Genius ISO XFR-100W, ISO XFR-75W Crt Auto Bias And Auto Bright Circuit, Function, Description

CRT AUTO BIAS AND AUTO BRIGHT CIRCUIT, FUNCTION, DESCRIPTION.

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

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.

SIMPLIFIED PICTURE TUBE VIDEO BIAS CONTROL CIRCUIT: (One channel shown)

VIDEO

+

R

G

Video

CRT

INTERFACE

Amp.

CA3224E

B

.1uF

927

10uF

G1

G2

Auto Bright

Beam

Blue input

Blue

+

FBT

Current

5K

A

hold

Amplifier

+4.2V

Screen

Buffer

921

B

895

4.0V

cap.

+

8

adj.

SW C

*

LM324

LM324

comp.

normal

B

33K

TC 9

4.0V

920

+

200Ω

.047uF

V ref.

G

33K

15.8K

169K

C8

GREEN CHANNEL

68.1K

R

33K

RED CHANNEL

900Ω

+6V

V sync

Counter, Decoder

Grid pulse

H sync

Program Pulse

1.8K

-21V

Control Logic

*

Adjust FBT bottom pot for 4.0V to 4.4 at pin 8.

Note: Chassis before rev. E4, set to 4.6V.

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).

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.

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

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

895stays unchanged.

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 895 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.

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