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I've been using my Macbook Pro M1 with my (old, but still going strong) Dell U2412M 1920x1200 @ 60 Hz for months and months, over a DVI-HDMI cable (monitor doesn't have HDMI or USB-C, laptop doesn't have DVI).

Yesterday I used another monitor with my Mac, and when I then went back home to connect my monitor as I always do, it was displaying in 1280x720. If showing additional resolutions, the only offered one is 1920x1080, which at least is a decent resolution, but aspect is wrong since 16:9 is applied to a 16:10 surface (the text I'm writing now has really tall and narrow letters for example). It looks absolutely horrible. So I've tried

  • Rebooting
  • Disconnecting, turning off monitor numerous times
  • Factory reset the monitor
  • Verified that the monitor is 1920x1200 @ 60 Hz natively
  • Disabled mirroring, external monitor is main display
  • Installed BetterDisplay and tried to configure a custom resolution, but after reboot it's gone - it refuses to offer 1920x1200.

In the control panel/Displays, the display is listed as "Unknown display", not as Dell U2412M, but honestly I don't know if it did that before as well, or if that's changed too. Option-clicking and using "Detect displays" also does nothing.

I tried connecting my old PC to the monitor (via VGA!), and it immediately went to 1920x1200 and offered numerous other scaled resolutions as well. Everything looks just fine.

Now I'm basically at a dead end. The monitor is old, so I'm leaning towards buying a new one, but it's really frustrating when it seems to work fine with my PC. Could the DVI-HDMI cable breaking cause loss of signalling from the monitor, with these consequences? Or is the monitor simply broken? Or is this a software bug with MacOS 12? I updated from 12.3 to 12.6 this morning as well since that had been postponed for a while, but that didn't help either (I'm not supposed to upgrade to Ventura yet as per corporate policy).

Any ideas whatsoever?

JHH
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    Leave the monitor connected, turn off the monitor and leave off for about 30 seconds. Turn it back on. It should re-dectect it properly. Also you should use an active adapter when converting to HDMI – Allan Jun 28 '23 at 14:58
  • This isn't a full answer, but there are free apps in the App Store that give you a lot of control over your monitor resolution if it's not letting you choose what you want. One example is EasyRes. – Br.Bill Jun 28 '23 at 19:41
  • @Allan Thanks but did not help. – JHH Jun 29 '23 at 06:56
  • @Br.Bill Already tried SwitchResX and BetterDisplays without luck. Possibly I could go all the way to disable SIP and experiment with custom resolutions forced upon the OS, but it seems really weird considering two days ago this worked fine - so either MacOS got screwed up by me using another monitor in between, or the monitor and/or cable broke yesterday. – JHH Jun 29 '23 at 06:58
  • If it just started, then perhaps your PRAM, NVRAM, or SMC need to be reset. It's pretty simple to do and often solves really puzzling issues, especially with hardware. Here's how to do these resets: https://www.macworld.com/article/224955/how-to-reset-a-macs-nvram-pram-and-smc.html – Br.Bill Jul 06 '23 at 18:05

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