An old monitor as a second screen

I have an Acer V223HQV monitor I would like to reuse as an external display for my 2012 MacBook Pro (Retina display). I’ve tried using a VGA to DVI adaptor connected to a DVI to Thunderbolt connector but I get no picture. The monitor is detected in System Preferences, but the screen stays blank. Can this work? Jack Maslin

Yes, but not with those cables. The Thunderbolt port on your MacBook Pro can output both analogue and digital video, but when you connect it to the DVI to Thunderbolt adaptor, only the DVI-D portion of the signal is carried, which is digital only.

The VGA to DVI adaptor plugged into that is expecting the DVI-A (analogue) part of the signal, but hears nothing, because those pins aren’t connected.

To fix this, you need a single Mini DisplayPort to VGA adaptor. Mini DisplayPort uses the same connector as Thunderbolt and is backwards compatible with it – you can use a Thunderbolt port to connect any Mini DisplayPort devices.

Apple sells these adapters for £25 but there are clones on Amazon for around £6. Connecting a VGA monitor to a Retina MacBook isn’t going to give you a fantastic picture though. The signal is converted from digital to analogue and back again and simultaneously downscaled from Retina resolution to whatever the monitor supports. Expect small fonts to look blurry.

Apple’s VGA adaptor cable seems overpriced, but there are some active conversion electronics inside and cheaper clones may have cut corners there.