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AUTOMATIC BRIGHT LEVEL ADJUSTMENT SYSTEM
The Automatic Bright Level Adjustment System employed in the VB7C
chassis replaces the mechanically adjusted Sub-Bright Level control used in
conventional systems. The primary difference between this system and
conventional systems is the addition of the beam current detection circuit and
the adjustment program incorporated within the CPU (C-003).
The advantages of this system include improved productivity and increased
accuracy of the bright level adjustment during production. This is due to the
computerized and digitized alignment procedure which allows remote
operation.
A block diagram comparison of the previous and present system is shown
below.
Note:
The automatic bright level adjustment system requires special
equipment and test signals. For this reason, this system should be
used only for production.
An additional adjustment mode is provided in the CPU (C-003) for
service. To enter the service menu, press and hold the MENU key
while connecting the AC power. Then select “NO. 25 SB” (Sub
Brightness) and adjust the data with the remote control. To turn off the
service menu display, press the MENU key again. Refer to the specific
Service Manual for your model for the complete Bright Level
Adjustment procedure.
The automatic bright level adjustment system is composed of the beam
current detection circuit and the automatic adjustment program in the CPU.
The fundamental operations are described below.
Fundamental Operation
The BUS data for the brightness control with the 7-bit control data up to 127
steps is output from IC801, the CPU. The 64 steps are used for customer
control and the remainder (63 steps) are provided for bright level adjustment.
The BUS data for the brightness control is input to IC101, the Signal
Processor, and coupled to the BUS Interface circuit within IC101. The BUS
Interface circuit transfers the 7-bit control data to the Brightness Control
Register in IC101. The 7-bit control data changes for the brightness control
are the same as those shown for the bright level on the screen.
Since the beam current required for displaying pictures on the screen is
supplied from the +B (130V) DC line within the beam current detection circuit,
the average beam current is detected and transformed into a DC level for
input to the CPU, pin 30 (A/D input). Maximum A/D input voltage is produced
at 0 beam current.
The average beam current is in inverse proportion to the A/D input voltage as
shown below. The beam current detection circuit has been designed to output
a certain DC voltage between 21/128 Vcc (0.82V) and Vcc (5.0V), at 0 beam
current. The CPU (C-003) has been programmed to automatically adjust the
bright level at a suggested beam current for a specified video signal input
during FACTORY PRESET mode.
