Thanks for posting in the forums! I'm sorry to hear you are unable to get a video signal when your television is powered off. It seems as iff you cable box is powering off when you turn your TV off. The Slingbox will always get a video signal unless the cable box is off.
With that being said, I would ensure your cable box is not powering off when you turn off the TV. You can avoid this by manually pressing the power button on your TV, rather then using your remote.
Have a great day!
The Sling Moderation Team