Released September 25, 2015. Model A1688/A1633. Repair of this device is similar to previous generations, requiring screwdrivers and prying tools. Available as GSM or CDMA / 16, 64, or 128 GB / Silver, Gold, Space Gray, or Rose Gold options.

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How can you effectively identify a component that is causing a short?

I currently own an iPhone 6S , which has a CLEAN logic board (no water damage and no visual physical damage) , and the iPhone does not power on at all. It might be a short in VCC_MAIN, but I have no idea how to identify the shorted component. How can I truly identify the shorted "line" and the shorted component on the logic board that prevents my iPhone from powering on? Any suggestions would be really appreciated. Thank you all for viewing my question! (PS; Basically, I am a novice at mobile phone repairing, and I would like to investigate and solve the problem by myself so that I would gain a bit of experience all in all)

UPDATE: I now know that VCC_MAIN is shorted, and I now seriously need to find an effective and time efficient way to identify the cause of the short. Correct me if I am wrong. Now, I am powering on my DC power supply and adjusting the voltage to almost 4 volts. I then attach a wire to an anode side of a capacitor (which is the rail side of the cap) and attach the probes of the DC power supply to that wire. I then use a freeze spray to spray the whole logic board in order to find an area that heats up. It is usually a cap that is shorted, so I would check if there is any cap that is heating up. If no capacitor heats up, then this would mean that I have an issue with a circuit that gets VCC_MAIN passing through. I should then check which component causes that short, and then try to remove that component of that circuit (which is usually the one that is being fed by VCC_MAIN). That's all what I have learned from my research. Am I doing anything wrong at this point? I need the advice of all of you people in this forum! Please help me here! Thanks a lot in advance.

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Finding a short on VCC_MAIN is the hardest short to find because there are capacitors on that line all over the board. All it takes is one bad capacitor to make all the others look bad. So you can't measure them in situ to find the bad one. Additionally, when you have a clean board, there's not always a visual clue, like heavy corrosion form water damage, to point you in the right direction.

The best bet is to question the owner of the device and ask lots of probing questions about how the phone failed (i.e. things got hot near the FCAM and suddenly the phone died), following what activities (I was hiking, water skiing...). Also ask what kind of chargers they use (could blow the charge circuit) and whether the phone looks scuffed up or not (drops, and physical abuse).

If that doesn't give you any leads, then a thorough visual inspection, preferably with heavy magnification, may give some visual clues. If you suspect a cap, then pull it out and test the VCC_MAIN again to see if it solves the short. If not, put it back and write it down. You can't conceivably do this for every component though. So I would start checking the logical failure points...the Charging circuit (Tristar/TIGRIS), the PMIC and the PMU_RF. If you start seeing shorts in secondary power rails generated by the PMIC or PMU_RF, then you can try to either find a downstream problem or it may be the actual PM chips themselves.

In the end, it's a lot of trial and error and experience. Everyone has their own technique. Just ask questions here and you will get some advice that may help you pinpoint the problem. The YOU can contribute to the site for all those following in your footsteps.

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Thank you Minho for your great reply! I will indeed follow your advice.

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My pleasure!

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Minho, could you please inform me with the names of the PMIC and the PMU_RF in the schematics. That would be really appreciated. I know I ask a lot here, sorry for bothering!

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The main PMIC (for Power Management Integrated Circuit) is U2000. On the schematic it is called the PMU (Power Management Unit). PMU/PMIC are pretty much interchangeable and if you search for solutions on Google, you may need to try both terms to find the info you are looking for.

The designation for PMU_RF is U_PMU_RF. The distinction here is that the PMIC is for the digital (computer) part of the phone and the PMU_RF is for the RF (cellular) part of the phone.

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Thanks again Minho! I have another question for you and everyone else. How do I check the voltage of a specific rail using the schematics and a multimeter? Thanks a lot!

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For a strong short that has no resistance to ground you can find the shorted line by measuring around randomly to find caps that beep --have continuity--to ground on both sides with a multimeter. Once you find the shorted line, use a schematic to identify all the possible components that could be the CAUSE of that line being short to ground--something with a path to ground that could have failed and turned into a wire to ground. Use visual inspection, or heat, to try and find the offending component and remove it.

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Thank you Jessa for all the information! I would just like to ask you a couple of questions. 1- Do I really need a D.C. Power Supply to test for continuity? 2- How could I diagnose both Tristar and Tigris ? 3- I am a newbie reading iPhone schematics. How could I find a "path to ground" using the schematics. Thank you very much for all your efforts in this forum Jessa. Nonetheless, your YouTube channel is really informative! Keep up the good work!

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No, you do not need a DCPS to check for continuity, you would need a Multimeter for that, just use one that has audible...as for checking for continuity with phone on...NEVER! Disconnect battery and power first... as for how to find ground, look for Sim card shield, it is ground...or the little copper color circles around screw holes or screw holes that raise off board. You would have to have a basic understanding of using schematics to be able to measure voltages around Tigris and TriStar the best thing to do is to match your symptoms to those others have had when TriStar or Tigers was the solution until you do get a basic understanding of the schematic. I too am fairly new at this and had an issue learning how to read the schematic, but once you've played with the schematic a little bit and follow these forums and ask questions and you'll get there I'm still working on it it's a slow process but a rewarding one :)

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Thanks a bunch Michael! Now at least I know how to begin analyzing the true problem.

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The iPhone now DOES NOT build up ANY heat ANYWHERE after removing the "possible" shorted capacitors. Is that normal? Still same symptoms. I feel like I bricked the phone right now. Could it just be a tristar problem? What "should" happen if the iPhone has a faulty Tristar IC and I plug in a charged good known battery? Any ideas?

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