Released on September 19, 2014, this 4.7" screen iPhone is the smaller version of the iPhone 6 Plus.

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c1200 is shorted and PMIC gets hot, needs PMIC replacement?

Hi,

Newbie here, just on my third iPhone repair (microsoldering) and wanted to get your feedback before I take off the PMIC and replace it.

I have a water damaged phone here, upon first inspection and after cleaning it didn't look so bad, but it was dead as a doornail. AMmeter does show 0.5a when I plug the logic board in (new battery, new dock connector, new screen). No sound/image though and the PMIC gets hot real fast.

Measuring caps along VCC main revealed a short to ground on C1200. I replaced that with a known good working one from a donor board, but the short is still there.

Seeing as PMIC gets hot as well, I think it's that that needs replacing or do you gurus have another thing I can try before I go that route ?

Thanks in advance for any tips !

Major update :

Turns out I was looking at the wrong spot the whole time. I thought the PMIC was getting hot, but in reality it was the CPU, a lot of caps near the CPU top row were also shorted out. Measured D1501 diode and only came out on 0.002, so the backlight diode was (one of the culprits), I suspect the backlight IC itself is also faulty. and maybe even the coil. I'll order a set of those and give it one last try.

I didn't think it would be 'just' a backlight issue, since I tried shining with a heavy duty flashlight through the back of the lcd and nothing appeared, but I guess a faulty backlight coil/ic/diode could cause these symptoms ?

This is really a rabbit hole, I don't think I'm a big fan of waterdamage repairs :)

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How long was it in the water and where did the water get most?

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Just dropped in the toilet and the owner grabbed it out rightaway. Fully submerged. Most water was in the top, near the camera connector. Upon doing a second sweep I found a couple of caps that are shorted near the PM8019 IC. And.. I also found a solderball... argh..

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Update : removed the backlight IC, the filter, coil and the backlight diode (since I ordered those to be replaced anyway). The shorts are still here. So no use in replacing the backlight parts. At this point I have no idea anymore where to check for shorts (thinking it might even be a short underneath/in the cpu itself), so I'm giving up on this one.

Better luck next time ;)

In the meantime I've been learning to use PADS. Can any of you pros give me a reason why I should buy the Extended version instead of keep using the free PADS X2.1 viewer ? It can handle the iPhone .pcb-files just fine, and with F5/F6 I can see which components link to which points/components.

Thanks Minho, you'll probably see me around here in the future :)

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There are a lot of caps on VCC_MAIN, you pretty much have to test them all until you find the bad one (if that's the problem).

The PMIC getting hot could be either the PMIC or a (or multiple) shorted line(s). Given that this is a water damaged phone, you really should test all of the voltage lines it drives to see if something downstream is pulling everything else down with it. First check to see if they are shorted and then test actual voltage.

Caps are a likely failure point but an oxidized IC anywhere on the board can also cause a short on VCC_MAIN. Water damaged phones are the toughest to troubleshoot because there could be multiple issues stemming from anywhere on the board. If the water damage was minimal and localized, you could focus your efforts there but otherwise, you will need to start at the beginning.

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Yeah, tested them all, c1200 was the only one that had a short to ground, but since replacing it doesn't fix that short, it might be an IC. And not really localized unfortunately. The owner put the iPhone in rice for a couple of days before calling me.. (yes, I know.. I have told her that was the worst thing she could do, but the rice myth is so popular on facebook, it's hard to get it out of peoples' heads..). Anyway, thanks, I'll go and check every part of the board under the microscope again to find the culprit(s)!

Ah, and to be fair, I replaced the cap (c1200) with a 15 uF one, same voltage though. But I doubt this could have any negative effect, if anything, it would be better.. no ?

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If you change a cap you can use a higher voltage rating but not capacitance. In this particular instance, it may not make a difference because there are 12 X 10uF caps on VCC_MAIN next to the PMIC so adding 5uF only changes the value by <5% which is well within the tolerance of the individual components. If you had changed C1271 from 100nF to lets say 220nF, then you end up changing the entire filtering effect of this bypass capacitor.

Bypass capacitors serve two main purposes. On one hand, they filter out noise on the voltage line. Engineers do lot's of testing to determine at what frequency they occur at and select capacitors to attenuate them. They also serve as a charge reservoir (mainly the bigger ones) in case the IC requires more current than is readily available due to inductance effects.

As a best practice, you should always replace components with the same thing. Generally speaking, the engineers put that value for a reason.

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Thanks Minho, I thought I would always be safe with higher capacitance, I mixed up. Thanks for the correction !

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No worries! You have a tough one in front of you and you will learn a lot. Don't hesitate to come back and comment/question on your progress. Take this opportunity to dig into the schematic and try to understand how the device works, at least the power/charging subsystems. Then learn about backlights and LCD driver circuitry. The more you understand, the easier it will be moving forward to troubleshoot. You don't need an engineering-level of understanding either, you just have to understand how the different components work (especially the passives).

One step at a time...

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If there is a short on VCC_Main wont every cap appear to be shorted?

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Dennis Meerveld will be eternally grateful.
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