The 'Green Screen' happens when an HDMI handshake fails to negotiate how the Humax and your TV will talk to each other.
A selection of possible things to try are:
Cycle through the Vformat settings using the v-format button on the Humax's remote control.
Make sure the HDMI connectors are making good contact.
Restart your av amp.
Make sure that the TV is switched on, and if possible have the TV looking at the appropriate HDMI socket, and then switch on the Humax. I.e. switch on the TV first then the Humax.
Ensure that you have the most recent software version installed on the Humax.
With a scart cable in place, navigate to the Humax installation menu (default password '0000') and action a system reset. This will not delete your recordings, but any schedule or favourites will need to be re-entered.
Try a different HDMI cable. (A half metre HDMI 1.4 cable is available from Poundliand for one pound. My HD-FOX-T2 connects up a Samsung just fine using one of these.)
Unfortunately there are far too many variations to be able to supply a solution, and you have not given many details. Providing more details would have helped isolate the issue.
E.g.
You do not state whether you have a 9300T, HDR-FOX-T2, HD-FOX-T2, Freesat-HDR, Freesat HD, Youview or freetime.
You do not state which version of software the Humax is using. (You can look this up via the Humax menus but you may need to add a scart cable so that you can actually see the menus!).
You do not state the make and model of your TV and whether its firmware is up to date..
You do not state whether the Humax is connected directly to the TV or if not what is in between e.g. AV amp, e.g. HDMI switcher.
You do not state whether you have always had a green screen when connecting the Humax via HDMI and what if anything you have changed.
You do not state what you have tried so far.
NS, September 2013
