My monitor, an old Toshiba 47wlg66, will display what my boards output (whether it's my old Raspberry Pi 1 A or new Raspberry Pi 4 B) only if it was powered first. Otherwise it will remain black..
It is troublesome because I power the 2 at the same time, and the Pi's must be quicker to send their first signal because in that case the screen remains black and I need to power off and on the Pi for the monitor to recognize the signal.
I usually use
dietpi, but have seen this behaviour with Raspbian as well. Is there a way to fix that, or at least some kind of signal I could send from a script that would tell the monitor to start listening as it does whilst booting?