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Old 04-15-2022, 11:06 AM   #1
grimacelord
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Default Apple iPad Pro 3rd Generation 12.9" (A1876) No Power/Charge When Screen Is Connected

Hello.

Working on an Apple iPad Pro 3rd Generation 12.9" that has a weird issue. When the display is NOT connected, the iPad will charge and power on normally. I know it's on because I can hear notification sounds and the charging notification sound.

If I connect a brand new display to the logic board, the iPad will not turn on at all. When I connect the iPad to an 18W wall charger and a USB-C ammeter, I can see that the iPad is attempting to charge, but constantly "resets" after just a moment or two.

For example, when I connect the iPad to the ammeter and 18W charger, the ammeter's display will initially read 5V and about 0.90A. The amps will move up to about 1.2A and then the entire ammeter's display goes blank (as if I unplugged it from the wall) and then the entire process starts over in an endless loop.

When the screen is not connected, the charging behavior appears completely normal for this model which is 8.93V and 1.87A holding steady.

I have experimented with the 4 display connectors to see which connector seems to be the culprit by removing one of the 4 cables at a time and retesting after each one. I found that the LCD connector (labeled Connector 1 in my image below) is the one that seems to be at fault. When I have the ribbon cable attached to that connector, the iPad will not power or charge. When I disconnect that cable, but still leave all 3 of the others, the iPad will power and charge normally (but of course I cannot see anything on the screen because that connector is for the LCD)

I have tried two different displays and they both behave the same way.

I then thought I would see if there are any diode mode readings of the pins for the LCD connector. I found some online, but my readings seem to be quite a bit different than what is shown on the image I found. I have taken measurements and edited a photo to show what my measurements read versus what I found on the logi.wiki website here. My measurements are the ones in red whereas the original photo's measurements are in white. If I haven't put a red measurement next to the white one, it is because I am reading the same. The only red ones is if I am reading a different measurement than what the original photo listed.

Thanks

EDIT: And just as a side note, my measurements more closely resemble the listed measurements on logi.wiki for the iPad Pro 4th gen. ZXW does have part/circuit names listed for the 4th gen, but not the 3rd. It seems like I could likely use the 4th gen boardview to help me with my 3rd gen.
Attached Images
File Type: png MYMEASUREMENTS.png (534.7 KB, 12 views)
File Type: jpg 1200px-iPad_Pro_1293_Overall.jpg (221.3 KB, 9 views)

Last edited by grimacelord; 04-15-2022 at 11:25 AM.. Reason: more info
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Old 04-15-2022, 12:11 PM   #2
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Default Re: Apple iPad Pro 3rd Generation 12.9" (A1876) No Power/Charge When Screen Is Connec

So it looks like Pins 29 and 27 are the I2C data lines for the display connector. I2C_TCON_SCL_1V8_CONN and I2C_TCON_SDA_1V8_CONN, respectively.

Pin 29 (I2C_TCON_SCL_1V8_CONN) connects to C4565 and R4561. The other side of R4561 (I2C5_SCL_1V8) connects to R0760 and BE47 pad underneath the SOC.

Pin 27 (I2C_TCON_SDA_1V8_CONN) connects to C4575 and R4571. The other side of R4571 connects to R0761 and BC47 pad underneath the SOC.

I have measured each resistor in both circuits and all of them are normal. There are no short to ground in either circuit. I am getting 1.8V on both circuits.

Am I looking at a SOC (CPU) problem here?
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Old 04-15-2022, 04:14 PM   #3
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Default Re: Apple iPad Pro 3rd Generation 12.9" (A1876) No Power/Charge When Screen Is Connec

What is the 820-xxxx part # on your logic board ?

Is it possible that the LCDs you are testing are incompatible for your logic board?

What is the part # for the LCD you are testing that causes the logic board to not boot ?
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Old 04-16-2022, 03:25 PM   #4
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Default Re: Apple iPad Pro 3rd Generation 12.9" (A1876) No Power/Charge When Screen Is Connec

Quote:
Originally Posted by mon2 View Post
What is the 820-xxxx part # on your logic board ?
820-01435-A is the logic board part number.

Quote:
Originally Posted by mon2 View Post
Is it possible that the LCDs you are testing are incompatible for your logic board?
No. The iPad Pro 12.9" 3rd Gen shares the same display assembly with an iPad Pro 12.9" 4th Gen. I have purchased 2 different screens from a reputable vendor that I always use for mobile device repair. The only thing that has remained the same is the LCD flex cables you have to swap from the original display to the new one. Perhaps these cables are faulty? I can't really understand how a flex cable like that would cause this particular issue, but I suppose it's an idea to explore. I can order some new flex cables and just see. I don't have any in stock anyway, so it won't hurt to order some.

Quote:
Originally Posted by mon2 View Post
What is the part # for the LCD you are testing that causes the logic board to not boot ?
I have tried an OEM refurbished display (OEM Apple LCD with new digitizer) and I have tried a 3rd party display, so neither one of them carry the original Apple part number. The refurbished one might, but I am sure they've covered up the part number with some tape or something. I will have to check on Monday.
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