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Repair information and guides for the iPhone 6S released by Apple on September 25, 2015. Models: A1688, A1633

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Why won't my phone not turn on?

Hello!

Bought an iPhone 6s of my friend that didn’t start, when you started the phone it was completely black . He said the problem was a bad battery. So I bought a new battery and replaced it, nothing changed. Then I tried to connect to iTunes and it popped up, the phone was in recovery mode (serial was N/A). I tried to restore, the first part worked and the phone showed an apple logo, then turned completely black except the backlight that was still on and the restore failed with unknown error. If I charge the phone it sometimes shows an apple logo (around every half an hour if you hold the power button) and then the same thing happen as if I connect it to iTunes. I tried to heat up the memory chip with a hot air station with no change. Which is the broken part(s) on the logic board? or how can I find out? I have some experience with microsoldering on MacBooks.

Thanks!

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Is this something that occurred suddenly, for no apparent reason or was it following some kind of event like a drop, coming into contact with water or repair attempt?

I would open up the phone and do a good visual inspection to see if anything is out of place or if you find evidence of water or other debris. When dealing with a “dead” phone, there is no "silver-bullet" solution. You have to probe the board to get a better understanding of what is working properly and what is not. It may be a simple solution or a complex one. Sometimes we see a visibly blown cap, replace and all is well. Other times, everything looks perfect yet the phone won't boot.

You have to start at the beginning and check PP_BATT_VCC, PP_VCC_MAIN and PP5V0_USB. I would start by checking to see if those rails are shorted to ground. If one of these rails is shorted to ground, then you will need to identify what is causing the short. It could be a bad decoupling capacitor, conductive debris or defective IC that is directly supplied by those rails.

If they are not shorted, then you can connect, preferably, a known-good battery (or a current limited DC power supply with the appropriate connector adapter) to see what voltage you measure. If the voltage is low or lower than the battery voltage (which you measured before plugging it in ;>), then there could be a short circuit on secondary subsystem that is causing the battery or DCPS to be current-limited.

If you are measuring the proper voltage, then you move onto the PMIC and check the voltage rails it generates. The PMIC generates a lot of voltage rails. They are all important (for obvious reasons) but the ones to check first are as follows:

  • PP_CPU & PP_GPU – These rails supply the CPU & GPU. They are low resistance rails so they may “beep” when you test them on your multimeter in continuity mode. It’s important to look at the reading and not focus just on the beep. You will typically measure something around 20-100 Ohms on these lines.
  • PP1V8_SDRAM & PP1V1_SDRAM – These rails supply the SDRAM (which is sandwiched with the SoC/CPU).
  • PP_SOC & PP_FIXED– These rails supply the rest of the System on a Chip. What we commonly refer to as the CPU is actually a SoC.
  • PP_CPU_SRAM & PP_GPU_SRAM - These rails supply the Static RAM
  • PP0V9_NAND & PP3V0_NAND – These rails supply the NAND chip.
  • PP3V0_Tristar – These rails supply Tristar chip.
  • PP1V2 - Supplies various subsystems including SoC, FCAM/RCAM
  • PP1V1 – Supplies the High Speed Digital Communications via the SoC
  • PP0V8_OWL - OWL subsystem on the SoC
  • PP1V8_ALWAYS – This is an “always-on” voltage rail that is used for the bootstrapping of the device

Once again, you should start with measuring the resistance while unpowered to limit any potential damage of excessive current going through the logic board. If there are shorts, you can connect the battery and look for hotspots (you can use the freeze-spray or IPA method). Do this with caution to avoid additional damage to the logic board.

Get back to us with your findings.

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