Please pardon me for the title of the subject. It does not help much, but I could not find a better one.
I am using an ELEGOO screen "shielded" on top of an Arduino Mega. I tried several test programs provided with the Elegoo screen without any problem. Same positive result with simple programs of my own. Everything works fine as long as I use the USB supply from the PC, but the screen stops working as soon as I switch on the normal power supply of the Arduino (I use a 6V battery). The screen then looks dead.
The problem does not look like coming from the power socket because the Arduino Mega alone seems normally alive. The led on the Arduino is on and the 3.3v voltage on pin 22 checks Ok. To make reasonably certain that the Arduino is ok, I tried a few simple programs like the "Blink" and everything works fine.
To make sure I was not dreaming, I tried the same setup with an Arduino Uno (Elegoo screen + Uno) and it works OK with both power supplies.
Thanks to anybody that could help.