Re: Monitors do not power down when screen blanks
It has been a month an a half and maybe your found your answer. If so what was the ultimate solution? I'm not sure why the other posters went off the rails about differences between OS' and didn't address you issue, but maybe this will help as I have the same issue:
Monitors go blank, but do not power down
Background: I received a donation of a small stack of identical Dell 5040's. I decided to replace three PC of various makes, all running dual monitors, all working before the swap. Each got a fresh install of the latest build of Ubuntu 20.04.2.0 Two of the builds work exactly as expected, but one exhibits the problem you are reporting.
Since I have a small stack of identical units, I swaped out the box but kep the already build HD. The problem was still there. Next, I booted to the USB installation media and selected "Try Ubuntu" and let it sit for 10-15 mintues. It work liked you would expect it to. So I figured something must have gone wrong in the original installation. Give it a fresh installation and ... I still have the problem.
So it's not that hardware, the monitors turn off properly if Ubuntu is ran from the installation USB, but will not turn off properly after a full installation. I also reran all updates. Later today I will receive new HDMI to Displayport cables and swap those out (for an unrelated reason) so maybe that will change something. But again, because the monitors turn off normally when booted from the installation media, this is not likely the cause. And of course it's not the PC because I changed it, and the other two are behaving normally. The only thing left is the monitors themselves. They are a pair of InnoView 24".
Years ago I had a thread going about an odd problem with my Toshiba laptop. Similar to this issue, when the laptop was booted to the installation media, the keyboard and touchpad worked fine, but once the OS was installed, they would stop working until I entered some 'no splash' and mux=1 strings in my grub config file. I have no idea why that worked, but I have always had to do that when installing Ubuntu to that laptop. Perhaps there is some weird combination going on with the InnoView monitors and the Dell 5040. I will try the 'no splash' trick later today and let you know if it changed anything.
A goat leads the lambs to slaughter......
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