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

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Dead iPhone 6 tried everything to repair suspect CPU

I have bought a broken iphone 6 from eBay to learn micro soldering and repairs. It is all going well but this device came to me and has been worked on before. Many of the capacitors have been ripped off the board, mainly on VCC_MAIN.

I found the main fault which was a shorted PP1V8_SDRAM, the short was caused by a faulty PMIC which has been replaced. When connected to a bench PSU the phone at power on draws around 45mA and does not boot. I have checked the following and they're all OK.

VCC_MAIN

PP_CPU 0.775V-1.0V 

PP_VAR_SOC 0.9V-0.95V

PP_GPU 0.9V-0.95V 

PP1V8_SDRAM 1.8V CPU

PP1V2_SDRAM 1.2V CPU 

PP0V95_FIXED_SOC 0.95V CPU

PP1V8 1.8V CPU 

PP1V2 1.2V 

PP1V8_GRAPE 1.8V

RESET_1V8_L

PP3V3-USB——A8 CPU 

PP1V8_VA_L9_L67——U0900、U1601

PP3V0_TRISTAR——U1700 USB

PP3V0_IMU——U1901

PP3V0_NAND——U0604

PP3V0_PROX_ALS——J111

PP1V0——A8 CPU 

PP3V0_PROX_IRLED——J111

PP1V8_ALWAYS——A8 CPU

PP3V0_MESA——U2100

Does anyone else have any ideas? Not sure what the next signal should be to the CPU to boot? I think it may be a dead or broken CPU :-( Do I need to replace all of the missing CAPs.... no long screw damage either.....everything else looks good.

Any help very much appreciated for someone learning.... loving the challenging though.....

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You should also check the outputs from the baseband PMU (U_PMICRF). If something is wrong there, the phone will not boot.

If too many caps are missing on VCC_MAIN, it could impede the boot process but there's no clear cut-off of how many (or which ones) you need. The bypass caps filter the line from noise caused by the internal "switching" but they also act as a reservoir of "juice" when an IC has a rapid need for more current. The inherent inductance of the copper traces supplying the power actually works against a rapid supply of current. That's why the caps are placed as close as possible to the IC's power pins as possible.

Was this phone water damaged? If it was, then you could have some issues underneath certain IC's. If the problem is CPU related, then it's pretty much done. Reballing a CPU is not for the faint hearted and has a low success rate. Take this opportunity to learn your way around the schematic and try again on another phone. What you learn there may eventually help you find a solution for this one.

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Thanks Minho, this is really useful stuff I will remeber to check these lines too. The phone was not water damaged. Sadly I was preparing the pads where the caps were ripped off all along the PMIC so that I could replace them, that's all.

Since then I now have another short of PP_1V8 SDRAM and this time its not the PMIC. Taken a few caps off on this line but suspect CPU. I will though put it in a draw and come back to it later perhaps as you have suggested.

I do have another iphone 6 that is water damaged and I have found a number of faults already (all around the speaker amp).. Badly burnt out PP_BATT_VCC with bad and shorted caps replaced. Phone draws current with DC PSU connected, so I replaced VIBE motor IC, now no current but when I switch it on it draws 70mA and then resets continually looping every few seconds 0 to 70mA. Check the same voltages and they're all good (well for the couple of seconds they are up)..... any idea on this one. I will check the PMU rails now....

Thanks once again

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