Service manual

1.3 POTENTIAL CONTROL (LATENT IMAGE CONTROL)
1.3.1 Concept
Latent image control adjusts the development potential to maintain the
density of the toner image on the drum. It does this by compensating for
variations in drum chargeability and toner chargeability.
Development Potential (V
DP
)
The development potential (V
DP
) is the difference between the development
bias voltage and the voltage of areas of the drum that have been discharged
by laser exposure at full power. If changes in this potential are not accounted
for, the color balance will be poor.
– Factors that change the development potential –
Drum Chargeability
After long usage following installation or PM, drum chargeability will
gradually change due to the following factors:
Dirty charge corona and grid plate
Changes in the drum characteristics with age (OPC wear) and
temperature.
Also, as the drum gets older, its sensitivity (response to light) gradually
decreases. This leads to an increase in residual voltage in exposed areas of
the drum after laser exposure, and on all areas after quenching.
Toner Chargeability
Toner chargeability varies with humidity, and this affects the image density
even if the toner concentration is kept constant.
When humidity is low, toner chargeability increases resulting in lower image
density. When humidity is high, toner chargeability decreases resulting in
higher image density.
Also, after long usage following installation or PM, the surface of carrier
particles is gradually coated with melted toner (this is known as filming). This
decreases the toner chargeability.
How the machine maintains the optimum development potential
The machine uses the drum potential sensor to detect changes in the drum’s
response to a standard applied voltage. It also uses the image density (ID)
sensor to detect changes in drum reflectivity and the density of a standard
sensor pattern. These tests are done during the process control self check,
which is preformed every 150 copies and other times, such as after the main
switch is turned on.
The machine calculates the current V
DP
from a series of M/A values
obtained from image density and potential readings off the 12 grades of the
V
SP
sensor pattern on the drum. To make sure that V
DP
is kept at an
optimum value, the machine refers to a look-up table of current V
DP
against
V
D
, V
L
, and V
B
(this is known as the pointer table).
PROCESS CONTROL
A166/A187/A189 2-4 SM
CÓPIA NÃO CONTROLADA
CÓPIA NÃO CONTROLADA