Chosen Solution
iPhone 5s, don’t know the history as a bought a few to do some practice with. Phone had a VCC_MAIN short, C263 cap was replaced as it was pretty much destroyed. It removed the hard short on VCC_MAIN but still suspect something is still shorted as it has a diode reading of .263. After replacing the bad VCC_MAIN cap, the phone tries to boot, the DC power supply seems to show a booting phone. The phone will reboot when it hits about .750 to .800 ma. (I do get an apple logo as well ) Once it hits the .750ma range it seem to shutdown. After probing around, I was able to find PP1V8_SDRAM was shorted as well.
All the caps seem fine, so I started to remove the caps I could find on that rail to see if I could remove the short. There was only about 5 or 6 caps on that rail and I ended up removing them all, the short is still there. So, I am thinking if VCC_MAIN diode reading is low and PP1V8_SDRAM has a short, I looked at which IC’s that has both lines running into it. U21 (Audio Codec) U7 (PMIC) I ruled out tristar because I don’t see VCC_MAIN going into it but PP1V8_SDRAM line do. I haven’t removed any chips as I want to be sure I haven’t order looked something. So, since I am learning, I am wondering what am I missing and or what step have I missed in testing.
Thanks!
Your VCC_MAIN diode reading doesn’t seem out of line to me; I wouldn’t worry about it for now. You need to figure out what is shorting PP1V8 and then check the rest of the PMIC power rails to see if anything else is shorted. PP1V8_SDRAM is generated by the PMIC and goes to the Tristar IC, Audio IC, Backlight IC and obvisouly the SDRAM. It also powers another minor IC and goes to the dock connector as well. So something in that chain is causing your issue. You could try to use freeze spray to see what heats up first, that might give you some clues. Otherwise, you have to start pulling components. Knowing the history helps in determining where to start but you don’t have this luxury :>). The other thing to do is to check the main power rails that the PMIC generates. By this I mean the ones that power the “compute” system; so the CPU/GPU/SOC/NAND/SDRAM lines as well as critical lines like ALWAYS, TRISTAR etc. This will give you a good idea if there is major damage downstream or not. Everyone develops their own troubleshooting style, take notes of your readings so that you can compare over multiple devices (and remember) and having access to a known-good device helps for obscure problems.
