"This machine is copying and sending fax with a line about 3 cm across straight down the paper. This is a thick black colour. Any ideas as to the cause? I have already checked that there is no paper jam or any block to the sensor.
(From: Robert Macy (firstname.lastname@example.org).)
I'm not familiar with the HP LaserJet FAX, but I assume the LJ scans in a page and then sends it, or copies it. Since the black stripe is unique to sending or copying a document (does not appear on incoming faxes), the problem is in the HP's acquisition system.
It can either be the illumination of the page over that small section (no light, looks black) or it can be a whole section of the linear CCD went out (the whole section is losing any charges built up, looks black) or it can be the memory where the image is shifted into (dead RAM section, looks black).
I don't know how the HP illuminates its pages. Is it a flourescent tube across the page, or a row of LED's? Knowing HP's penchant for reliability I would almost assume the engineers went after LEDs. And the section that lights up that 3cm width could have died. However, if it's illumination that died, usually the edges are "soft".
If it uses flourescent light (remember I said I'm not familiar with this scanner), then it sounds like the CCD, or memory, has gone funky on you.
If the memory where the image is stored has a section that died, then you could end up with such a stripe also. Even my ScanMan Plus by Logitech has occasionally put a black strip on the first 10% of the pixels. To solve that I have to do a full power OFF reset (not reboot, but full power OFF)
You can look for the illumination uniformity with your eyes. Just make sure there is no section that's darker by a noticeable amount (not necessarily black, but just a lot dimmer)
Put a sheet of white paper in the machine and put a scope on the output of the CCD & sync from the line reset. Make sure that the CCD is supplying active light information across its whole field of view. If not, you can go from there.
If the data coming out of the CCD looks good, then it's probably the memory holding that line. You can probably change the line storage RAM and fix it.
These are just guesses based upon how I perceive the HP LaserJet to work. Hope somebody that KNOWS this unit jumps in with more specific help.