I just bought a used z3tc still running Android 4, and it can no longer be upgraded directly from the settings. (The final official version for the device was Android 6.)
Do I need to get a copy of the stock Android 6 from Sony before I can proceed with the custom ROM?
I’ve posted a question in XDA forums as well. Waiting for someone there to advise.
Thanks.
Yes, First install latest available stock android version for the device is a requirement !
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Thanks, @piero, but…
I previously tried the links to Flashtool and Xperifirm for Linux. The Flashtool link returns “server can’t be found” in my browsers, and the installation instructions for Xperifirm on Linux required adding invalid/untrusted certificates to my computer, and anyway resulted in an error when I tried to launch it.
Any other options for upgrading the device software/firmware?
I don’t know if this will help but …
I have one version of FlashTool (Flashtool 0.9.33.0) which works on my 'daily-driver` 2017 MacBook Air running MacOS Monterey, and which also ran on earlier versions of MacOS and earlier hardware. I tried a later version (0.9.36.0) but it fails to start in the same environment.
I was never able to get FlashTool working on my Linux development machine (2010 Macbook Pro running Linux Mint).
I don’t think I ever managed to get Experifirm working on either MacOS or Linux. I found the firmware files I needed by serachong the sites linked above. For my Z3 Tablet Compact (WiFi-only version), I found the firmware on AndroidFileHost, and was ablet o flash it using my working copy of FlashTool
Good luck!
@Taurus What is your model number ?
When xperiafirmware leads to nowhere, xperiastockrom is still a good source
I found android 6.0.1 download page (Flash-Tool for windows x64 is included into the .ZIP)
Sony Xperia Z3 Tablet Compact SGP611 Stock ROM Firmware (Flash File)
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Sony Xperia Z3 Tablet Compact SGP612 Stock ROM Firmware (Flash File)
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Sony Xperia Z3 Tablet Compact SGP621 Stock ROM Firmware (Flash File)
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Sony Xperia Z3 Tablet Compact SGP641 Stock ROM Firmware (Flash File)
And the HowTo use Flash-Tool for Sony devices
@petefoth
Oddly enough, I’ve used Xperifirm and Newflasher once before, a few years ago, successfully on this very laptop running Linux Mint. At the time, I needed to flash a different regional firmware to my XZ1 Compact. I seem to recall that it was fairly easy. I’m not sure what changed with Xperifirm (or Mint) since then, and I didn’t retain the software for future use. This was before I had ever installed any custom Android ROM.
I saw the SamSony files on AFH when I was searching around, but I couldn’t tell if they might be what I needed.
SGP621
Yes, I visited both those sites, too, and found this at Xperiastockrom:
But I wasn’t sure if I should install something labeled “Germany_Generic.” (I’m in the U.S.)
… and I don’t have a Windows machine.
But…
In the end, I went to a local phone repair shop, explained the situation, and they agreed to install Sony’s Android 6 ROM for me, using the Emma software, for a reasonable fee. If they don’t have any problems, I should get the upgraded device back in a couple of days. 
Normally, I much prefer the DIY approach, but I just got tired of going around in circles all over the internet, and didn’t want to risk installing a bunch of random software on my computer. If Sony supported more operating systems than Windows, the process would be much simpler and safer.
Anyway, thanks to both of you for the information. I’ll report the results later.
May be some friends, neighnoors…
Bad news/good news:
The shop that was going to do the upgrade to Marshmallow for me was not able to do it. The stock build is no longer available in Sony’s archives, thus they couldn’t use Emma (via Windows) and weren’t experienced with newflasher either, so couldn’t figure out how it works.
But then I learned that pre-Marshmallow Sonys need xflasher instead of newflasher, so I took the device home, found a stock ROM from a developer site, and, using Linux Mint, flashed Marshmallow. It took some trial and error, but I figured it out.
Good news/bad news:
So now I’m finally on stock Android 6.0.1.
But after flashing the boot.img and the PBRP image, I couldn’t boot into recovery, so couldn’t install the custom ROM. Besides that, the tablet wouldn’t even power on, so I had to flash stock again for now. I’ve gone through the process several times, but no luck.
Does anybody have any suggestions?
[Marking this as Solved, since I was able to upgrade stock Android, even if I’m struggling with the custom ROM for now.]