Date: 10/02/98 R762839
RGB Output/ABL+CRT
R762839
14
Technical Description
Technical description Video Amplifier R762839
Video Amplifier
The video signal arrives at pin 2 of J1 and is terminated into 75
Ω
the resistors R1...6.
The first half of IC2 switches the base of Q2 between this video signal (no ABL-
Automatic Black Level) and the reference voltage on pin 1 of IC10 (during ABL).
Via buffer Q1 and D3, the signal is fed to the input of a non-inverting amplifier formed
by Q2 and Q3. The diode D3 serves as protection for Q1 against high inverse Vbe
voltage during blanking. The resistors R13, R14 form together with R15 and R104 a
divider that is chosen to limit the voltage at the emitter of the transistor Q1 during
overdrive conditions of the input. This prevents saturation of the amplifier.
The amplified signal arrives at pin 4 of the VPH08. Feedback for the previous amplifier
is taken from pin 3 of the VPH08. The potentiometer P1 determines the GAIN and C1/
R21 improve the frequency response.
IC1 (VPH08) is a cascode class A amplifier with built in buffer stage and external load
resistor, formed by R32...35.
Via pin 2 of J3 and J4, the signal goes to the IBCL (Individual Beam Current Limiting)
and ABL measurements stages and finally arrives at the cathode of the CRT. The
components D4..D7,C49, R103, D28, B1, B2 and R95 protect the amplifier against
arcing.
IBCL measurement
With the transistor Q4, the mean cathode current is measured. The current flows via
D12, D13 and Q5 and through Q4 to R45. There, the current is converted into a voltage
and is sent to the RGB driver module. The capacitor C10 prevents high frequency
currents to flow through Q4. The resistor R46 and zener Z4 form a protection network.
ABL meas
Switch
During the non-ABL period, the base-emitter junction of the transistor Q6 is shorted by
the transistor Q5. The cathode current flows through R51 and Q5 to the IBCL
measurement stage. If the cathode current is high, the voltage drop across the resistor
R51 increases and D12/D13 also come into conduction. The gate of Q5 is held at +5.6V
with Z1 and R49/R50.
During the ABL interval, the optocoupler IC7 comes into conduction and shorts the
gate-source of Q5. Now the cathode current can flow through the transistor Q6, R52
and R53 to the ABL control loop. By shorting the transistor Q6 outside the ABL interval,
smearing is prevented.
The optocoupler is controlled by the circuit around the transistors Q10 and Q11. During
the blanking, the BL-signal (Blanking) is negative (-15V). The pulse is integrated by
R80/C22. During the blanking, the BL-signal is wide enough to get Q10, Q11 and IC7
into conduction until some time after the vertical blanking.
ABL Control Loop
This circuit has to keep the cathode current just above the black level constant. The
ABL measurement is performed at the end of the vertical flyback, when the electon
beam is moved outside the phosphor screen of the CRT in order not to see these
measurements lines.
The ABL interval is initiated by
a 12V pulse of 20
µ
s, which is AC coupled on the IBCL
line. The pulse is coupled via R54 and C13 to the base of Q7. The resistor R55 keeps
the diode D15 into conduction if there is no pulse to prevent false triggering. The pulse
at the emitter of Q7 and MP7 is used to perform the leakage current measurement and
is now called leakage pulse. With the trailing edge of this pulse, the transistor Q8 starts
to conduct through the network C14, R59 and R60 for about 20
µ
s. This pulse at MP6
is now called the measurement pulse. The total ABL interval is 40
µ
s wide and is
electrically formed by D17, D18 on R61 and C15.
Summary of Contents for GRAPHICS 1209S
Page 4: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC SAFETY NOTICE...
Page 9: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC GENERAL INFORMATION...
Page 28: ...BARCO PROJECTION SYSTEMS GRAPHICS 1209S 90 00972 230V AC 90 00977 120V AC SERVICE SHEETS...
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