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iPhone 6s U4500 ic problem? Or worse?

Hi! Whoever you are your already much appreciated for reading and attempting to help me out here.

So I have a water damaged iPhone 6s. i cleaned it with 90% isopropyl alcohol after removing the shields and replaced the screen as well as the battery. No screen response. I can feel some mild heat (meaning normal) when I place my finger on top of the connected display terminals as well as the connected battery terminal, when I press the power button or plug it in. I don't know if that's helpful but to me that means that there is power flowing through the board and to the display. I "charged" it for a while and then swapped the battery to another iPhone 6s and their was no improved charge on it. I replaced the lightening cable to see if that was damaged by the water. No change. So clearly this is a U4500 charge ic problem but is it more than that? Because even when a fully charged battery is connected to the phone there is no display response. This has already been a pain in the ass and I don't want to Solder a new U4500 ic to the thing only to find out it's still dead. Basically can a messed up U4500 ic kill charging and display capabilities? Or just charging capabilities. I know it's a logic ic as well but that's where my deeper understanding ends.

Thanks so much

Ryan

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Certainly, charging issues are related to the Tristar IC (U4500) and certain non-booting issues as well. However, rather than just replacing IC's for no apparent reason, I would suggest you do some troubleshooting first. Every time you start removing IC's, you always run the risk of doing some collateral damage.

Check some of your main power rails first, like PP_VCC_MAIN, PP_BATT_VCC & PP5V0_USB (when plugged in). Then check the PMIC and the dozen or so power rails it generates. If you find shorts, then look for bad components. Check the backlight driver (U4020) and related circuitry.

The problem with water damage is that there could be multiple problems to resolve. You could get lucky and solve it all with a Tristar replacement. But if you are going to do more of these types of repairs; and being able to remove shields shows you have tools and skills, then spend some time looking at the circuit (the iPhone 6S schematic is available in the wild) to understand what is going on.

It'll be fun!

EDIT

Here is a snapshot of the Tigris Chip (U2300). It is responsible for generating PP_VCC_MAIN. By default, the battery (PP_BATT_VCC) supplies this. If the Lightning port is connected, then Q2300 is "opens" and PP5V0_USB is used to generate PP_VCC_MAIN via the built-in Buck converter.

Assuming those rails are okay, then look at Tristar. The problem with Tristar is that there is no easy way to see if it's working unless you use a scope. Maybe @cyberzero has some ideas here. I'd love to hear them.

Otherwise, you need to start looking at the PMIC voltages. Rey's trick will help you do it "blind". Once (or if) you find something, the look at the PMIC (U2000) .

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@refectio You and I have similar concepts of fun! Hah I like it. Thank you for the advice. Yeah water damage seems to be a roll of the dice as there are so many factors that could contribute to which componets get damage. (Plus phone owners likely do some "engineering" of their own in an attempt to get it up and running again...) I've looked at the logic board schematic for other phones before I just have a hard time understanding it all. My real shortcoming is understanding the dos and dont'ts of cont. testing. Do you have any realiable reasources you could suggest to me that would give me some guidance? Google is typically shallow when attempting to find information on this level of detail.

Thanks again!

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@refectio what would you think of posting some PCB images that will show the rails and the components connected with each? You think that might come in handy for some of the questions?

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minho is rigth, i have one solution for water damage to detect the shortage faster. first you need power supply and set the voltage to 2.8 then solder the positive wire in positive area in logic board and nigative wire in ground area in logic board then use your finger to touch the logic board and you can feel where is the parts getting hot. if you feel the parts getting hot that is the shorted one. hope it helps

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@cyberzero Just like that? Interesting. On the opposite sides of the board? Seems like you couldn't exactly depend on proper voltage distribution throughout the board. For example wouldn't certain components recieve more voltage than others causing them to heat up? Though I am assuming this solution has worked for you?

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yes it is working, the minimun voltage of battery is 3.7 and i am using power supply by setting on 2.8. lower than battery voltage. i am repairing for almost 7 years in mobile level 3 chips level.

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Well, first check for burnt components, see if the is cont. on capacitors, etc. Now, after placing burnt components, I would replace the power IC, if still won't work, then I don't know what the heII is wrong with it.

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Thank you I will give that a shot!

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