Xerox M15 mixed mode as default

[landspeeder]

I have a M15 under contract that has a slight vertical line when copying off the glass...its not dirt but the scanner starting to fail...if you push the original mode button on the copier and put it into the "mixed" mode from the "text" mode the line disappears but after copying it goes back to the default "text" mode. I cant see anything in the ser. manual about setting it, mixed mode, as the default but maybe I missed it.....anybody know how??? Or a way to get the scanner some more life?
Thanks and Merry Christmas !!