CIRCUIT DESCRIPTION
1. Main Board
l-l Power
Circuit
This circuit generates the regulated + 5V DC, +
12V
DC and
+ 24V DC from 22OV/240 AC.
1-2 Video Amplifier Circuit
The composite video signal fed to base of
46
from control
board through pin
1
of CN3.
The
video
signal from the emitter of
96
IS fed through VR8
(CONTRAST), amplifier
Q7,Q8
and
Q9
to cathode of CRT.
1-3 H.V Deflection Circuit
This circuit generates
vertical
(V)
deflectlon
sawthooth
current for V scanning of the beam inside the cathode ray
tube (CRT) and horizontal
(H)
deflection sawthooth current
for horizontal scanning of the beam inside the cathode ray
tube.
1-4 High Voltage Circuit
This
circuit generates high voltages for cathode ray tube
(CRT) electrodes.
The flyback pulse generated by
horizontal
(H) deflection
circuit is supplied to the primary winding of the
flyback
transformer
(FBT)
in high voltage pack T3 to step up the
flyback
pulse to the necessary level. The boosted pulses
obtained at the secondary winding of the transformer are
rectified to generate high voltages.
2. Control Board
2-1 Camera Power Circuit
The video monitor
WV-EM90
has four camera power circuit
which supply the DC power to the specified cameras through
the coaxial cable respectively. Since these four circuits are
exactly same, the power supply circuit for camera-l will be
described.
This circuit consists of a constant current circuit, and a
misconnectlon protection circuit.
0
Constant Current Circuit
In order to compensate the difference of voltage drop due to
the coaxial cable length between the video monitor and
camera, this circuit applies the constant current to the camera
regardlessof coaxial cable length.
+ 24V DC supplied from Main board through at pin 8 of El is
divided by zener diode
D2,
VRl (DC CURRENT ADJ) and R3
-
T
R5.
The divided DC voltage is supplied through operation
amplifier
ICl
(a) to base of impedance converter 471 for
controlling the camera DC power to steady state.
The variation of load current due to coaxial cable length is
detected by R6 and R7 as a voltage change and the changed
1,.
_
voltage is supplied to operation amplifier
ICl
(a) which
compensates the voltage differences between pins 2 and 3 of
ICl. Therefore, even if the cable length is changed, current
fed to detecting resistors R6 and R7 is kept constant and the
constant current is supplied to the camera.
l Misconnection Protection Circuit
The video signal from the camera is multiplexed on the
power line, the rnisconnection protection circuit protects the
power circuit from a open or short circuit.
When the power switch on the video monitor is turned ON,
Ql
is turned ON at beginning and low potential at collector
of
Ql
issupplied to pin 2 of operation amplifier
ICl
(a).
At this
time,
approx. +
22V
DC set from + 24V DC by dividing
with
D2,
VRl
and R3
-
R5
and supphed to pin 3 of
ICI
(a) is
compared with the potential at pin 2 so that the output at
pin 1 of
ICl
supplied to base of impedance converter 471 is
high level and
Q71
is kept OFF.
At the same time,
43
is turned ON momentarily and approx.
+4V
DC set by
zenner
diode Dl is supplied through
Q3,
R8
and Dl to the camera. In the camera, approx.
40KHz
pulse is
generated by the
+4V
DC and multiplexed on the power line.
This pulse is supplied to
Q2
for turning it ON and
Q?
OFF.
Due to
Ql
OFF, collector potential is increased and thus the
output of
ICl
(a) is decreased therefore, the impedance
converter 471 is turned ON and converts the impedance of
power line from high to low. As a result, the DC power is
supplied from the constant current circuit continuously
through
471.
While camera power circuit supplies constant current to the
camera, voltage difference appears at both end of R6 and R7
so that the
Ql
is turned ON and
Ql(CONTROL
SUB BOARD) is
kept OFF.
If this video monitor is connected to other CCTV cameras or
products, no pulse is fed back to the starter circuit and
Ql
(CONTROL SUB BOARD) is kept ON. Therefore, no DC power
is supplied to the connected products in order to protect
them.
When the coaxial cable is disconnected in the operating
condition, no current is fed through R6 and R7, voltage drop
of R6 and R7 reduces and Ql is turned OFF. As a result,
Ql
on
the Control sub board is turned ON and potential at pin 2 of
ICl
(a) decreases, pin 1 of
ICl
(a) is increased and
471
is
turned OFF. In this way, the no DC power is supplied to the
power line.
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