This board looks very similar to, though not identical to, the motherboard for a New 3DS XL, which I have experience working on. So please take what I say with a grain of salt as it may not necessarily translate over to to your device, but it should at least be close.[br]
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That "pop" sound, while commonly attributed to the ribbon cables for the top screen being disconnected or damaged, actually is a power discharge that happens when ***any*** component has a power problem that forces the console into an emergency power off state. It is basically not a diagnostic of anything other than that something is amiss.[br]
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I have had several devices in my shop that demonstrate that pop sound, and all it took to fix it was to disassemble it fully and then put it back together. Even when I found no issues with any connections, that has solved the pop sound issue with 2 different devices in my shop, so I would recommend trying it. I know the 2 ribbon cables that go onto the back corner of the motherboard is a pain and it always is scary like you're going to rip the one that goes sideways and has to twist, so please be careful, but the first thing I would recommend is completely disconnecting the motherboard from everything and letting it rest, inspecting every cable and (if you find nothing amiss) putting it all back together carefully and trying again.[br]
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The bottom screen going black and the device not reading SD cards is suspicious. If I am looking at the board correctly, your MicroSD card reader plugs into the open press connector that is basically centered in photo 4 from your 2023/06/28 update. Is that correct? And the touchscreen connects via the ribbon cable on top in the second photo from the same date?[br]
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Based on no concrete knowledge and nothing but proximity, that Texas Instruments IC (and surrounding components; resistors, caps, etc.) is between those two things that are giving you issues. If you can find the schematics for that chip online, even if not the whole motherboard, I'm pretty sure that cluster of test pads is for testing that chip. Even if you have no diagram of the motherboard, if you know that, for example, 10 lines on that IC are supposed to offer 0.2Ω resistance and you get 0.0 readings on everything, then you likely know that chip is shorted somewhere, somehow. [br]
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This is just a shot in the dark and I don't even know if that is how you would test that chip, but the idea I think has merit and someone better at these things (*cough*OldTurkey*cough*) may be able to take that idea and give you a concrete direction to go with it.[br]
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To possibly go along the route OldTurkey recommended originally, I did a bit of digging and found these wonderful folks who tested and verified some contact points on the motherboard: https://gbatemp.net/threads/n3ds-nand-backup-possible-i-hope.381506/[br]
To be quite honest, it is a bit above my head so I am not sure if it is the right things you need or not. (Make sure to scroll down. This thread has both the XL and nonXL on it, and you have to scroll past the XL to find the nonXL photos.)[br]
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If you absolutely want to buy a new mainboard, this is the only one I could find: https://www.aliexpress.us/item/3256801587761245.html[br]
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