1 iPhone X PP_BATT_VCC short
Hi, this is my first time asking a question here, i'll be grateful if someone could answer some of this questions that i've been searching online for a days and just couldn't find the answer.
To start with, i'm completely new to logic board level repairing after watching dozens of videos on youtube regarding the topic.
I apologize if there are any mistake i made since english is not my first language.
To start this journey out of curiosity, i found this dirt cheap iPhone x (like a few bucks) on my local marketplace that's apparently won't boot. The owner stated that it happened after he attempting to replace the back glass by chiseling it, only noticing that the phone is dead right before assembling the new back glass. The metal housing is absolutely need a full replacement due to scratches and dents. Enough with the backstory, i want to focus more on the logic board
The first thing i tried before hook up my crude power supply setup (which is just 4v 18650 battery and multimeter with current measurement), i notice that the battery rail which is PP_BATT_VCC line is shorted to ground. I tried measure the VDD_MAIN on the J5700 connect and find out the diode reading is around 0.25 (red to ground and black to vddmain), indicating that only battery line is shorted while the VDD_MAIN is... fine? I found online that diode reading for VDD_MAIN is around 355, i wonder if 0.25v is acceptable. I'm planning to check the capacitor near the battery connector first if there a short before attempting anything fancy. My usb microscope, proper power supply and some other stuff is still on its way, so i would like to ask few questions before it arrive.
1. Normally, can you "inject" or plug in 4v directly to VDD_MAIN? I've seen another answer suggesting to "inject" 4v directly to VDD_MAIN to find the bad components, but i don't know much about it.
2. With BATT_VCC is shorted, is it not a good idea to plug in 4v directly to VDD_MAIN, even if the VDD_MAIN itself is not shorted? Maybe it would cause a damage to the ic that is connected to both vdd_main and batt, like U3300?
3. If i have a normal diode reading like the one that i found online, how much can it vary until it's considered too low or too high? With 0.355v for example, is 0.25v too low or it can be considered as normal, or acceptable?
Additionally, maybe some general advice for a complete beginner like myself that is often hard to find on google?
Since this is my very first attempt to fix the board, i'm not expecting i'll be fixing this board and able to successfully boot it, but hopefully it does.
I apologize if this post is getting too long
Thanks!
Update (01/06/24)
For those who are wondering, this is the picture of the capacitor near the battery connector looks like. Sorry for low quality since it literally captured with telescope eye piece. Looks like something happened there
Bu iyi bir soru mu?
@geceiv Can you upload a photo of the multimeter you are using? This will help us with giving you the right multimeter settings.
Adding images to an existing question
Andrew S tarafından
@andrewsawesome Thanks for your response. I'm using the diode mode on my rather old multimeter. Just to clarify that i'm pretty comfortable working with electronic stuff, so feel free to be technical with me :)
Geceiv tarafından
@geceiv Before plugging in power, I would find what's actually shorting the battery line to ground. If you switch your meter into resistance mode, you'll see a certain resistance between the PP_Batt_VCC and ground. If you can find another point you can test them at and you see that the second test point set has lower resistance, you know you're getting closer to where the short is. (Unfortunately, I'm not sure how easy this would be to do, since most of the ICs are BGA).
Just clarifying: you checked the diode both ways and it only let current through one way, correct (regular diodes should test "open" in one direction)?
Andrew S tarafından
@andrewsawesome Thanks for the tip about the resistance getting lower as it nears the short, although it didn't work for me since the resistance is too low and my multimeter simply goes 0.0Ω. It probably work with a meter that can measure up to micro-ohm.
As for the diode, I'm aware that diodes allow current flow only in one direction (unless it hits breakdown voltage) and also the diode curve. Thanks for pointing that out.
I also plan to use rosin to identify the shorted capacitor between batt-gnd first once the 'microscope' arrives.
Until then, I'm seeking a clarification regarding the VDD_MAIN questions as mentioned in the original post. I'm removing the 4th question since i've already figured it out.
Geceiv tarafından
@geceiv I'm not an expert, but I would wait until you get your bench power supply. That way, you can inject voltage at a low current instead of not being able to control the current.
Andrew S tarafından
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