A couple of days ago the headphone jack on my MacBook Pro started to glow an eerie red, and my internal speakers stopped working. Below, I’ve provided a brief explanation of what happened, and the steps I followed to resolve the problem. Apparently it’s a common issue on MBPs.

Clever folks that they are, the Apple folks designed the headphone jack on MBPs to take both standard headphones and digital audio out. It detects which jack is plugged in by the shape (differently-shaped jacks trigger different switches inside the port) and automatically disables the internal speakers when an external audio out device is plugged in.

The problem with this design turns out to be the switches inside the port. Because they’re apparently entirely mechanical, once a switch has been triggered by plugging a device in, it doesn’t release control of audio to the default speakers until it’s been triggered off. And, it turns out, if you unplug your standard headphones at the wrong angle, it’s possible to toggle on the digital out. The digital out is transmitted by an optical signal, hence the red light from the headphone jack.

Fascinatingly, plugging in a different audio device (aka, headphones) overrides the digital out, but only until that device is unplugged, at which point the digital out takes control once again.

So how do you resolve the situation? Manually.

Take a toothpick or another small hard, non-metal object, and trigger the digital-out switch, which is in approximately the 6-7 o’clock position in the port. Metal objects don’t seem to cause any damage, but when I tried using screwdrivers to trigger the switch, the port detected them as headphones and turned off the digital out only when the screwdriver was in the port. A toothpick did the trick within seconds.