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When there are black streaks on photocopies, there are several causes, and several possible cures. These cures vary from the less expensive, to the more expensive and difficult. I hope the cure for your copier's problems is the less expensive type.
Tell me please: do you do photocopies from both the document feeder, and from the glass sheet? if not both, try the type that you do not normally use, and see if the black streaks are evident on photocopies made that way also.
And let me know, please, by Reply To Expert, and I will advise you further. Thanks.
Horizontal streaks meaning, across the pages rather than along them?
And also, do you print to this machine from a computer, ever? if so, do the black streaks show up then?
That is unfortunately an indication that the scanner, or the light-sensing part in the scanner, has a problem. The problem is not mild, and will require that the printer be service-repaired, I'm sorry to have to say.
If the problem had only shown up when the feeder was used, or if the computer-to-printer results had streaking also, repair-servicing would not have been needed.
Would you be interested in the location of a nearby choice of Brother Authorized Service Centers that you could take the Brother machine to for this repair? if so, please reply with your local zip code, and I will give you the listings for them.