14
Contrast and brightness controling generator includes a line counter which controls the
Both adjustment blocks consist of electronic potentiometersblanking and measurement pulse insertion stages.
and are controlled via the IIC-bus. The contrast control has
an influence on the amplitude of the RGB signals and theSwitch-on delay circuit
brightness control on the DC level of the RGB signals rela-After switch on, all signals are blanked and a warm up test
tive to the black level.pulse is fed to the outputs during the cut-off measurement
lines. If the voltage on the cut-off measurement input ex-
Blue stretchceeds an internal level, the cut-off control is enabled but
The blue stretch channel gives additional amplification ifthe signal still remains blanked. Signal blanking is stopped
the blue signal is greater than 80% of the nominal signalwhen the cut-off control has stabilized.
amplitude. In such cases, the white point is shifted towards
a higher colour temperature so that white parts of the pic-
ture seem to be brighter.CRT module
Measurement pulse insertion and blankingVideo output amplifiers ICh1, ICh2 and ICh3
During the horizontal and vertical blanking time and theThe video output stage consists of three separate output
measurement period, the signals are blanked to an ultraamplifiers, TDA6111. The circuit has a high slew rate and a
black level, so the leakage current of the picture tube canlarge 16 MHz bandwidth and is thus suitable for 100 Hz
be measured and compensated for automatically.applications. The circuit is protected against CRT flashovers
Measurement pulses are generated in the timing genera-and electrostatic discharges (ESD).
tor and are inserted into the R channel during line 20, the
second is inserted into the G channel during line 21 andThe RGB signals from the RGB video processor are fed via
the third is inserted into the B channel during line 22. Theselow pass filters to the inverting input pin 3 of the amplifi-
measurement pulses are fed with the RGB signals to theers. The non-inverting input pins 1 are connected to a volt-
video output amplifiers (ICh1, ICh2 and ICh3), and feed backage level of +3 V. The amplifiers have two outputs for the
the voltage levels to pin 19. During the cut-off measure-picture tube cathodes, pin 8 for the DC currents and pin 7
ment lines, the output signal levels are at the cut-off meas-for the transient currents. After these output pins the am-
urement level. The vertical blanking period is timed usingplified RGB signals are fed to the picture tube cathodes.
a sandcastle pulse. The measurement pulses are triggeredThe feedback information is output from pin 9 and con-
by the negative going edge of the vertical pulse of thenected via a resistor to input pin 3. Monitoring of the black
sandcastle pulse and start after the following horizontallevel of the cathodes is done via output pins 5. These out-
pulse.puts are connected together via resistors and the final re-
sult is fed to the RGB video processor, pins 19 (cut-off con-
White point adjust, automatic cut-off control andtrol) and 15 (beam current). The required supply voltages
output stagesare fed to pins 2 (+12 Vp) and 6 (+200 V).
The nominal signal amplitude can be varied ?50% by theA negative flyback pulse from the diode split transformer
white point adjustment using the IIC-bus. During the leak-Mk1 (DST) is fed to the control grid G1 via connector Xh1-
age measurement time, leakage is compensated in order2, capacitor Ch19, and resistor Rh28. This intensifies the
to get a reference voltage on the cut-off measurement in-horizontal blanking during the line flyback.
put pin 19. This compensation value is stored in the exter-The blanking of the screen after switching off is done by
nal capacitor on pin 17. During cut-off current measure-transistors Th1 and th2. During normal operation, capaci-
ment times for the R, G and B channels, the voltage on thistor Ch27 is charged to +12 V and capacitor Ch18 to +200 V.
pin is compared with the reference voltage, which is indi-When the set is switched off, Ch27 discharges to the base
vidually adjustable via the IIC-bus for each colour channel.of Th1. The transistor conducts and discharges Ch18 as
The control voltages so derived are stored in the externalnegative charge on to the control grid G1. Also, capacitor
feedback capacitors on pins 21, 23 and 25. Shift stages addCh28 is charged to +12 V and when the set is switched off,
these voltages to the corresponding output signals. Finally,the base of th2 goes low and the transistor conducts. Ch28
the RGB signals are amplified to the nominal value of 2discharges to the non-inverting input pin 1 momentarily
Vpp and the signals are output on pins 24 (R), 22 (G) andcausing about +200 V to be fed to the outputs. This pre-
20 (B).vents the increase of the beam current until Th1 has had
time to block the picture tube.
Beam current and peak drive limiting
The circuit is provided with two kinds of signal limiter. An
average beam limiter, which reduces the signal level if a
certain average value is exceeded and a peak drive limiter,
which is activated if one of the RGB signals even briefly
exceeds the IIC-bus determined threshold. The beam cur-
rent limiting voltage is taken from the cathode current in-
formation and fed via transistors tt4 / tt1 and capacitor Ct29
to pin 15. If the voltage on the pin 15 decreases below +4 V
due to the charge of capacitor Ct29, the internal limiter starts
to reduce the contrast. If the voltage decreases below +2.8
V, the limiter also starts to reduce the brightness.
Sandcastle detector and timing generator
The two level (2.5V / 4.5V) display sandcastle pulse (DSC)
from the deflection controller on the Feature box is fed to
input pin 14. The sandcastle detector separates the
sandcastle pulse into combined line, field pulses and clamp-
ing pulses, which are fed to the timing generator. The tim- |
ACEC
