_the_Professa
                Jessa_the_Professa commented
                Editing a comment
                No Freeze Spray! Freeze spray is a wonderful tool, but it is somewhat dangerous and it asks a very specific question "Which component is the cause of a short circuit, when I know I have a short circuit that consumes high current" You do NOT have a short circuit. Do not put freeze spray on that board.

                I read through this problem again. This is a data problem, not a power problem.

                When you told us "The first voltage test on VCC_MAIN was probably a mistake and I may have read it wrong or not had a good connection. Every time I have tested it afterwards, it tests a 1.79V" above this must have been a mistake. Can you confirm that you meant that vccmain measures battery voltage 3.8-4.3v?

                If so, then that makes this a data/information problem. Some line that is important for the CPU to send or receive instructions has been damaged, which may have resulted in software corruption. In either case, the boot sequence begins and either there is missing information (software corruption) or a line for issuing instructions (data bus) is damaged.

                How did this happen? We need to identify MOST LIKELY and make a plan to rule out these possibilities.

                Look for data lines that are between the CPU (marked on the schematic as AP) and several other chips, for example, the i2c lines. Find some that go to the areas where you have done work---tristar and backlight area. See if you can find a problem such as a nudged resistor on these lines.

                You can rule out software corruption by forcing the phone to boot to recovery mode and attempting to update or restore.

            • #9
              VCC_MAIN voltage is 3.8V with DCPS set to 3.8V. When jumping the power button pin to ground, the amperage draw fluctuates between 0.00 and 0.07. I pulled the tristar back off and put on a new one after verifying everything around it was properly soldered and no solder balls shorting anything. Still nothing

              Comment


              • #10
                follow above instructions

                Comment


                • #11
                  Originally posted by Jessa_the_Professa View Post
                  follow above instructions
                    • How did this happen? We need to identify MOST LIKELY and make a plan to rule out these possibilities.

                      Look for data lines that are between the CPU (marked on the schematic as AP) and several other chips, for example, the i2c lines. Find some that go to the areas where you have done work---tristar and backlight area. See if you can find a problem such as a nudged resistor on these lines.

                      You can rule out software corruption by forcing the phone to boot to recovery mode and attempting to update or restore.

                  Comment


                  • #12
                    I don't know how this happened on the phone.... it came not charging and would not boot (As stated in my first 2 posts). I did the normal steps.....check charge port, battery, and then pulled the board and checked for shorts (As stated in my first post). Found a short on C4023 and removed it using 350F Hot air and my JBC station. I have quadruple checked the area around the CAP replaced for any damaged, bumped, poorly connected parts and found nothing wrong. The short was removed and I then attempted to power on the device using the DCPS and this is when I was getting the constant pulse of amperage reading. Since it still would not boot and it seemed to be a power issue, I replaced the tristar which did not fix the issue still. I have fully checked all the components around where I have worked and around the edge of the board where the cover plates where removed to ensure nothing has been damage.... No damage was found.

                    The phone will not boot to recovery or DFU....

                    Comment


                    • Jessa_the_Professa
                      Jessa_the_Professa commented
                      Editing a comment
                      So you have TWO DIFFERENT phones that both can not be forced to boot into recovery mode? That only means one thing---you need to look up how to boot a phone into recovery mode and refresh your memory.

                      I am sure that this phone will boot into recovery mode if done properly and then you can see what happens when you restore.

                  • #13
                    "So you have TWO DIFFERENT phones that both can not be forced to boot into recovery mode? That only means one thing---you need to look up how to boot a phone into recovery mode and refresh your memory." Wow!! I did not know I was that dumb?!?!?!?!

                    1 phone will not show anything / will not boot (This one) and the second phone if you read the post, turned out to be a shorted home button and entered recovery mode now, but will not restore. (Fails at 19%)

                    Comment


                    • Jessa_the_Professa
                      Jessa_the_Professa commented
                      Editing a comment
                      Okay so you're down to one phone that will not boot to recovery. Most phones are able to boot to recovery, so troubleshoot that--it requires usb detection (tristar), 5v at tristar from dock, good home button/long flex/connector. Put it on DC power supply, press home and then connect usb---what happens?

                  • #14
                    OK, finally got back working on this one again. Got the device on the DCPS with a known good charge port and a known good home button. Tried entering recovery by pressing home button, then connecting USB to computer, but the DCPS still cycles between 0.00 and 0.07A with nothing being recognized on the computer.

                    Comment


                    • #15
                      That is not normal, a phone that only had a short on backlight anode line should be able to boot into recovery. So this is a problem that you created during troubleshooting. If you are sure that dock and home button are good, and backlight driver is still off, then retrace steps. You changed tristar on this phone. That is in the MOST LIKELY position right now—change it again. A phone with this history can always boot in recovery mode. Troubleshoot the cable, dock, dock connector, lines from dock to tristar, home button, connector etc until you find the problem.

                      Comment

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