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The external monitor used to be working before, but now, there's no video being sent. The monitor is sleeping when I connect, even though the mac says it's connected.

I am using a SyncMaster390b as the monitor.

I tried turning it on and off and reconnecting everything, but still no luck.

Any help?

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I'm guessing it's actually a mini-DisplayPort to VGA adapter you're using. Is it a 3rd-party item or genuine Apple? Either way it's quite common for these to be recognised by the host Mac and yet not output anything to the display. The reason for this is quite simple, mini-DisplayPort to DisplayPort/DVI-D/HDMI connections are digital and pass straight through to the connected display, the only difference between them is the form factor of the connector. Mini-DisplayPort to VGA adapters have a digital to analogue converter in the main body, and while the host Mac may be able to communicate with the DAC (as it would if connected to a digital display) it will be unaware of any faults downstream of the DAC. If there is an issue with the DAC (or the post-DAC analogue section) the result is frequently that the Mac thinks a display is connected, and will show mirroring and extended desktop options in Display Preferences and yet output nothing to the display.

Genuine Apple units are more reliable but more expensive. We've many (cheap) 3rd-party units around our campus, and we've had many fail after a few months, giving the same symptoms as you describe. Assuming you've checked the VGA cable is securely connected the other tests you can try are as follows...

Hook the display up to a Windows laptop if you can. Even relatively modern ones have a VGA connection. This will let you test the cable and display function.

If they both check out then you need to either try the adapter on a different Mac, or create an OS X installation on an external disk to make sure it's not your OS X install that's having issues. Having said that, it sounds like your Mac is correctly identifying and communicating with the adapter as you've already stated it thinks a monitor is connected.

A quick look for your model didn't bring up any obvious results but many Samsung's have a DVI connection. If yours does, get a mini-DisplayPort to DVI adapter as they're far more reliable.

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The problem that you are facing is simple to fix. Actually there is no inbuilt Thunderbolt to VGA drive inside a MacBook/Mac mini. So what you should do is install a Mini DisplayPort to VGA Firmware Update v1.0. After installation restart the system, then connect your thunderbolt to VGA connector. It will work perfectly fine.

Here is the link for the driver where you can download it:

https://support.apple.com/kb/DL799?locale=en_US

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Same issue... I have a SyncMaster 2232bw plus. used for about 1 year. one day the video just stopped beeing sent. first thing i did was to buy another mini display to vga adaptor. nothing happened. i have 2 macbook pros, one with mountain lion and a brand new with yosemite, none of them sending signal to the monitor. but when i connect it to my friends imac... it works perfectly. oh, and both macbooks recognize the monitor. I'm trying to find the monitor driver, but there are only windows versions.

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With all the research I have done and how many people are having this same issue (myself included), the problem is either with the Apples operating system or the Macbook Pro itself just do not detecting VGA connected monitors after sleep mode. The ONLY solution is to restart the computer and/or in my case what worked as well is to unplug the VGA to Thunderbolt, plug into my sons Macbook Air, then remove and replug into my Macbook Pro retina. My DVI to HDMI works flawlessly but my second monitor (VGA to Thunderbolt) does not. As another test to prove my theory, I connected my sons second monitor with DVI to Thunderbolt and that too works flawlessly as a second monitor on my set-up...even after sleep mode over and over again.

Solution is to not use VGA for MacBooks! Unless you restart and/or shut down and reboot every time.

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