The red light proves that the converter is powered up, but it doesn't nevessarily prove that the light is being modulated, i.e., containing an audio signal.
I suggest using a process of elimination if you can. For instance, If you have a DVD player with a coaxial output, see if the converter will decode that source. If yes, you know the problem is in the tuner.
By a systematic method of making sure each component in a system works by using substitution, you can narrow down the problem until you are left with the one weak "link".
Larry
www.fineelectricco.com