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4. Controlling Grid Bias Voltage
Q7
goes
OFF
and
the comparator
output is "H".
Los goes ON.
Current does not flow into trans-
former T4 and Q5 goes OFF.
Bias is applied to the grid.
The surface of the drum
is
charged.
The voltage applied to the grid is fed
back to the comparator which functions to
maintain the grid voltage constant.
Leas goes ON and comparator circuit
output is "L".
Q6 goes OFF and current flows into
T4.
legias is not applied to the grid.
In the above way, the surface of the Z
drum is not charged, hence toner does not '
adhere to it (blanking).
COPYRIGHT © 1988 CAWON INC,
CANON HP-1215 REV.0 MAR. 1968 PRINTED IN JAPAN (IMPRIME AU JAPON)
E. CONTROLLING DEVELOPING
BIAS
a S
1. Outline
The circuit shown
in Figure 3-21
con-
trols the developing bias. Its main functions
are as follows.
(1) Switching AC component of the devel-
oping bias ON/OFF.
(2) Regulating
voltage of the DC compo-
nent of the developing bias.
The copy density can be controlled by
varying the voltage of the developing bias
to suit the following variables.
(1) Settings
of copy
density
cursor
and
recalibration dial
i
(2) AE sensor output
(3) Type of developing unit
When there is a black developing unit
in the copier, J605-1
and J605-4 are con-
nected electrically to inform the copier that
a black developing unit is in place.
The photosensitive drum gradually dete-
riorates with use, causing the potential (Vi)
of light areas of the drum to increase, so that
copy
density
is not
correct
for a given
setting
of the density control
cursor. To
compensate
for this,
a COPY
DENSITY
RECALIBRATION knob, which can be turned
to raise the DC bias by exactly the increase
in Vi, and thus produce clear copies again.
2. Operation
a.
Switching AC-component of the devel-
oping bias ON/OFF
The square
wave
generator
operates
continuously when the copier is switched
ON.
;
(1) When ACBON
=
1
AC bias switch circuit goes ON.
LE Gatpat from the square wave gener-
ator is cut off.
So AC bias is not supplied to the devel-
oping cylinder.
(2) When ACBON
= O
The AC bias switch circuit goes OFF.
So the output of the square wave oscilla-
tor goes to the T1 drive circuit. This causes
the AC high-voltage transformer to gener-.
ate a 1300VAC AC bias, which is supplied
to the developing cylinder.
3-19
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