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Hey guys, I don't have a question about a specific phone, but I am wondering what kind of tests you do when a phone won't power on or boot normally.
I look for signs of water damage and corrosion, damaged components or obvious shorts, component failure and such, but what do you do when there are no signs of damage to the board? I'm looking to learn some deeper diagnostic tricks, so please post anything you think could help.
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You pretty much have to go through all of the voltage rails that the PMIC generates. Just because BATT_VCC & MAIN_VCC are ok doesn't mean that all is fine with the board. Chances are you will have an issue with an important rail like PP_CPU, PP_GPU, PP1V8 or PP1V2. Those are created by the PMIC.
I'm not sure if this is the kind of answer you are looking for. If not, please be more specific.
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This is exacly what im looking for. How do you test these? Compare voltages to the schematic? Test for shorts?
The first thing I would do is test for shorts when the logic board is not powered. Be careful because PP_CPU & PP_GPU are low resistance lines, typically in the 20 - 80 Ohm range so they will "beep" in continuity mode on some meters. You need to look at the value and not just rely on the beep. If you see something less than 1 Ohm on these lines than you probably have a short.
If there doesn't appear to be shorts, then I would power the board and measure for voltages. Obviously, PP1V8_SDRAM should measure 1.8V and so one. If the NetName doesn't include the voltage, it is usually written somewhere on the schematic. Do a search on the NetName and cycle through until you find it.
What you're looking for now is lower voltage than expected, which would point to either excessive current draw which forces the voltage regulator (could be an LDO or Buck or Boost regulator) into current limiting mode or it could be a defective regulator that just can't deliver the juice.
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