Ricoh VT3500, Service Manual

Page: 110 / 330

6.11 INK DETECTION
[Ink Detection Circuit]
The detecting pin [B] works like the electrode of a capacitor to detect
electrostatic capacity. Through this detecting pin, the time constant of the
pulse, which is generated in the ink detection PCB, is different when ink is
present and when ink is not present. The detecting pulse is compared to a
standard pulse to detect whether there is ink in the drum or not .
(1) The standard pulse is output from OUTPUT 1. The pulse length (T
0
) can
be adjusted by adjusting VR901.
(2) OUTPUT 2 is the detection pulse. The time constant is determined by
C908 and the electrostatic capacity between the detecting pin and the ink
roller [C] or doctor roller [A] (ground). The detection pulse is triggered by
the dropping edge of the standard pulse that is input from TRIGGER 2.
When ink is present, electrostatic capacity increases and the pulse length
(T
1
) becomes longer. On the other hand, when ink is not present, the
pulse length (T
2
) becomes shorter as electrostatic capacity decreases.
(3) The pulse length (T
1
or T
2
) is compared with the standard pulse (T
0
).
When the time constant (T
2
= No ink) is shorter than the standard pulse
(T
0
), the output of CN902-2 goes low.
TP105
CN002-2
-12V
VR901
1
2
8
-12V
D904
D902
D903
TP1
TP2
5
9
CN002-1
C908
12
CN104-20
Main PCB
THRESHOLD
OUTPUT1
OUTPUT2
TRIGGER2
THRESHOLD2
INK Detecting PCB
[B]
[C]
[A]
Ink is present No ink is present
T0 T0
T1 T2
(1) Standard Pulse
(2) Detection Pulse
(3) Output of CN902-2
1 December 1993
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