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51.1 Errors on 4100 Printers

10 Oct, 2001 By: Steve Geishirt imageSource

51.1 Errors on 4100 Printers

the last couple of months, we’ve been taking calls from all over the country
on a 51.1 error on 4100 printers; the error is defined as a laser beam detection
error. The errors appear to be intermittent, separated by 1000s of pages, or as
little as a hundred. Part of what makes this error unique is the whole laser
scanner system on the 4100 which contains two laser beams, not just one. In this
article, we’ll take a closer look at the workings of the 4100 laser scanner,
and what needs to be done to fix the 51.1 error.



off, the fix to the 51.1 error is typically the replacement of the laser scanner
unit, part number RG5-5100-000. Word has it that one of the two lasers is not
functioning correctly causing the 51.1 error. In some cases, the extra step of
replacing the engine board has also fixed this, but the jury is still out on the
exact cause. Typically when a 51 error occurs, the print on the delivered page -
if it delivers - is either distorted, prints a completely black page, or there
is no print whatsoever. In all of the cases reported here, when the 51.1 error
occurred, the print looked fine on the page. This may have a lot to do with the
second laser diode in the laser scanner assembly.


Laser Diode

The second laser diode is placed right over the top of
the first one. They both bounce off the same six-sided spinning mirror, optics,
and deflection mirror in the laser scanner unit. After that, however, the two
beams split up, focusing one beam onto a separate line over the top of the
other, as they scan they separate lines across the image drum, see figure
. This dual line scanning enables the printer to create its true 1200
dpi image and keep printing at 25 ppm. The laser has no problem switching on and
off at the needed frequency, thereby creating a 1200 dot per inch across the
drum. In fact, the lasers in these laser scanners are only used to 1.3% of their
true capability. The hold up recently in printing true 1200 dpi has been the
drum rotation speed, which correlates with the paper path speed. In the 4000,
for instance, when printing 1200 dpi, the drum motor needs to take twice as many
steps as it did in 600 dpi to create the 1200 lines (down the image drum in an
inch), meaning it prints 1200 dpi at half print speed. In the 4100, the two
beams are offset by a factor of 1/1200th of an inch, which allows the
drum to turn at its 1/600th of an inch step but be scanned at 1/1200th;
that’s a brilliant idea that produces speed and quality at the same time.




weak link
in relation to this 51.1 error revolves around the laser, beam
detection loop. This is the feedback system that initially helps synchronize the
laser beam and keep it synchronized. When a print is first created, the laser
beam is turned on which allows it to bounce off the beam detect mirror and
reflect over to the beam detect sensor. This sensor turns the light impulse back
into an electrical impulse, which the engine controller uses to locate and
synchronize the beam. Without th
is, the print on the page could wrap off one
edge of the page and then back onto the other side, with the white edge of the
boarder somewhere down the middle of the page. Once the beam is synchronized,
the engine controller keeps track of its timing throughout the print

to make sure the image is correctly imaged. This is
done by turning on the laser when it is focused off the image drum and onto the
beam detect mirror which again sends it to the beam detection sensor as seen in figure
. The question that comes into play here is what part of this loop is
failing causing a 51.1 error. The possibilities include:


One of the lasers.

The laser beam detection sensor.

The engine controller, monitoring this laser/detection loop.

The cables to the beam detect or laser PCA.


at the possibility that only one of the laser beams fail, this could explain why
print on the page looks fine and the 51.1 error occurs. An unanswered question
in this equation is, whether only one laser is used when printing in 600 dpi or
two? While carefully working with the laser on our 4100, (do not try this at
home as it can be dangerous) in 600 dpi mode, print was either fully present on
the page or not there at all. When it was switched to 1200 dpi mode, changing
the position of the laser in the laser scanner caused variations to gray areas
of the page. This leads me to conclude that the second laser is only used in
1200 dpi mode. Could it be that the only time the 51.1 error occurs is when the
printer is in true 1200 x 1200 mode via the printer menu, or driver menu? This
could explain the intermittent occurrence of this error and is now a question we
are asking our customers when their printers are displaying a 51.1 error.


Detection Circuitry

second area of possible failure is the beam detection circuitry. This area
detects the beam and turns it back into an electrical pulse that the engine
controller uses to synchronize the laser beam as described before. One of the
questions is whether the feedback of this unit measures the intensity of the
beam or not. Monitoring the voltage of the beam detection circuitry, the feed
back was a constant reading regardless of whether it was in 600 dpi, Fast REs
1200 or true 1200 x 1200 dpi. This seems to indicate the beam detection unit is
only concerned with finding a beam that is strong enough to trip its feedback
trigger to the engine controller. That prompts yet another question does it
actually monitor both laser beams, even in 600 dpi mode, or does it monitor one
beam and assume the other is present? While I don’t have a definitive answer
for this, common sense would indicate that both beams are present at the beam
detect sensor. The risk of not monitoring this is too great, and it would be
easier, not to mention less expensive, to monitor the double pulse than it would
be to monitor the current or on/off pulses in extra circuitry on the laser pca.
By monitoring both beams at the same time, if one of the beams were not present
when the pulse was sent to the beam detection sensor, the beam detect sensor
should send a lesser voltage signal. The engine controller could interpret it as
a beam detect error and possibly create a 51.1 message from this. This theory
again points to a laser beam failure.


of you who have worked on HP’s LaserJets for a number of years are likely
thinking that the printer should be producing a 41 error, not a 51 error. To
clarify the difference between the two, the 41 beam detect error has
traditionally been a loss of synchronization for a period of two seconds or
less. A 51 beam detect error has been a loss of synchronization for more than
two seconds. A 41 error has been considered a recoverable error, while the 51
traditionally has not. The book does list a 41 error but not as a “printer
error.” Suggested parts replacement focuses around the laser scanner, cables,
and engine controller so it appears 41 still means a beam detect error.
Strangely enough, non of the printers encountered had a 41 error listed in the
event log (error log). Thus, it appears the firmware may be programmed to jump
directly to the 51 error over the 41 under the circumstances that it is



third possibility here is the engine controller, which keeps track of the
synchronization process by sending laser pulses to the beam detect mirror, which
then sends them to the beam detect sensor to check to see if the pulse came back
to it. If there is an error in this process, a 51.1 beam detect error could
occur. I note this because our repair department attempted to fix a 4100 with an
intermittent 51.1 error and after replacing the laser scanner, the unit returned
with the same problem. The next step of replacing the engine controller appears
to have fixed the problem; it has not returned for repair again. Most of the
customers we are talking to indicate the laser scanner has been the fix for
their printers. However, they too may have a short term fix replacing the laser
scanner and they are yet to come back from a still unhappy customer, time will
tell. The interesting note of a possible failure of the engine controller in
this instance has traditionally been either a hard failure or it works fine. The
likelihood of an intermittent 51.1 error created by a flaw in the engine board
would not be considered common.



are listed as the fourth possibility but the likelihood of them causing the
failure is small. Considering many of the 51.1 problems, replacing laser scanner
has repaired the machines, and sometimes the engine controllers, indicates it is
not likely the cables – particularly on a new printer. However, it is
interesting to note that with two lasers, the laser data is sent to the laser
PCA via four independents wires. The merit of using four data lines either has
to do with splitting up the data for each laser to check data, less likely, or
they were concerned about the speed of transmission and possible cross talk in
the wires messing up the laser pulses; interesting. Another possibility in using
two lines for each laser means you can split up the speed of transmission and
use less expensive chips to communicate the data – this would make a fair
amount of sense. Thus cables are not very likely the cause of this error. Most
of the roads seem to point to the laser scanner as the culprit of this 51.1



last note, those reading this article and are thinking about taking a closer
look at the laser out of curiosity, don’t. As I tell the students who attend
our laser printer repair classes: “This printer contains Class 1 lasers,
normally about 5 mW output each. That’s not a whole lot of power, but unknown
to most, they operate at a range of light that is invisible to the human eye. A
5 mW laser can slowly damage your eyes and eventually cause blindness without
you even knowing it – don’t play with the laser.”


contact Steve Geishirt, call (800) 886-6688, or email him at sgeishirt@partsnowllc.com.



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