A customer bought a used 624 but I'm not familiar with them. Everything works normally except CO1; the input is dead. I've defaulted the system, checked my ground, etc, etc but no luck. When I pulled the cover, it appears that one of the components (MMXX B8M)for CO1 is fried. Any help would be appreciated.

Otis, lets review.
You have a confirmed live CO line with a good line cord going into CO 1 on a defaulted system.
You can not access line 1 on any of the system sets? Have you tried an slt or your butt set
and dial 81?
If you see something that appears fried, it probably IS.
I can give you the Info. for the repair center in GA. if you'd like to send it in and get it checked out.