PC to TV via RCA video cable problem

I have a Mitsubishi 60" 1080P DLP that works fine except the HDMI connectors don't work. (internal board is shot and can't be replaced.) Picture looks fine hooked up to cable via coax. Also Xbox games look fine when hooked up via RCA cable connections.

Trying to hook up PC to this TV to view video is another matter.... here is how it is hooked up....
Computer has video card with DVI/VGA out and S-Video out.

Video card out via S-video to RF Modulator.....Modulator to TV via yellow RCA cable.

Had to change PC display resolution to lowest setting avail ( 800X600) anything higher was too blurry to read any text. Still is blurry but readable at least. Video is ok but just barley acceptable. What is the weak link here?

I have read that composite video is only capable of 480i. But I still don't understand why the display looks fine with detailed Xbox games via RCA but not video from PC???
 
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