Also tried a not with tapping with different timings, but nothing appears on the screen.
If I press the power button alone for a short time nothing happens, if I press it longer the tablet vibrates every ~3 seconds. Tapping or pressing Volume-Down does not change anything. I also re-charged the battery during fastboot to rule out an emptied battery.
Only fastboot mode works, but it has no debug facility (e.g. console or so).
Iāve tried many things, but it keeps coming back in the 2.2 update. External Storage keeps stopping (and giving on screen failure messages that canāt be stopped). So I (unfortunately) needed to go back to Peteās last/latest build from July 2023 (something). I hope @ronnz98 will dive into this. Thanks!
I happened to see this while looking for something else:
https://xdaforums.com/t/sgp611-dead.3732742/post-75152796
Maybe it can help you get your tablet operational again.
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Thank you for the hint!
Iāll try this and report here, but may take some time because I do not have any windows PC here which is needed for the companion software.
Note the simpler solution that worked for someone, that was linked from that thread: https://xdaforums.com/t/discontinued-rom-z3tc-aicp-android-ice-cold-project-o8-1-oreo-for-sgp621-sgp611.3731197/page-6#post-75360150
(Well, maybe simpler. It may recover just from holding the yellow button for a little while.)
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I have the same problem with my z3tc (SGP621) now. Canāt power on, canāt boot into recovery. Fastboot works, though. I was able to fix it by reflashing a stock ROM a couple of times, but now Iām even having problems with that. Currently trying again.
If youāre talking about flashing a stock ROM, be aware that Sony has removed the ROM for the z3tc and z3tcw and some other older devices, so you wonāt find it there. Check Android File Hosts instead, or elsewhere on the internet.
You donāt need Windows to install stock ROM, anyway. The following general procedure worked for me, but obviously, this is at your own risk:
Either manually download the stock Android, or use Xperifirm to get it from Sony (if they still provide itā¦ and they donāt for this device). Use either xflasher (for Z-series devices) or newflasher (for XZ-series devices), both from the same developer. See this and this. The instructions are the same for both xflasher and newflasher. [EDIT: Note that the 2nd link (an individualās blog) says to first delete all .ta files, but the 1st link (the developer of this application) says deleting files is not needed for a clean flash of the stock ROM.
Tip: Even if you think itās not working, let it run to completion. The terminal will close itself after about 10 minutes, when done. After disconnecting and powering on, it may seem like the device is stuck, but itās just preparing the OS. Let it finish. (Although, I actually did the button-combo reset once or twice at that point, and that worked out fine, too.)
Now to see if I can unbrick mine againā¦ ! [EDIT: It wasnāt bricked, just my own error. Still canāt get into flash mode yet.]
@MaxO , could you elaborate on that? Why and how. Maybe it will help me with my problem. (Canāt boot into recovery or into system, requiring me to flash stock again in order to get device working.)
This is the SGP621 / z3tc LTE.
I cannot really tell you why as I am not really understanding much of all this, I am just a lucky amateur user who spent some evenings on thisā¦ 
You know where you find Petefothās builds? pf-custom-roms - Browse /z3tc at SourceForge.net
Thinking back I guess I have done something wrong flashing it with /e/ first and the did it right when I flashed the LOS 4microG, but even that is a guess. Sometimes I tried changing the drivers in the device manager and that has to be done separately in adb and fastboot mode, but you probably know more about this then me I guess.
I personally have had more success with OF recover then PB, but it shouldnt be a problem eitherā¦
Sorry, but I think you need somebody who really understand that stuff 
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Ahā¦ OK. You got farther in the beginning than I have so far.
Every time I flash Peteās PBRP recovery image (to FOTAKernel) and the boot .img from the build, the device locks up, wonāt respond, and then I have to reflash stock (6.0.1) to get it back. Iāve tried several different things, based on XDAforums help, including OF-recovery, too.
Doubtful! 
Thanks, anyway!
I never had to do something like you say.
I āiustā flash the recovery via fastboot with āfastboot flash boot recovery.imgā
And then sideload via adb with āadb sideload image.zipā
Did you try it like this already?
Do you mean fastboot flash recovery recovery.img?
Yes, Iāve tried that instead of, or in addition to, FOTAKernel, but with flashboot flash boot boot.img, according to the recommended instructions. Maybe I should try without boot.img.
My problem is SOLVED after following this guide to (re)flash stock Android: https://xdaforums.com/t/androxyde-flashtool-09-23-1-linux-32-bit-for-use-of-fsc-script-also-adb-and-fastboot.3765076/#post-75930721
After using that guide instead of the other tools and tutorials I had tried, I was able to progress to the custom ROM installation, which succeeded. 
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There is a build of LineageOS for microG available at https://sourceforge.net/projects/pf-custom-roms/files/z3tc/
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