I've been running Linux Mint 17.3 with 3 1920x1080 monitors (HDMI, DVI, VGA) with no issues for 2 years. I installed Ubuntu 20.04 on an additional SSD yesterday, and the monitor on the DVI is black. It is detected, and Settings > Displays allows me to configure it, and I can move windows to and from it, but the monitor itself remains blank.
Reboot to Mint, and the monitor works again. Reboot to Ubuntu 20.04, and it's blank.
I upgraded 20.04 to the ppa:oibaf/graphics-drivers drivers, but the video to the other 2 monitors is scrambled any time the DVI monitor is enabled, yet the 3rd monitor remains blank - so it's disabled at the moment. The AMD proprietary drivers aren't available yet for 20.04, so I can't check those.
I'm low on options. Any suggestions would be greatly appreciated, as I'd like to stick with Ubuntu for this LTS cycle.
lspci | grep VGA
shows
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Turks XT [Radeon HD 6670/7670]
xrandx
shows
Screen 0: minimum 320 x 200, current 3840 x 1080, maximum 16384 x 16384
HDMI-0 connected primary 1920x1080+1920+0 (normal left inverted right x axis y axis) 510mm x 290mm
1920x1080 60.00*+
1600x1200 60.00
1680x1050 59.88
1280x1024 75.02 60.02
1440x900 74.98 59.90
1280x960 60.00
1280x800 59.91
1152x864 75.00
1024x768 75.03 70.07 60.00
832x624 74.55
800x600 72.19 75.00 60.32 56.25
640x480 75.00 72.81 66.67 59.94
720x400 70.08
DVI-0 connected (normal left inverted right x axis y axis)
1920x1080 59.93 + 60.00
1680x1050 59.88
1280x1024 75.02 60.02
1280x960 60.00
1152x864 75.00
1024x768 75.03 60.00
832x624 74.55
800x600 75.00 60.32 56.25
640x480 75.00 59.94
720x400 70.08
VGA-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 476mm x 268mm
1920x1080 60.00*+
1680x1050 59.95
1600x900 60.00
1280x1024 60.02
1440x900 59.89
1280x800 59.81
1280x720 60.00
1024x768 60.00
800x600 60.32
640x480 59.94
720x400 70.08