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Journey with AllWinner A31 SoC a.k.a. born to die

!!!Allwinner A31!!! Quad-Core 64 Bit ultra cheap SoC for everyone!!
Yeah sound good...but the truth are awful is some case . First of all do you know about the Banana Pi M2?The Banana Pi M1 (A20) was the killer of the first generation Raspberry pi . After the second generation Rpi was released the Bpi m2 was released too . But where is it ?
The Bpi m2 was a huge fail (M3 was changed the M2 very soon) . Nobody care about the A31 SoC linux support . No community,no projects,hack or any interesting thing about this mini ARM board.
And what about the android support? Little bit better but not in all case.
My new target is a A31 SoC based tablet with nice screen and big battery . But something wrong because I can't turn on.And my journey started here.
I took it apart and try to locating the wrong parts.I started on the power rails :
Sorted Main Board
WorkBench
 APX221 is a dedicated Power Management Unit for A31 SoC. You can find a lot of informations  on sunxi linux wiki.
Lets try measuring voltages on the rails!Nothing....
After a while I discovered an i2c bus between the SOC and the PMU. Luckily I have a logic analyser so I can check what is going on the i2c bus. Nothing except 1 packet (not documented)
Sniffed i2c bus

I am big fan of Louis youtube chanel.So what I learned? Locating the power rails and try to find which one is bad. I desoldered the inductors one by one and I tried to powering up the device and I measured the rails voltages again.

After a while I detected one of power rail is shorted to ground. But which component?Not easy because we have a lot. on this board :D
Louis trick again: the shorted component generate a lot of heat if I flowing high current through it.
And this is what I'm  exactly do!I easily located the shorted heat source component (cap) and I removed it from the board:
Removed Caps


As you can see I removed 3 capacitors. I don't really know which one was bad but I think this is a decoupling caps and not so critical parts.
I assembled the power unit because I desoldered a lot component when I troubleshooting the shorting issue. Luckily I soldered every component in a right place. Except the "hard bricked" wifi module :D that I completely fucked up.
Power Domains
And voila! Working again!
Booting UP

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