BLANKING, MASTER GAIN, AND FAULT CIRCUIT DESCRIPTION.
The master gain control 485 is connected
to the video gain line through a 1K resistor 062 . The voltage range of the video gain
line is programmable via resistors | 064 | , | 076 |
and solder bridges at S , T , & | U . |
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The solder bridges may connect resistors
244 , 245 , 258 , and 260 to the video gain
line. This arrangement permits a variety of input signals and picture tubes to be used with the same monitor PCB.
Horizontal blanking (HB ) is added
to the gain line by transistors 072 . This transistor pulls down on the gain line when the flyback pulse is high. Capacitor 081 is charged by diode 080 and resistor 093 such that, as soon as the flyback pulse starts going positive the NPN transistor 072 turns on and horizontal blanking starts. The time constant of capacitor 081 and resistors 078 and 093 is chosen such that the capacitor will lead the FBP on the downward slope and turn the horizontal blanking transistor off just at the end of the FBP. This advanced timing compensates for the turn off delay of transistor 072 .
Vertical blank time is started when a low going pulse from the LA7851 pin 16 causes the output, pin 1, of the dual comparator
210to go low. Capacitor 207 is discharged through resistor 208 at this time. After the end of the LA7851 pulse, the capacitor
207holds the output, pin 7 of the comparator, low until the bias active pulse recharges the capacitor 207 through diode 253 . During the high time of the bias active pulse, the comparator output pin 7 is still low, because of the voltage drop across the diode 253 . The end of vertical blank time occurs when the bias active line returns to it's high impedance state. The capacitor 207 holds the charge from the bias active pulse until the next vertical blank time. The advantage of this type of vertical blanking circuit is that, if the CRT auto bias IC fails to produce a bias active pulse, the screen stays blanked. This action protects the CRT when the vertical deflection system fails since the output of the LA7838 boost pulse is needed for the CRT auto bias vertical sync (CA3224E pin 8).
The video gain line will source up to 32mA during blank time, which is the reason for buffering the vertical blank comparator with a PNP transistor 212 .
Resistors 251 and 252 supply a voltage that is midrange relative to the LA7851 pulse for maximum noise immunity.
Resistors 248 and 250 also supply another midrange voltage for the bias active pulse and the, vertical blanking, hold capacitor 207 to work against. Resistors 211 and
368are used as jumpers.
The beam current limiter circuit uses the base to emitter voltage of a darlington
transistor 036 to set the maximum beam current. To sense the beam current, capacitor 010 integrates the current pulses produced by rectifying the high voltage
flyback pulses. The beam current is |
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converted to a voltage across resistor |
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This voltage is applied to a long time |
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constant RC circuit, resistor |
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011 | |||||||||
capacitor |
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014 | |||||||||
darlington | transistor. The sharpness of the | ||||||||
limiting response is set by resistors |
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| 012 | , |
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065 and 066 . Transistor 071 then,
reduces the video gain by pulling down on the master gain line upon excessive beam current. The beam current is also reduced if the FBT temperature sensor exceeds 74°C. Resistor 020 sets the temperature at which this circuit becomes active. The resistance of thermistor 180 decreases with increasing
temperature until the voltage at the cathode
of diode | 018 |
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transistor |
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036 |
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071and darkens the screen.
The fault circuit senses the temperature or beam current line with a, comparator
connected, OP Amp. 033 at pin 2 (– input). The + input of the OP Amp. is biased to
3 volts by a voltage divider, resistors 034 and 037 . The output of the, OP Amp. is connected to a low pass filter, resistor 017 and capacitor 035 to insure that the fault
circuit does not become active on power up. Transistor 008 conducts the fault signal to the over voltage protect input of the power
supply IC. Resistor | 005 | protects the voltage | |
translator transistor |
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| 008 | ||
supply controller IC. |
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