Removing extraneous color channel information

[Steve Rickaby]

FrameMaker 7 for Mac, print to .ps, Distiller 6.

I am currently packaging a  book to be printed in b&w using the same workflow that I've used successfully on lots of other books in the past, either b&w or spot color.

Although I have applied all the processes used for previous books (reducing photos to 8-bit grayscale, applying Illustrator's 'convert to grayscale' filter to line art, making sure all included graphics are CMYK and not RGB), I seem to be unable to remove information in the C, M and Y channels in the press-ready PDF. The problem manifests itself with inserted photos and drop-shadows on line art. In preflight, if the C, M and Y channels are switched off, the photos look bad.

Rather than dump this problem on the printers, I'd like to resolve it here, but I am out of ideas. Can anyone advise me?

-- 
Steve