Hi Robin,
The reason you are encountering this problem is that you are feeding the units an HD signal but composite analog video is only SD and so can only display NTSC or PAL video. Both of these converters will be able to convert your HDMI or HD-SDI signal to component analog HD video but they don’t have down-converters so nothing will output on the composite output.
I do have some good news for you. In today’s press release Blackmagic Design Announces Major Mini Converter Software Update, we have announced a free software update for the Mini Converter SDI to Analog model as follows.
In Converter Utility 1.0, an update is included for the Mini Converter SDI to Analog model that adds a new high quality hardware down converter. This down converter allows HD to be converted to SD in letterbox, anamorphic 16:9 and center cut 4:3 modes. Analog component video output can be set to directly output HD or, by selecting the bypass switch, will down convert to SD component output. If an HD-SDI input is ever connected to Mini Converter SDI to Analog when the analog output is set to S-Video or composite, the HD down converter is automatically enabled so a picture can always be seen.
Converter Utility 1.0 software will be available free of charge in May 2009 from the Blackmagic Design web site at https://www.blackmagic-design.com/support.
So please keep a look out for the software update in May which is not far away!
Regards,
Luke Maslen
Blackmagic Design