Hitachi 51SWX20B Training page 57

DP-2X ABL CIRCUIT EXPLANATION
(See ABL Circuit Diagram for details)
The ABL voltage is generated from the ABL pin (3) of the Flyback transformer TH01. The ABL pull-up resis-
tors are RH27 and RH28. They receive their pull up voltage from the SW +115V B+ line for Deflection gener-
ated from 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 9V 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 9V line.
ACCL TRANSISTOR OPERATION
The ABL voltage is routed through the PPD2 connector pin 3 to the Power Supply PWB, then to PPS2 connector
pin 3 to the Signal PWB. Then the ABL voltage is routed through the acceleration circuit RA54 and D404 to the
base of Q413. Under normal conditions, this transistor is nearly saturated. Q413 determines the voltage being
supplied to the cathode of D403, which is connected to pin 53 of the Rainforest IC, I401. During an ABL voltage
decrease due to an excessive bright circumstance, the base of Q413 will go down, this will drop the emitter volt-
age which in turn drops the cathode voltage of D403. This in turn will pull voltage away from pin 53 of the Rain-
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forest IC, I401. Internally, this reduces the contrast and brightness voltage which is being controlled by the I C
bus data communication from the Microprocessor arriving at pins 30 and 31 of the Rainforest IC and reduces the
overall brightness, preventing blooming as well as reducing the Color saturation level to prevent color smear.
ABL SWITCH QH03
New for this chassis is the ability to change the brightness level of 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 age-
ing 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 now do this. When
the Side panels are turned off, the overall average ABL level for the image is reduced. To compensate, QH03
ABL Switch is added. When the customer selects Black Side panels, the Microprocessor I001 tells the Rainforest
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IC I401 via I C communication to output a high from the DAC2 line pin 36. This high is routed through the
PPS2 connector pin 2 on the Signal PWB to the Power Supply PWB. Then from the PPD2 connector pin 2 on
the Power Supply PWB to the PPD2 connector on the Deflection PWB and finally to the base of QH03 turning it
On.
With QH03 turned on, the Resistor RH29 connected to the collector is added to the ABL pull up circuit and the
ABL level drops slightly to compensate for the side panel loss of brightness.
The Difference between chassis for the ABL circuit relate to the values of Resistors RH24, RH25 and RH32.
See diagram for details.
Gray Side Bars
Black Side Bars
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