Epson EPL-N1200, Service Manual

Page: 73 / 149

2.1.2.5 Bias Voltages and Laser Drive Timing
Figure 2-21 is a diagram of the drum charge bias voltage, image transfer bias voltage, doctor blade
bias voltage, and the developing bias voltage control circuit. These bias voltages are generated
from the +24 VDC from the high-voltage supply board (HV1). If the printer detects the top cover
open condition, the interlock switch is set to OFF, which cuts the +24 VDC, which, in turn, cuts the
bias voltages.
These bias voltages are controlled by the engine controller board (PWB-A). The HV-T signal is the
image transfer (roller) bias voltage control. While this signal is LOW, the image transfer roller is
charged to 3.2K VDC by the high-voltage supply circuit. And while this signal is HIGH, the image
transfer roller is charged to –0.8K VDC. The HV-CT.R signal controls the drum charge. While this
signal is LOW, the PC drum is charged to –1.2K VDC.
The HV-B.VR signal is an analog signal for developing bias voltage control. This signal controls the
bias voltage level (–175 V to –375 VDC) using analog data. The image density is controlled by the
developing bias voltage level.
There is a three position slide switch on the PWB-S board. The switch can select one of three
image transfer bias current values. The standard position is M.
If the printer is used in an environment with high temperature and high humidity and an image
transfer problem occurs, especially with small size of paper, it is recommended that the switch be
positioned at H.
If the printer is used in an environment with low temperature and low humidity and a toner smear
problem occurs at the age of the transparency sheets, it is recommended that the switch be
positioned at L
Each position’s current value is shown below.
H: 18 uA
M:
8 uA
L: 5 uA
+24 VDC
Interlock
SW
Power Suppry
Board
(PU1)
+24 VDC
HV-T
HV-B.VR
Image Transfer
Bias
3.2K or -0.8 K VDC
Developing
Bias
-175 to -375 VDC
Drum Charge
-1.2 K VDC
Engine Controller
Board
(PWB-A) High Voltage Supply
Board
(HV1)
Doctor Blade Bias
-550 VDC
HV-CT.R
PWB-S Board
H
L
M
Figure 2-21. High-Voltage Supply Block Diagram
Operating Principles EPL-N1200 Service Manual
2-14 Rev. A