iPad Mini low voltage on VCC_MAIN

[madf0x]

Hi there, Im a tech working at a repair shop and I've volunteered myself into the position of learning board level repairs. We have an iPad Mini that was sold to us and the plan was to refurb it for resale. Unfortunately while another tech was attempting to replace the LCD he KIA'd it. So now I've taken it as my pet project to practice diagnosing the board and fixing it, at this point the knowledge gain is more valuable than harvesting it for parts. I should note that the board looks super clean, no obviously missing components, no clearly burnt out ones, etc.

I started measuring the power rails because the device wont boot up at all. No image, backlight, no sound, not recognized by itunes. I found PPVCC_MAIN has a voltage reading of 3.66V and according to the schematic it should be at 4.8V. It was recommended to me to find the voltage regulator for the power rail, as a damage there could cause a voltage drop without causing a short to ground.

Now my main problem is Im not really sure how to recognize it when I see it in the schematic. I've seen example circuits but translating that into a real practical schematic has been an issue for me.

I've found a possible candidate , which will be the attached image. It looks fairly similar to descriptions of a dc to dc boost converter which would make sense as the battery outputs at 3.72V and needs to be upped to 4.8V for PPVCC_MAIN. Its also the only spot I've found where I can see the battery power rail PPBAT_VCC touch upon PPVCC_MAIN. In my head this also makes sense cause if the circuit simply failed in a way that the voltage never got boosted then 3.66V would be a sound reading to receive on PPVCC_MAIN. Only problem is I havnt seen any example circuit where the schottkey diode has one end going to ground, so I could be totally offbase in thinking what this segment of the circuit actually does.

So ultimately by questions boil down to: Am I on the right track at all? Is this a boost converter or is it something else? If its something else could what it actually be still be responsible for my voltage drop? If it is potentially responsible for my issues how would I go about testing the components on the board(namely I dont know how to test diodes, n-channel mosfets like this one, or testing inductors)? I found a few resources but they didnt make sense in the context of diagnosing a board.

I greatly appreciate any help and direction provided!

[BlueMidnight]

Re: iPad Mini low voltage on VCC_MAIN

You have to take a few of the voltages listed on Apple schematics with a grain of salt. It is sometimes a maximum design voltage rather than the typical voltage you will see there.

Or to put it more directly... the battery is 3.7V, so you're not going to see more than that on the main power rail. Even when the battery is charging, there still can't be more than 3.7V. Otherwise, there will be all kinds of pretty colors and flames and explosions.

I've never had to work on an iPad logic board, but I can read schematics, so here goes...

The circuit you found is the source of PPVCC_MAIN. However this type of circuit is called a (synchronous) buck converter. You'll definitely want get yourself familiar with those (how they work and how to spot them). The MOSFET Q8104 plays no part in it. Q8104 is there to allow power to be directed into the battery in order to charge it.

PPVCC_MAIN is itself powered from either the battery or PPVBUS_PMU, depending on whether the device is plugged in.

See attached picture.

The second step in troubleshooting a board (the first step is to look for obvious burn marks and damage), is to make sure all the power rails are working. You are already aware of that, so just continue on with what you were doing. We know we have voltage on the main rail, so now it's time to start checking all the other rails that feed off of that one.

If you look on the right side of page 43 of the schematics , you will see a bunch of coils/inductors attached to U8100. You'll want to check that you are getting proper voltages on all of them.

It also helps tremendously to measure every voltage of interest and write it down. You can write it directly on the schematic using a capable PDF reader, or take a screenshot of the page and use an image editor, or even just print it and do it by hand. It makes it so much easier to understand and troubleshoot circuits compared to the method of just writing lists of pin numbers and voltages. Make sure you understand why each voltage is what it is. Research electronics theory as necessary.

Hope that helps.

[madf0x]

Re: iPad Mini low voltage on VCC_MAIN

Cool that just the kind of assumption break I was looking for. I had assumed that since I knew boost converters were a thing that if the schematic said 4.8V that it was being upped to that voltage but yeah now that I think about it if you wanted your main power rail to be 4.8V itd be easier and more cost effective to simply have your power source as 4.8V

And yeah I pretty much stopped checking rails as soon as I thought something was off with PPVCC_MAIN so Ill continue with checking other power rails.


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So I've been going through some of the other power rails and PP5V25_GRAPE has a really low voltage of .23V and I found a missing resistor R1750 and it looks like part of the touch system. Unless there is something Im overlooking I dont think this would cause it to not boot at all, just that the digitizer wouldnt work. Of course I could be wrong.

Next I found that PP6V0_LCM_VBOOST has 0V but is not shorted to ground. I didnt see it listed along with the other power rails on the alias page but I suppose thats a new lesson for me, that not all power rails have an alias and can be spotted from that page alone.

Looking along those lines it seems like L8229 may be burned. I had assumed it was just some mark during my visual passover but after examining it closer while investigating PP6V0 it looks burnt to me. The cap next to it, C8238, also looks a bit unusually darker. They aren't connected save in the sense theyre both connected to U8100 which at this point Im assuming is the PMC. Im not entirely sure what the purpose of C8238 is other than it seems to be for a reference voltage of sorts, no idea what that means but Ill look it up later. At this point Im worried if those two are both bad, that might indicate the PMC itself could be bad. Of course I could just be leaping to conclusions.

Im going through and testing the other power rails and the inductors like you advised, but sadly Im not gunna be able to swap out L8229 anytime soon. The equipment I have here is pretty crap, especially for something right next to a major chip like that. Me and the boss are trying to get the owners to invest in better equipment within the next few months so maybe I can come back to this after getting practice on real equipment. Still this has been a very enlightening process to practice board diagnoses and I appreciate the help you've given so far.