Could you check a few other devices? For instance, do you have video game console or DVD player? If you could test your other device on the same port as the STB, it could narrow down the possibilities.
Also, I presume you've tried the STB on more than one port, and also, on more than one connection type. If you haven't, please do so as well.
My Samsung LN32B360C5D television has audio but not video when connected to my cable tv service. When I connect the tv to my computer using the monitor cable, i have video. What could be the problem?