There's no way to set the wallpaper to "no wallpaper" or a solid color. To get a black background, I had to turn the flash off and take a picture of a solid black surface, then use that photo as wallpaper.
I think the problem with the treadmill is the controller board. I have power to the incline motor, but I see no voltage on the spades for the belt motor. Damn. That's about a $300 part. Still, given that it was like a $2500 treadmill when it was new (though I didn't pay that) it's probably worth it.