I'm back here on the forum to report something that happened to me: I got at the stage of having the tubes installed on the board (4 tubes) and I wanted to connect the RTC module as next step. So far the Arduino was in "test pattern" mode and LEDs and tubes were working as expected.
Lacking of female dupont connectors, I soldered the DS3231 directly to the 4 pins for the I2C bus (top left of the board). As soon as I connected the power to the board (12V in the PCB connector) I saw that for a second everything was on then everything went off and some smoke came out from below the PCB. I unplugged the power immediately and I started looking for burn stains. The PCB looks alright, hopefully I did not burn anything there. The Arduno as well but I see that the 7805 voltage regulator is outputting 8V instead of 5V so I guess that is the component that went to heaven.
With this post I just wanted to double check if I did something stupid, Has it happened already to anybody?
After searching around, I believe that the issue might be unrelated to the fact that it happened just after I connected the RTC module. I was wrong, the Arduino clone I am using uses the AMS1117 voltage regulator. I believe it just failed by itself.
I am going to try with a 9V power supply and a new Arduino board.