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

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Bad tristar chip (U1700)?

PP3V0_TRISTAR & PP1V8_SDRAM are having 0 volts and no short to ground. Does this mean the tristar chip is is bad? I already checked the main rails off course. They don't have a short but the voltage was too low (0,5 volts on VCC_MAIN)

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@ivar3000 did you ever get it figured out?

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@techwarrior Yep. Had to do with a short on one of the lines of the pmic chip (ldo10 & ldo12) which apparently causes it to not be able to charge the phone or do anything (I think that is because the pmic chip supply's power to chips like tristar). I'm pretty sure it's a chip which is connected to both of these lines but i'm not sure if it's the cpu or the pmic.

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You don't say much on the history of this device and why it is being repaired. Logically speaking, if PP3V0_TRISTAR is giving you 0V and you don't measure a short on that rail, then it is more likely to be a problem with the PMIC (U1202). The same applies to PP1V8_SDRAM.

However keep in mind that the PMIC may be current limiting the output because of shorts on the other rails. Ideally, you want to test the main rails it produces before replacing the PMIC. I would double check my measurements and in the case of PP1V8_SDRAM, I would even consider removing L1216 to "break" the rail. That way I could measure on both sides to see if the problem is upstream (PMIC) or downstream (Tristar or Mesa Boost).

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@refectio

First of all, thanks for the response. Let me explain myself a bit better. So I started microsoldering / fixing dead phones not long ago because i'm going to study electronics in september and wanted some more experience in this subject. I recently bought a few dead iPhone's to fix them. Now I came to a point where I have 3 iPhones with a short on pp_ldo10 and pp_ldo12. First I thought it supposed to be shorted to ground or have a very low resistance or something till I started measuring on another iPhone and saw that it doesn't suppose to have a low resistance.

My (new) question is: should i replace the pmic (pm8019) or the cpu (mdm9625)? And is it even possible to replace the cpu chip and make the phone work again? On the schematic I saw both the lines (ldo10 and ldo12) go to these chips.

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That's awesome that you will be studying in electronics!

What resistance are you measuring on PP_LDO10 & 12? If it's truly short, then that's a good catch because this is on the Baseband side of the phone, not the traditional non-RF side. Just so you know, the MDM9625 is the Baseband CPU not the main CPU and the PM8019 is the baseband PMIC not the main PMIC.

Because you bought dead phones, they are probably massively dead and likely not fixable. Still, this is great experience in following the schematics and properly probing the board.

Let me know what you measure on those two rails.

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Yea really looking forward to use the things I learn there in fixing stuff like this.

Thought they were 0.4 - 2 Ohms. Will get back to it tomorrow for the precise numbers.

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All the iPhone's have 0,5-1 Ohms on LDO10 and 12 except one iPhone. That iPhone has 2 Ohms on LDO12. Btw I don't think they're massively dead because nobody opened them before

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Is it maybe a good idea to reflow or reball the basband cpu and pmic? If not, why not?

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