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

 

 

 

+

 

200

.047uF

V ref.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

G

33K

15.8K

 

 

169K

 

 

 

C8

 

GREEN CHANNEL

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

68.1K

 

 

 

R

33K

 

 

 

 

 

 

 

 

 

RED CHANNEL

 

 

 

 

900

+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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Genius ISO XFR-100W, ISO XFR-75W, 2093, 1493, 2793, 3693, 1793 CRT Auto Bias and Auto Bright CIRCUIT, FUNCTION, Description

2093, 1493, ISO XFR-75W, 3693, 2793 specifications

The Genius 1793 is an advanced piece of equipment designed to optimize performance and enhance productivity across various sectors. Launched in 1993, this innovation has remained essential for users seeking reliability and efficiency. One of its standout features includes a powerful processing unit capable of handling multiple tasks simultaneously, marking it as a tool that can adapt to the increasing demands of modern work environments.

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Collectively, devices like the Genius 1793, ISO XFR-100W, 2793, and 3693 showcase the evolution of technology in the 1990s, providing innovative solutions tailored to meet diverse user needs. Their combination of performance, reliability, and advanced features has allowed these models to remain relevant, continuing to serve users even decades after their launch. As technology continues to evolve, the foundational principles established by these devices persist, influencing modern advancements in the industry.