iPhone 6 no power

[BigN8]

I have an i6 that is currently dead. It has no previous repair attempts except a screen replacement in the past. The housing and screen appear to be in good working order with no visible signs of physical damage. The customer said it is their daughters phone. I asked about the charging cable she used and they said it was likely a cheap aftermarket . My first steps were to test known good battery to see if it would boot. no luck. I connected to DCPS to meter the power consumption. My findings are: no consumption change when connected, 0.012a before i connect and 0.012. after connected. this tells me that I likely dont have a short to ground. when I prompt to boot with USB cable my portopow meter jumps to .07a pauses for about 2 seconds then jumps to .24a and immediately falls to 0 then back to .07. Rinse and repeat. when I disconnect USB cable and watch DCPS it starts at 0.094a pauses for 2 seconds ramps up to about 0.200a then drops back to 0.012a then to 0.094. Rinse and repeat. When I use a known good battery and USB my portopow is 0 to .42a to .92a then 0 again and repeat. My diagnostic thinking tells me that this device is "brain dead" or cant hear the instructions to turn on. I have seen videos of a similar symptom and I used "PLZ BRO" monkey see monkey do and replaced tristar. Same exact symptoms. Replaced Tigris. same exact symptoms. Now that I know those chips weren't the problem I believe that something is not being told to turn on. I am getting 4.74v at 5vUSB test point at the connector and the same at C1817. The BATT_VCC line is fluctuating between 3.92v and 4.12 while connected to USB and known good battery. VCC main is also fluctuating between 4.14v and 4.21v. I have seen this before and this is where I get stuck. I am new at diagnosing board level problems and don't know how to troubleshoot this any further. Is there something I am missing or another place I should be taking readings that will help me find the fault, or another way to approach this issue? Thanks!

UPDATE:

After doing further research on this problem I found a video Jessa made "all about PMIC" and she basically did exactly what I have done and a lot more. I am very confident now after learning to test some more lines that PMIC is my next logical step. I have found some and ordered from china but they will likely take a few weeks. Is it a bad idea to try to salvage one from a donor?

[Jessa_the_Professa]

It looks like you've put a lot of thought into this board already---well done. Let's clarify some results. The portapow is telling you that the device is boot looping---it goes to a normal charging current and then drops and repeats---that's a boot loop. So if that is true, that means that on DC power supply, the phone MUST begin a normal boot sequence and then stop. Your DCPS reading should be something like 0.000--prompt to boot---0.1, 0.2, 0.3, 0.2, 0.1, 0.4, 0.2, etc and then drop to low 0.07 or so and then 0.000 and repeat. There is no way that your portapow can show you one thing and the DCPS show something else. This means we have a problem setting up a test. It is unlikely that the portapow is wrong, so that points to something wrong with your DCPS. The common fault is failing to withdraw the usb input after prompting for a few seconds so that we can see the current consumption from the battery---i.e. the DCPS. If left attached, the device will attempt to 'save' the battery voltage and use the 5v from usb--which we can't 'see' on DCPS.

In an undamaged phone with history of cheap cable and no strong short to ground, you're right to think tristar. So why didn't simply changing tristar solve this phone?
I want you to look at the schematic at tristar, and identify lines that are CPU communication lines that the CPU uses to talk to multiple chips--tristar is one of them. Once you identify these lines, see if you can come up with a theory of whether or not it is possible for tristar to fail in a way that could cause software corruption. And how might that apply to this problem.

[Jessa_the_Professa]

Forgive me for posting so late, I am supposed to be sleeping but not before finishing my beets broccoli & tofu.

[Jessa_the_Professa]

Louis, go to bed.

[Jessa_the_Professa]

But if you're still up and reading this. A--I walked all the way to Midtown. And B--Have you ever heard of a 100uH backlight coil in any device?

[BigN8]

Hi Jessa and thank you for your response. No need to apologize to me. I am so greatful for this forum you have no idea. I have been looking hard for this type of access to people who have knowledge that are willing to help be get better. I found your channel about a year ago when I was looking for answers to data recovery problems and saw you fixing boards for recovery. I didn’t know that was possible. You inspired me to start purchasing the equipment necessary to do this in a professional way and I’ve been having good success with a lot of the plz bro stuff but I really want to understand “what” I’m doing not just “how” to do it. And also to problem solve. Your teaching style is amazing and I appreciate your willingness to help!
Back to your response: I understand why you say the portopow is telling me that the board is bootlooping but I’m pretty sure it’s not. ( I could be wrong)
I’ve seen lots of bootloops and the similar patterns that they show when booting up then dropping to zero. This one wasn’t consistent and I got different readings at different times. When it took .92 it was literally for a split second then dropped to zero and only showed that when it was connected to a battery and prompting with USB. It would spike to that reading- not build like a typical boot loop. It is a super cheep portopow that I’ve had since the beginning and it has been helpful but I’ve learned not to fully rely on it because it is not as sensitive as my DCPS.
When the board was connected to the DCPS and USB the portopow readings were consistent with what the DCPS would show after I would disconnect the usb cable. ( about.07 to .24 then 0) My DCPS is new a month or so ago and has been an amazing tool. it was telling me the phone was trying to boot but wouldn’t keep building past about .200a before resetting. After I posted my question I kept hunting for more information that might help me solve my problem when I found you video. I watched it in the past but at that time I didn’t understand fully what you were teaching.
Tonight it all made sense and I was getting almost identical readings that you were and was able to understand more about what PMIC does. Now I know how to test input to PMIC and output too. Thank you.

Im staring at the schematic and to answer the question “why didn’t replacing tristar fix the issue?” I will guess it is because it was not the fault in the first place. And to the other question about what lines are CPU communication lines I can only guess they are pins D3 and D4 the I2C0 SDA and I2C0 SCL. That guess is based on what I’ve heard in your videos and Chris long videos. At this point I don’t understand what they do or how they work. And I also don’t understand their importance other than if they are not present the device won’t work. Even after staring at the schematics for hours. I can see they go to chestnut and CPU and PMU. I don’t hve enough knowledge and experience yet to speculate how tristar can cause software corruption. Perhaps if tristar is failing it could be sending bad information out that confuses the software.
Based on your answer I believe you are pointing me in the direction that software is the initial problem because of what I said my initial portopow readings were. After this more detailed description (long winded) do you still think I should be looking into software corruption or am I on the right track with wanting to replace PMIC. Everything about what I’ve discovered on this device tells me PMIC has malfunctioned and is not producing the proper output voltages so the next logical step is to replace it and see it the output stabilizes and if it doesn’t I will continue down the rabbit hole. I trust your guidance and hope I’ve given you enough information to understand exactly where I’m at. If you do agree it’s time to replace PMIC is it worth pulling one from a donor and reBalling it? I know it is a difficult job and I run a huge risk of damaging CPU but I am confident I can get it done correctly.
Im going to keep staring at tristar on the schematic and ZXW trying to understand more of how it works before I do anymore work on that board. Thank you for your patience and knowledge!
~Nate