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I have a machine running Ubuntu 20.04 which was connected to a 4K display (Samsung), and is now connected to a different 4K display (Dell). Since rebooting the machine, just before it gets to the login screen, the display goes blank and the display notifies me

No HDMI 1 Cable. The display will go into standby mode

Why would a different display cause this and how do I fix it?

Note : I've just inserted an Ubuntu Live USB device. Afer checking the disks I get the same thing, just a blank screen and the message:

No HDMI 1 Cable. The display will go into standby mode.

Note: As requested, the Dell monitor model is S3221QS

Matthew Layton
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  • I have a Philips HDMI monitor and I recall that there is a button on the back of the monitor which allows the user to set the input. Does your monitor have similar settings? – graham Jun 08 '21 at 11:41
  • @24601 The input on this Dell monitor can be selected similarly through the monitor config. Currently I have three different devices plugged into it; one DP, and two HDMI. It's definitely selected on the correct HDMI port as it displays the BIOS and Ubuntu loading screens. – Matthew Layton Jun 08 '21 at 11:44
  • Just out of curiosity, have you got a lower default resolution on the samsung display? – Irsu85 Jun 11 '21 at 10:24
  • @Irsu85 The card only supports 4K @ 30Hz, which is what loads up on the Samsung display, but I changed it to 1920x1080x60Hz. All modes work on Samsung, none on Dell. – Matthew Layton Jun 11 '21 at 10:29
  • Maybe this is an EDID thingy with hdmi compatibility... https://www.youtube.com/watch?v=0abWMwwCtQw – Irsu85 Jun 11 '21 at 10:32
  • Can you update your question with the results of running xrandr in a terminal. Also please state the exact model of the Dell monitor. – codlord Jun 11 '21 at 14:31
  • @codlord See edit + it also boots with safe graphics. – Matthew Layton Jun 12 '21 at 10:19
  • Did you have a chance of testing what was suggested? – sancho.s ReinstateMonicaCellio Jun 15 '21 at 10:07
  • Please use the wireless info script as in https://askubuntu.com/questions/425155/my-wireless-wifi-connection-does-not-work-what-information-is-needed-to-diagnos @MatthewLayton – Tejas Shetty Jun 17 '21 at 14:30

1 Answers1

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Diagnosis

This problem will require back-and-forth interaction (much in the same way as in forums).

Just a few general questions:

  1. Please post the complete models of your monitors.
  2. Are you using the same HDMI cable to test both monitors?
  3. Did you try connecting both monitors at the same time? If not, please do so. Try exchanging them as primary/secondary, and switching the ports you connect them to. Report what you get.
  4. Do you have Windows in your system, to test the same as you are using now but with Win? Please do so if you can. Report what you get.
  5. "I tried this in safe mode"... Is it this answer? Which of the two alternatives? (It usually helps a lot if you give as precise info as possible).

In addition to the suggestions above, consider a few others, and diagnostic commands. Please copy in your OP each command with the full output you get.

  1. Post the output of

    2.1. xrandr (with one and two monitors connected if that makes a difference in functioning).

    2.2. sudo lshw -C display (with one and two monitors connected if that makes a difference in functioning).

    2.3. uname -a.

    2.4. dpkg -l | grep nvidia-driver.

Possible solutions

In addition to the suggestions above, consider a few others. Please report in your OP what you get.

  1. Try the sequence below, you would need the other monitor connected for this (source, albeit for a somewhat different problem).

    1. Turn off the PC.
    2. Disconnect the HDMI cable.
    3. Turn on the PC (without HDMI).
    4. Log in.
    5. Reconnect the HDMI cable.

    See also other answers in HDMI port not detected on Ubuntu 18.04.

  2. This would be worth trying if everything else fails. Reinstalling with (source)

    $ sudo apt-get install --reinstall gdm3 ubuntu-desktop

    or probably less invasive

    $ sudo dpkg-reconfigure gdm3
sancho.s ReinstateMonicaCellio
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