Nearly all set top boxes in Australia only have A/V output on them & Aerial input, the other RF socket is for antenna signal loop. It doesnot put out analoge RF signal. What ive used is an old RF converter out of an old VCR & fed A/V into it & fed the RF out to the old TV that didn't have an AV input.

 

Yeah,I thought so,but was too lazy to dig one out & have a look!