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HP LaserJet Quick Reference Service Manual Page 270

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The problem will typically resolve itself after 10-50 pages
have been run through the duplex portion of the printer. This process
cleans the surface of the oblique roller. If you must replace the roller,
prior to installing the new roller in the printer, be sure to rub the surface
of the new roller with some clean paper to remove any excessive build-
up of the preservative. Do not clean the roller with alcohol or other
cleaning solutions. Doing so would remove the preservative altogether,
leading to premature failure of the new roller.
If the oblique roller (RG1-1356-000CN) requires replacement, be sure
to also check the inside portion of the white plastic alignment guide.
Paper being run through the duplexer and forced into the side of the
alignment guide can eventually wear a groove into the plastic. If a
groove has developed, replace the entire lower duplex guide assembly
(RG1-2498-000CN). If it is not replaced, subsequent duplex paper jams
can result when paper gets caught in the groove. (Contrary to the
implication in the service manual, note that the lower duplex guide
assembly does not include the oblique paper alignment roller.)
Misalignment of Duplexed Images
Sometimes, a customer may notice that the images on
the front and back of a duplexed page may not exactly align when held
up to the light. This is most noticeable when printing on pre-printed
forms or when the document has a border. The most prevalent reason
for this misalignment is the process of fusing the toner into the page.
The first time paper passes through the fusing assembly, it is heated
and almost all the moisture is removed. This causes the paper to shrink.
Therefore, the physical size of the paper and the printed image are no
longer the same when the second image is placed on the page. This
results in the second image being slightly larger than the first.
The manufacturing tolerances for the registration of the paper also
cause misalignment between the front and back images. The current
registration specification for these printers is 4.2 mm (+/- 2.5 mm) from
the leading edge of the paper to the first printed dot. Reducing this
tolerance would increase the cost of the printer exponentially.
268 – Summary of service notes and technical tips


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