Digital output not activated

[eetu]

I'm having a hard time with this problem. I have an optical cable to plug the iMac with a a/v receiver. I plug the cable but the iMac won't recognize it and in sound preferences it will show that it's headphones and no sound is coming. Same cable and same setup works perfectly with the Macbook. As soon as I plug the cable to it, it shows that it is digital output in the sound preferences.

Any ideas about how to solve this problem?

Thank you in advance.