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iPhone 5SE Short VCC MAIN

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  • iPhone 5SE Short VCC MAIN

    This phone came to me with a note that it was liquid damaged. My portpow gave a zero reading and my DCPS will pull maximum amps (2.00) as soon as its connected. These symptoms lead me to believe I have a dead short on VCC_MAIN. I pulled the board out and noticed it is in pristine shape. The outer housing looks brand new and the insides have absolutely no signs of liquid. I removed all the shields except for the one covering CPU (because ZXW doesn't show vcc main under it.) The first place to get hot was Q2300 and because it doesn't have a foot on ground I removed it and bridged V_BATT and VCC_MAIN so I could inject voltage through the battery connector. Yes I did test the line before and after removing Q2300 . My readings are 0.5ohm on VCC_MAIN. I connected DCPS and gave it 4.2v @ 2.00a and felt the board for heat. I used the finger detection method, the face detection method and the lips. I can not feel the board getting hot. it feels slightly warmer on the bottom half of the board. I turned up my amps slowly to 4.5a and still not finding a hot spot. I have used freeze spray an both sides too and do not see it melting anywhere. My thinking now is I have a short under an IC because there is not an obvious hot spot. Is there a common IC that can have this problem? Also are there ones that are more likely to fail in this way that I should pull first? Thanks!

  • #2
    That is a great description of this phone and your thinking. It sounds like you are on the right track, and you are right to stop there and ask "hey has anyone seen this before"

    Yes.

    I have seen a difficult to detect main short on an SE a few times, the time that was most similar to your presentation the short was (surprisingly) a bad vccmain cap that feeds audio ic. I've also see bad main under/within audio ic---both seem to occur in a pristine phone with a history of failure when customer was powering some kind of external speaker.

    I've also seen a vccmain short in SE be a single bad cap near pmic---it had incredibly minor physical discoloration and I was able to find it with close visual exam. Tigris is another possibility, although tristar and tigris are challenging in the SE.

    You could try injecting a voltage into main at some cap on the opposite side of the board to see if heat will make it declare itself. You can also play around with voltage and current---your goal is to create watts. You can never exceed the voltage rating for the line, but you can turn it up or down to create a wattage (current x voltage) that will make heat. In general, I don't allow the current to exceed 2.5A when detecting shorts---some components are sensitive to amperage just as others are sensitive to voltage.

    The best you're going to get here is a combination of razor sharp visual exam, experience, hunch based on history, and whatever hint heat may give you---after that, start guessing!

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    • #3
      Your power injection cable isn't getting hot it is ?, as the power may be dissipating through heat out of the cable if the cable is too thin. That's why I tend to use two old multimeter probes for injection instead as it means the cable itself isn't heating up and also you can just inject power where you want without having to de solder anything or solder on wires. The easiest way is to put a alligator clip on ground then use the probe to inject anywhere you want to, that way it's a one handed process.
      i know before I did this when soldering on a wire, if the wire was too thin what would happen was that the wire would get hot and not the board.

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      • #4
        By the way, I think this is vcc main, but on the SE iv seen this a crazy amount of times now. But there is a capacitor to the right / under, of the rear camera connector that iv seen smoked to oblivion many many many times, I think it's the iPhone se version of the wifi cap on the 6. So probably a good place to start for a short hunt.

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        • #5
          Hi everyone, and thank you Jessa, it took me a few days but I did get this solved. Unfortunately it took way too long but the good news is that if I see something like this again I will have a better chance of solving it sooner. I have learned a lot because of this one. You were right Jessa about the places to look and it turned out to be the little capacitor near PMIC. I used a full can of freeze spray on this and 3 sessions of work. I understand what you mean about creating wattage and once I figured it out I was able to find the bad cap. Next time I will know better. The funniest part of this fiasco was I only quoted the customer 100 dollars because I was confident it would be easy. I removed 3 IC's and several caps including Tigris (a huge nightmare). Then I had to put it all back together. By the time I factor in my time and cost to solve it I will have made like 5 bucks an hour. LOL . I decided to film it just in case anyone else has a similar or same issue and hope this will help them and save them time. Thanks a bunch! https://youtu.be/MsPcZF2zBaE

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          • AaronH
            AaronH commented
            Editing a comment
            Great video! I absolutely HATE working on Tristar/Tigris on the SE. Great work, glad you finally found the dumb capacitor!
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