Phil Marlowe wrote in a message to Roy J. Tellason:

 PM> Black areas are the result of light striking the drum.

I would've thought that it would take more than the amount of light that
was present in the room here,  but maybe not.  I guess I'm just too used to
thinking of "laser" as meaning more powerful/significant/whatever
than the room light would be.

And the dark bands that came and went *were* about as far apart as the
circumfrence of the drum might be,  approximately.

 PM> A defective or improperly seated beam detector could send false 
 PM> scan timing signals to the main logic.
 PM>
 PM> The laser would make its scan line while main logic waits to send 
 PM> its data, as shown in the illustration.
 PM>  
 PM> At the beginning of each scan cycle, the laser beam strikes a 
 PM> detector. The detector carries laser light thru an optical fiber 
 PM> to a circuit that converts light into an electronic logic signal 
 PM> compatible with main logic.
 PM>  
 PM> Main logic interprets this Beam Detect signal and knows that the 
 PM> scanner mirror is properly aligned to begin a new scan.
 PM>
 PM> Main logic then modulates the laser beam on and off corresponding 
 PM> to the presence or absence of dots in the scan line.
 PM>
 PM> POSITIONING AND ALIGNMENT ARE CRITICAL.
 PM>  
 PM> If the beam detector is misaligned or loose, the printer's motor 
 PM> vibrations may cause the detector to occaisionally miss the beam.
 PM>  
 PM> Main logic responds by activating the logic full time in an effort 
 PM> to synchonize itself again. Reseat or replace the beam detector 
 PM> and optical fiber.
 PM>
 PM> A loose or misaligned scanning mirror can also cause this type of 
 PM> problem. Vibrations in the mirror may occaisionally deflect the 
 PM> beam around the detector. Realign or replace the scanning assembly.
 PM>  -----------------END-QUOTE-----------------------


 PM> ----------ILLUSTRATION
 PM>  
 PM> ------------
 PM> LASER-DIODE--------------------> /\
 PM> -------------                    \/ MIRROR
 PM>   |                            /
 PM>   |                           /
 PM> --------                    /
 PM> LASER-                _________
 PM> DRIVER-                  BEAM
 PM> CIRCUIT               DETECTOR
 PM> ----------            ----------
 PM>   |                   |
 PM>   |                   |    |=========================|
 PM>   |      optical      |    |======DRUM===============|
 PM>   |      cable------->|    |=========================|
 PM>   |                   |
 PM>   |                   |
 PM>   |                   |
 PM>   |             -----------------
 PM>   |             LIGHT-TO-VOLTAGE
 PM>   |             SIGNAL-CONVERTER
 PM>   |             -----------------
 PM>   |                   |
 PM>   |                   | |--------------------------|
 PM>   |                   | |                          |
 PM>   |                   |-|-----BEAM-DETECT-SIGNAL   |
 PM>   |                     |                          |
 PM>   |                     |                          |
 PM>   |                     |      MAIN-LOGIC          |
 PM>   |                     |                          |
 PM>   |                     |                          |
 PM>   |---------------------|--LASER-CONTROL-SIGNAL    |
 PM>                         |                          |
 PM>                         |--------------------------|

Hm.  I'm going to have to have another look at things.  There isn't much
that's apparent to me when I open the door on the side of the unit. 
There's that assembly that contains the drum,  which pops out fairly easily
and seems to have some mechanical coupling to the rest of the printer via
plastic gears. This would seem to indicate that there oughta be some
optical coupling as well,  maybe more.  At least this gives me some more
idea of what's going on in there...

(Looking)

Ok,  I just popped it open again.  The assembly with the drum contains some
mechanical bits,  assorted plastic gears.  There appears to be a roller as
long as the drum on the inside of the door that could stand to be cleaned. 
And one other,  below that whole assembly that pops out.  As soon as I can
get my hands on something fairly lint-free,  I'll have a go at dealing with
those,  see if I get improvement there withough having to tackle the drum
iteslf.

Looking at the back side of this assembly I see what appears to be the
toner stuff.  There's a filler hole,  and a point apparently used for
recovery from the drum,  where it goes back into the bin.  There's also an
assembly with wires running across an opening.  Other than the drum itself,
 I don't see anything in there resembling optical components,  I think
they're still in the box when I pull that bit out.

I had a book on this printer somewhere,  it'd be pretty nifty if I could
find it...    :-)

Maybe the toner refill kit I have on hand here (but have never had to use)
would offer some further clues.  Imagine that,  had this printer close to 4
years now and I've never had to refill it.  I guess that shows how much
printing I do these days.  Mostly,  my grandkids use it. 

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