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12
7.RGB Out Blanking
RGB Out Blanking
The purpose of The RGB blanking circuit is prevents burn in
of the phosphor of CRT.
There are three blanking transistors, Q7730 (blue), Q7728
(green) and Q7729 (red) to cut off the beam of the CRTs.
The collectors and bases of the three transistors are
connected together so that if one drive fails, all of the beams
will be turned off.
The collectors of each of the three transistors will go low
turning off the RGB drive transistors.
The RGB transistors are used in the horizontal blanking,
vertical stop blanking, vertical over deflection, power off
blanking and protection circuits.
If the power off blanking and protection circuits is activated,
the PTV is turned off at the same time.
The purpose of The RGB blanking circuit is prevents burn in
of the phosphor of CRT.
There are three blanking transistors, Q7730 (blue), Q7728
(green) and Q7729 (red) to cut off the beam of the CRTs.
The collectors and bases of the three transistors are
connected together so that if one drive fails, all of the beams
will be turned off.
The collectors of each of the three transistors will go low
turning off the RGB drive transistors.
The RGB transistors are used in the horizontal blanking,
vertical stop blanking, vertical over deflection, power off
blanking and protection circuits.
If the power off blanking and protection circuits is activated,
the PTV is turned off at the same time.
Horizontal Blanking
During horizontal retrace, a series of horizontal pulses is
supplied from horizontal output transistor Q551 to the base
of Q7738 for horizontal blanking, to prevent the projection
tubes emitting light during the fly-back interval.
During horizontal retrace, a series of horizontal pulses is
supplied from horizontal output transistor Q551 to the base
of Q7738 for horizontal blanking, to prevent the projection
tubes emitting light during the fly-back interval.
Vertical Stop Blanking.
If vertical deflection stops, the remaining horizontal line will
burn the phosphor of each CRT. The vertical stop blanking
circuit is designed to activate the blanking circuit by cutting
off the RBG drive to the CRTs.
The vertical pulse from IC451 vertical out pin8 is applied as
forward bias to the cathode of D402. During normal
operation, the pulse is applied to the base of Q400, keeping
Q400 turned on. If the V-pulse is not present to the cathode
of D402, Q400 turned off.
The collector voltage of Q400 increase, and supplies
voltage to the RGB blanking circuit, which turns off the
CRTs.
If vertical deflection stops, the remaining horizontal line will
burn the phosphor of each CRT. The vertical stop blanking
circuit is designed to activate the blanking circuit by cutting
off the RBG drive to the CRTs.
The vertical pulse from IC451 vertical out pin8 is applied as
forward bias to the cathode of D402. During normal
operation, the pulse is applied to the base of Q400, keeping
Q400 turned on. If the V-pulse is not present to the cathode
of D402, Q400 turned off.
The collector voltage of Q400 increase, and supplies
voltage to the RGB blanking circuit, which turns off the
CRTs.
Vertical Over Deflection Blanking
If during the deflection period the electron beam goes
beyond normal range and hits the neck of the CRT (over
deflection), the beam can burn through and destroy the
CRT. The over deflection blanking circuit is designed to
prevent this.
IC508 is composed of comparator. When abnormal level
V-pulse input, the pin 4 would go high and applied to base
of Q7738, it will turn on Q7738, to control the RGB out
blanking circuit.
If during the deflection period the electron beam goes
beyond normal range and hits the neck of the CRT (over
deflection), the beam can burn through and destroy the
CRT. The over deflection blanking circuit is designed to
prevent this.
IC508 is composed of comparator. When abnormal level
V-pulse input, the pin 4 would go high and applied to base
of Q7738, it will turn on Q7738, to control the RGB out
blanking circuit.
H-pulse
IC1101
MPU
59
60
CLK
DATA
16
1
2
B BLK
IC7707
DAC
9
G BLK
R BLK
14
15
+9V
+9V
Q7730
Q7729
Q7728
1
2
B
9
G
R
IC7703
RGB out
Q7718
Q7717
Q7715
CRT-B
CRT-G
CRT-R
H-out
Q551
Q551
V-pulse
Protection Detection
IC508
Vertical over
protection
Relay
Q7738
Q773
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