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