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Bootloop, no USB communication

Hi,

So I have an iPhone 7 boot looping (boots, shows apple logo for about 40 seconds, apple logo disappears for a second and repeats this process until battery is drained.

Charging the dead battery at around 5.2V, 0.6A. This value stays the same while boot looping until at some point it discontinues charging until the battery is drained and the process starts anew.

I tried DFU and restore mode, both unable to connect to iTunes while other phones do. Tried connecting a different dock, battery, screen, disconnecting the front flex to no avail. It must be the logic board being faulty. But where?

I am just starting out microsoldering. The phone is mine and has no sensitive data on it. So I really do not care if the phone will be dead when learning (as a matter of fact, I do, because that would mean I fixed the phone, but hopefully you catch my drift). I have not done any visual inspection or measurements yet.

Hope you could give me some pointers as to where I should be looking at. Could this be tristar although the phone charges up to a certain point?

I have water damage red spot at the logic board, not on the screen. No rust or corrosion of any kind though, the board looks brand new.

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I should add: after around an hour of boot looping there are no hot spots on the back of the phone or when touching parts of the logic board.

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Hi Bart,

Just to confirmed, where you able to place it and recoery/dfu and the PC would not detect it, or where you not able to place it in those modes at all? I would just like to clarify that before talking about motherboard repairs

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I am able to put it in recovery mode, iTunes will not detect it. I tried different lightning cables. DFU mode: I’m not sure, I tried a few times, but because screen stays black and I could not connect iTunes I assumed the same problem. On other phones I can activate DFU without too much of a hassle.

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@diddledee If you need help you will need to supply the phones history and the equipment you have on hand for the repair.

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History (what I’ve been told) is that the phone started boot looping after an update. For what I’ve seen is that the phone was opened before. Pentalobes were missing, screen went off easily, it wasn’t as stuck like a virgin phone. The protective stickers don’t look like they have been tampered with. Original screen, original battery, original everything. But that is about it. I have a professional microscope, hot air station, soldering station, preheater stone, iFixit repair business toolkit. I am waiting for my DT880 to arrive. I don’t have a DC power supply.

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I suggest you invest in a good multimeter and DCPS. The DT-880 is very limited on its uses as sometimes you have to inject voltage into the board to find a shorted component.

You will also need to purchase some needle point probes and an original power squid or an iPower pro. I prefer using the squids as they show true low amperage.

The first rule in board repair is: To completely rule out parts. In the newer phones this will mean putting the logic board into a known good housing. With the older boards you can remove the board from its housing and connect known good screen, battery and dock connector. That is what I would do with the 7 series.

The second rule is: Do not create variables with your repair. In other words do not start removing components from the board until you are sure they are faulty.

For this repair I would rule out a prior audio IC repair as this can damage the baseband CPU and produce these symptoms. If no prior repair has been done you will need to diode some lines and compare to another known good board or ZXW.

·         PP_VDD_MAIN

·         PP_BATT_VCC

·         PP3V0_NAND

·         PP0V9_NAND

·         PP1V8

·         PP1V8_SDRAM

If all those lines are good I would consider replacing Tristar first as your phone should be recognised in DFU mode by iTunes if all the above circuits are good. If you had a Tristar tester it would be the first thing you would use. The next component I would look at would be Tigris.

Remember to test the phone every time after replacing a component.

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Good advice Chris.

That's pretty much what I wanted to say to.

To add on that,

Iphone 7/7plus have many, many issues with th Audio IC. The pads on the motherboard tends to get thorn (kinda like the touch disease of the 6 plus). Audio IC issues sometimes causes the phone to bootloop for up to 5 min before turning on.

There has been alot of problems with one of the power IC for the reception that tends to develope an internal short, and think that causes the phone to bootloop as well. I never worked on a board with that issue, but I heard of others that have seen it commonly enough.

Thats two area maybe worth investigating too, but I would start with what Chris said

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Thank you for your very detailed response! I will reply more in detail later. Yesterday I started dioding some lines and got really awkward results. So I decided to get to some solid ground and test the display connector to confirm I tested properly. The display connector seems way off with what ZXW tells me to the point that I find it strange to get any image at all. It could be touch that’s off, I didn’t check yet. I was never able to test touch or home button due to boot looping. When turning the phone on I receive proper image: apple logo on boot and connect to itunes when in recovery mode. So, next, I decided to pick the front facing connector. Again to get to solid ground and to confirm I’m testing properly. Front facing connector is reading within its ranges. Didn’t really get to the lines you mentioned because I was kind of flabbergasted by the readings. I will post a detailed response with my readings later!

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@diddledee Remember red probe on ground. ZXW is always out on diode readings but what you are looking for will be a shorted or open line.

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@imicrosoldering I know about the red probe on ground ;), but didn’t know zxw readings are always off. Basically what you’re saying is to keep a separate list of readings measured on a known good phone and use those?

Shorted reading is always very close to 0 or 0?

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@diddledee ZXW diode readings are good if you need to know if it is ground or open but yes it is always a good idea to get them from a known good board. Even same model boards can be different as long as its close enough it is usually good enough.

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Hello Bart!

I can see you are having a logic board problem. It might be that one of the logic board’s components have failed/need replacing. It might be with a component. Have you tested the logic board from a donor phone? Soldering would damage it if it has failed. Try and see if you can switch boards.

Regards,
RivRob

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Do you mean testing the phone with a known good logic board to rule out a failure on any of the flexes? I did disconnect all the flexes except the on/off button and replaced the battery and screen with known good ones multiple times with no result.

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Yes, it means you need to test the logic board by getting a test one.

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Bart will be eternally grateful.
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