I've seen, on two occasions, where Ubuntu 16.04.5 has completely ruined laptop monitors by never turning them off. These were laptops my Uncle owned, where he left them on all the time.
I'm now experiencing the same issue in Ubuntu 16.04.5 on a laptop with an expensive 4K monitor. Upon timeout, it initially turns off all monitors, but upon screen-lock my laptop's built-in 4K monitor comes back on and stays on forever (yet the two external 4K monitors remain off as desired).
I desperately need a work-around for this bug that Ubuntu deems fixed, but it is not fixed for me and I cannot seem to get them to open it back up. I found this question, which is the same issue I'm having, but even there, they say it is fixed (not so for me).
Here's a work-around that I'm attempting without success:
I can manually turn off all 3 monitor with this command:
/usr/bin/xset dpms force off
And, I've attempted to set this command to run every 10 minutes via this cron job:
*/10 * * * * /usr/bin/xset dpms force off
I first tried setting this cron job by editing the root cron:
sudo crontab -e
Later, I tried doing it via user-level cron:
Neither method had an effect.
Why does this command work great when I type it into the terminal, but not work via cron? How can I make it work via cron?