"One of my fax machines produces some black stripes in copy mode and on the documents received on another machine. Are these stripes caused by a malfunction of the optical diodes bar, some other electronic failure or by dirt?"
(From: Paul Grohe (email@example.com).)
Probably by dirt or a paper shreds stuck in the optical path.
A common cause is Liquid Paper. Somebody makes a correction and sticks the not-yet-dry correction into the fax. Then the still-wet correction fluid makes a streak on the pickup window as it passes by. Subsequent faxes will now have streaks.
I had the same problem. It turned out that the ink from a faulty page stamp had dripped down onto the mirror.
Generally, the paper is moved and the optical path does not move. So if there is something blocking the optical path, it will show up as one continuous image, or stripe. The "blockage" can be a paper shred, ink, dust or even a loose wire.
The image is usually bounced off a long, wide mirror and then into the CCD pickup somewhere in the bottom of the machine. If anything is in this path, it will cause that problem.
Open er' up and just look around inside for dirt and loose debris. Then inspect the ENTIRE optical path. BUT DON"T REMOVE ANYTHING!!! You don't want to mess up any alignments.
If the fax is old, or is in heavy use, you'll probably find that it is in need of a good cleaning. Clean the mirror and blow out any dust or junk.
Also check the automatic paper cutter (if it has one) for paper shreds.