Genius ISO XFR-100W, ISO XFR-75W, 2093, 1493, 2793, 3693, 1793, 1993 manual

E

The CRT Auto Bias IC is a combination of digital and analog circuitry. The digital part is a counter and control logic which steps the analog circuits through a sequence of sample and hold conditions. The analog part uses a transconductance amplifier to control the voltage on a 10uF capacitor (one per gun). This voltage is buffered and sent to the video amplifiers as the bias voltage. In monitors without CRT auto bias, this voltage is adjusted manually using a setup procedure to set the color balance. With CRT auto bias, the color balance is set during the end of each vertical blanking time.

The control sequence is:

1.The cycle starts with a sync pulse from the vertical oscillator (15KHz) or from the vertical sync delay. 15H later the grid pulse starts.

2.The grid pulse on G1 causes cathode current which can be seen as the three faint white lines at the top of the screen. This cathode current is transmitted by the beam current feedback to the beam current buffer where it is converted to a voltage and applied to the CRT auto bias input pin. At this time the CRT auto bias IC outputs a reference voltage at its input pin which sets the voltage across the coupling capacitor. This coupling capacitor voltage is directly dependent on beam current.

3.After the grid pulse is over, the program pulse matches the voltage from the beam current buffer. If the voltage from the beam current buffer, during the grid pulse, is the same as the voltage from the program pulse, the bias is correct and no bias adjustment is made for that vertical cycle.

F

The timing of the auto bias IC is synchronized to the vertical oscillator and the flyback pulses. For horizontal frequencies higher than 15.7KHz a Vertical Sync Delay may be needed to position the grid pulse, generated 3 gray lines, at the top of the screen. The need for the delay circuit is dependent on the particular CRT vertical retrace time.

G

The aging of the picture tube (CRT) not only affects the balance of the cathode cutoff voltage, which is corrected by the auto bias circuit, but it also affects the gain of the CRT. The Auto Bright circuit actively corrects for CRT gain changes by sensing any common bias voltage change, from the auto bias circuit, and adjusts the screen voltage to hold the average bias voltage constant. The lower adjustment on the flyback transformer which is the screen voltage, is used to set the auto bright voltage to the center of its range. Therefore, the auto bright circuits sets up a second control feedback loop to reduce picture variation due to CRT aging. The auto bright circuit is also used to turn off the beam current when the monitor power is turned off.

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