Sony KP 48S70, Руководство по обучению

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17
CRT Drive (CG)
Overview
The section discusses the CRT drive using the CG board. The CG board
is similar to the CR and CB boards that drive the red and blue tubes.
These boards use a discrete transistor circuit to drive the tube cathode.
In addition to drive, each C board contains a circuit that monitors cathode
current and converts it to a voltage. This voltage will be used in a loop
between the YCJ and the CRT. We will also discuss the other inputs that
are used to bias the tube.
CRT Drive
The output from the YCJ (not shown) is input to CN731/7 on the CG
board. When this signal is input to Q731/B, the transistor begins to con-
duct. As current flows through Q731/C-E junction, current also begins to
flow through Q722/B-E. Q722 is a common base amplifier and it passes
the signal through its C-E junction. This signal is then sent to Q733/B.
Q733 is a current amplifier that drives the cathode of the tube. When
Q733 conducts, a voltage divider is formed between R739 and R736, and
R741, Q733 and R743. As Q733 conducts harder, there is less voltage
present at the cathode of the tube. When the voltage level of the signal
from Q733/E goes lower, the tube is driven harder, making the picture
brighter. D732, D733, D734 and C735 are present to prevent damage if
the tube should arc.
Tube Bias
In addition to high voltage, the CRTs need other biasing to properly dis-
play a picture. First they need a heater voltage, which is developed by the
FBT (not shown) on the G board. It is input to the G board at CN503/6
and 7. From there it is split to the CR and CB boards. The heater is
needed to heat the cathode so that it can emit electrons. If it is missing,
the cathode will not emit electrons and consequently there would be no
picture.
The G1 input on the tube is a control grid that is used for shading. Each
of the tubes has a signal applied to G1 that makes the picture darker
during certain portions of the picture. If this shading input is missing, you
may see an imbalance in color on either the right or left-hand sides of the
screen.
G2 is also a control grid used to limit the acceleration of electrons as they
travel through the neck of the tube. These changes in the acceleration of
the beam change the picture brightness. Each color has a G2 control that
is preset at the factory.
There is an input for the focus grid on each tube. This input is from the
electrical focus control VR on the focus assembly. It should be set using
a dot pattern for optimum focus.
IK Return
All Sony projection TVs employ an AKB (Auto Cathode Balance) circuit to
automate the white balance (black balance) by forming a loop between
the YCJ and the CRT. This loop compensates for losses in cathode cur-
rent due to aging.
The YCJ (not shown) outputs three reference pulses, each 1H long, that
are delayed by 1H from each other for each field. The sequence for these
pulses is Red, Green and Blue. These pulses cannot be seen on the
screen since they occur in the over-scan region. When these pulses
drive the tubes, current is monitored and converted to a voltage. This
voltage is input to a window comparator whose output is used to adjust
the drive level for each color.
R743 is used to monitor the tube current on the CG board. As Q733
draws more current, there will be a rise in the voltage drop across R743.
There are similar resistors to R743 on the CR and CB boards. All three
pulses are combined on the CG board. All three boards contain a block-
ing diode similar to D735. These pulses are input to Q734 Buffer and
then to CN731/1 which is connected to the A board. The IK return signal
is then input to the YCJ (not shown). If a problem should occur anywhere
in the CRT drive or tube biasing circuits that causes one of the color’s IK
return pulse to be incorrect, the picture will be blanked. This is indicated
by the Self-Diagnostics as the Standby LED flashing five times.