Hitachi 46W500 Training page 44

(See ABL Circuit Diagram for details)
The ABL voltage is generated from the Flyback transformer TH01 ABL pin (3). The ABL pull-up resistors are
RH27 and RH28. They receive their pull up voltage from the SW +115V which is the B+ line for Deflection cre-
ated in the Power Supply.
ABL VOLTAGE OPERATION
The ABL voltage is determined by the current draw through the Flyback transformer. As the picture brightness
becomes brighter or increases, the demand for replacement of the High Voltage being consumed is greater. In
this case, the Flyback will work harder and the current through the Flyback increases. This in turn will decrease
the ABL voltage. The ABL voltage is inversely proportionate to screen brightness.
Also connected to the ABL voltage line is DH16. This zener diode acts as a clamp for the ABL voltage. If the
ABL voltage tries to increase above 10V due to a dark scene which decreases the current demand on the flyback,
the ABL voltage will rise to the point that DH16 dumps the excess voltage into the 10V line.
ACCL TRANSISTOR OPERATION
The ABL voltage is routed through the PPD3 connector pin 3 to the Power Supply PWB, then to PPS3 connector
pin 3 on the Signal PWB. Then the ABL voltage is routed through the acceleration circuit R528 and D501 to the
base of Q511. Under normal conditions, this transistor is nearly saturated. Q511 determines the voltage being
supplied to the cathode of D502, which is connected to pin 78 of the Rainforest IC, I501. During an ABL voltage
decrease due to an excessive bright circumstance, the base of Q511 will go down, this will drop the emitter volt-
age which in turn drops the cathode voltage of D502. This in turn will pull voltage away from pin 78 of the Rain-
forest IC, I501. Internally, this reduces the contrast and brightness voltage which is being controlled by the I
bus data communication from the Microprocessor arriving at pins 29 and 30 of the Rainforest IC and reduces the
overall brightness, preventing blooming as well as reducing the Color saturation level to prevent color smear.
BLACK PANEL SWITCH QH05
This chassis has the ability to change the Side Panels when watching a NTSC 4X3 image. When a 4X3 images is
displayed on a 16X9 set, the sides do not reach the edges. To avoid excessive ageing at the 4X3 display area, the
side panels IRE levels are raised. However, sometimes the customer may want to turn the side gray panels off.
Through the Video Advanced features Menu the customer can do this. When the Side panels are turned off, the
overall average ABL level for the image is reduced. To compensate, QH05 Black Panel Switch is turned on. The
Microprocessor I001 tells the Rainforest IC I501 via I
23. This high is routed through the PPS3 connector pin 9, PPD3 connector pin 9 the base of QH05 turning it
On. This adds Resistor RH38 to the ABL pull up circuit and the ABL level drops slightly to compensate for the
side panel loss of brightness.
Gray Side Panels
ABL SWITCH QH04:
This switch adjust the ABL voltage when the Color Temperature is changed to high by the customer, 10,500K.
During High Color temperature, the overall average brightness is slightly higher, QH04 switches in RH36 to the
ABL pull up resistors. This slightly lowers the overall range of ABL during High Color Temperature.
•
When color temperature is high, SW QH04 is OFF
•
All other color temperatures, SW QH04 is ON.
RH32 manipulates the trigger point of shut down dependant upon the ABL level avoiding false triggering.
DP-3X ABL CIRCUIT EXPLANATION
Black Side Panels
Turned on by the
2
C communication to output a high from the DAC2 line pin
customer
Black Side Panels
PAGE 03-08
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