Can't get secondary display on my iMac

Hi All,

Looking for help as this has got me stumped and Apple technical didn't offer much help over the telephone.

I am trying to connect external display to my iMac 21.5-inch, Mid 2011 2.7GHz Intel Core i5 using MDP (male) to HDMI (male) cable without any luck, I just get the no signal message even upon trying a few different monitors/TVs.

Can't be a faulty cable as I also have a MacBook Pro 13", Early 2011 2.3 GHz Intel Core i5 which works fine with external displays using this same cable and any of the monitors I have tried.

Both iMac and MacBook are running Mountain Lion 10.8.2

Apple offered very little help over the telephone just advising me to take this to a repair centre not the most convenient thing as this is about an hour away!

Any help would be greatly appreciated.

PW