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Main monitor flashes blue over and over with 2nd monitor connected

My old 23" cinema display died, and I have a new 24" DGM 1920 x 1200 display with DVI input.

I have a Thunderbolt to DVI male cable which connects from the iMac to the DGM display. It was working fine earlier, but when I woke up from sleep, the login screen flashed blue then login then blue, over and over about once every 3 seconds.

Now if the second monitor is plugged in the main iMac screen flashes blue every 3-4 seconds, over and over.

Any ideas?

iMac (27-inch, Late 2012), OS X Mountain Lion (10.8.3), 1TB Fusion drive, iPhone 5, dual G5

Posted on Jul 8, 2013 11:27 AM

18 replies

Jul 10, 2013 1:15 AM in response to ileob

I also noticed a new effect last night...

When I put the iMac to sleep, both monitors go dark and power drops down low (I have a meter).

But if I unplug my iPhone or a USB device, the iMac wakes up "a bit" although its screen remains "black", but the 2nd monitor comes on "with signal" although it too is "black display". It never falls back asleep; never loses signal. Power usage is up a bit.

Aug 15, 2013 1:29 AM in response to William Donelson

There appear to be at least 20 threads regarding Thunderbolt/DisplayPort problems, so I assume Apple still have not worked out good drivers for the variety of monitors and display resolutions out there.

Generally, turning the external monitor off for 15 seconds seems to work for me, but it's a real hassle...

Very frustrating.

mrbofus

Level 2

257 points

Aug 15, 2013 11:28 PM in response to William Donelson

In my case, it's not an issue of the iMac's screen flashing blue, the iMac's screen just simply randomly turns off. I posted an update in the thread I started [ https://discussions.apple.com/thread/5148923 ] where the issue seems to go away if I don't let my computer go to sleep. If I don't let the iMac go to sleep, then the screen turns off and on as it should.

Main monitor flashes blue over and over with 2nd monitor connected