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The auto bias circuit performs all of its
sensing and bias corrections during the
sixteenth to the twenty first horizontal cycle,
after the vertical sync pulse. Before the
sixteenth cycle, the SW in the auto bias IC is
open ( SW in "C" position).
If the CRT cathode is too far into cutoff, less
beam current flows at the grid pulse time.
This causes the beam current buffer to output
a smaller negative pulse and less voltage is
stored in the coupling capacitor. The program
pulse amplitude (which is constant) is now
larger than the stored (beam current) voltage
and the channel amplifier will add current to
the, bias voltage, storage capacitor
8 95 thus
correcting the low bias voltage which caused
the cathode to be too far into cutoff. After the
program pulse is over, the SW is switched to
the open position again and the next time the
bias voltage can be adjusted is during the
next vertical blank time.
which, if the bias was correct during the
previous cycle, exactly balances the voltage
stored in the coupling capacitor and no
difference is sensed at the channel input. The
channel amplifier, in this case, does not
output current and the voltage of capacitor
895 stays unchanged.
During the 16,17, and 18 horizontal cycle,
the CRT is brought out of cutoff by the grid
pulse. The resulting beam current produces
a voltage at the beam current buffer output.
This voltage is applied to the coupling
capacitor
921 . At the other side of the
coupling capacitor is the channel input, which
is clamped to V ref. (SW in "A" position). The
voltage amplitude of the amplifier output with
the cathode current information is then stored
in the coupling capacitor 921 during this
time.
The auto bias circuit is a control system that
forms a closed loop for controlling the CRT bias
voltage. It generates a set of conditions where
the current near the cutoff voltage of each gun is
measured, and then adjusts the bias voltage of
the video amplifiers, to set the correct black level
voltage for each gun. This color balance
adjustment is necessary, since each gun in the
color picture tube can have a different cutoff
voltage, which also, will change as the CRT ages.
If the picture tube gain changes, the auto
CRT AUTO BIAS AND AUTO BRIGHT CIRCUIT, FUNCTION, DESCRIPTION.
SIMPLIFIED PICTURE TUBE VIDEO BIAS CONTROL CIRCUIT: (One channel shown)
Blue
Blue
CA3224E
895
927
+
10uF
input
hold
cap.
SW
normal
GREEN CHANNEL
RED CHANNEL
Counter, Decoder
Control Logic
V sync
H
sync
Grid pulse
Program Pulse
V ref.
4.0V
G1 G2
CRT
VIDEO
INTERFACE
Video
Amp.
comp.
4.0V
LM324
LM324
33K
33K
33K
15.8K
A
B
C
Auto Bright
Amplifier
Beam
Current
Buffer
5K
200
68.1K
.047uF
.1uF
921
C8
169K
900Ω
+6V
8
G
B
R
G
R
B
*
*
Adjust FBT bottom pot for 4.0V to 4.4 at pin 8.
Note: Chassis before rev. E4, set to 4.6V.
FBT
Screen
adj.
+
+
+
Ω
bias circuit would adjust all three guns in the
same direction to maintain constant black
level. This effect reduces the auto bias
voltage range which is needed for the cathode
differential voltage adjustment. To prevent
this occurrence a second control loop is added
to the system. This second control loop is
called the auto bright circuit and corrects for
CRT gain changes. The auto bright circuit
senses any common bias voltage change and
controls the screen grid (G2) to hold the
common bias voltage constant.
During the next three horizontal cycles (19,
20, and 21), the SW is switched to pass current
to capacitor 895 which is the bias voltage
storage capacitor. At the same time a
program pulse is applied to resistor
C8
-21V
1.8K
920
+4.2V
TC 9
921
895
C8
921
895
895
71