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It is possible there is something wrong with the hdmi board in the tv preventing the connection.As a workaround, you can consider using component video (there is a dongle for the xbox for this, it should have came with it.)Component video is a set of three cables, red/green/blue, with rca connections. Since it is video only, you'll need another cable for the audio. Your xbox dongle will have built in stereo audio cables (red/white connectors) as well as a port for an optical digital audio cable. You'll get the best sound quality from the optical cable, and should use that if you can for the dvd player as well.
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