appletv not being recognised

I have three appletvs all the same model, v3

two of them are connecting with my hdmi tv, when I connect the third one I continually receive the message no HDMI signal, I know that the cable is working because the other two are fine

I have also done a complete reset in itunes,

sadly when I try to contact apple there is no way of getting hold of the serial number, because it is printed in black on a rubberised black background in tiny letters, whose brilliant idea was that!

can anyone suggest anything else that I can do?