Intermittent Mini Displayport - VGA signal drop-out

[Joe Jackson]

I have a very annoying problem when trying to connect a VGA via the Mini Displayport of my MacBook Air 13 (Mid 2012).
The monitor usually shows the appropriate secondary screen for a variable length of time, between a split-second to hours. It occasionally appears that quickly pulling a window onto the secondary screen makes it stay on. If I just plug in the cable into the port, the VGA monitor just lights up a second or so then goes into "No signal" mode. I'm pretty sure that cable is OK, as it gives a perfect picture once things are working. There is also no dodgy connection - the picture doesn't change if I slightly manipulate the plug at the Mini Display port.
It seems that for some reason, the Macbook switches off the signal to the port. It briefly (sometimes for longer) comes back on when I unplug and re-plug the cable at the port. Is there a way to "force" the signal to stay on?

 

[bobtomay]

I would be willing to place a wager the issue is with the adapter.

 

[Joe Jackson]

Mini displayport - VGA no signal

Thank you for the initial feedback. I cannot quite see how it can be the cable. It is a passive adapter, and electrical connection seems to be solid. If I shake each end while it's working ok, there is no problem.
It seems that through some evil magic, the Mac OS thinks that I don't want to have my signal output anymore and switches off. There has been some definite sign that keeping activity on the secondary screen, like moving the mouse around in it like wild, can help keep it on, but not always.

 

[bobtomay]

Adapters just seem to fail for no reason. Many of them begin with needing to hold your mouth just right in order for them to work.

The second thing I would suspect would be the monitor.
What happens when it's not working if you turn off the monitor and turn it back on?
If it works in that scenario, then I would guess it's a pretty old monitor and one that should be turned on after the connection is made for a reliable connection.

Based only on experience, I have no reason to suspect the MBA until ruling out the adapter / cable / monitor.
Only way to do that is by using a different adapter / cable / monitor.

Also, a mdp to VGA adapter will not be passive - it will be active (or should be) - that is it will have a DAC in it that does the conversion from digital to analog. When that DAC starts going, directionality (e.g. moving the cable around) will not have any effect on getting the signal to work/not work like we are use to seeing related to an open in the cable. The Apple mdp to VGA adapter is most definitely active as are most others I've seen.

 

[Joe Jackson]

Thanks. I had a look at the original Apple MDP-VGA adapter, which has a "box" at the side of the VGA port, so that must contain the DAC. I'm pretty convinced though that my cable has no electronics - there doesn't seem to be anything at both ends apart from the plugs.
I suspect it might be that my VGA is a bit old, although when it switches into No-Signal mode, the Macbook definitely senses that and flickers between blue screen and single-display mode.
I have much less trouble connecting the Macbook through MDP to my LCD TV. So probably just get a new monitor for my Macbook …?