Hitachi CP2896TA, Руководство по обслуживанию

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14
Contrast and brightness control
Both adjustment blocks consist of electronic potentiometers
and are controlled via the IIC-bus. The contrast control has
an influence on the amplitude of the RGB signals and the
brightness control on the DC level of the RGB signals rela-
tive to the black level.
Blue stretch
The blue stretch channel gives additional amplification if
the blue signal is greater than 80% of the nominal signal
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.
Measurement pulse insertion and blanking
During the horizontal and vertical blanking time and the
measurement period, the signals are blanked to an ultra
black level, so the leakage current of the picture tube can
be measured and compensated for automatically.
Measurement pulses are generated in the timing genera-
tor and are inserted into the R channel during line 20, the
second is inserted into the G channel during line 21 and
the third is inserted into the B channel during line 22. These
measurement pulses are fed with the RGB signals to the
video output amplifiers (ICh1, ICh2 and ICh3), and feed back
the voltage levels to pin 19. During the cut-off measure-
ment lines, the output signal levels are at the cut-off meas-
urement level. The vertical blanking period is timed using
a sandcastle pulse. The measurement pulses are triggered
by the negative going edge of the vertical pulse of the
sandcastle pulse and start after the following horizontal
pulse.
White point adjust, automatic cut-off control and
output stages
The nominal signal amplitude can be varied
±
50% by the
white point adjustment using the IIC-bus. During the leak-
age measurement time, leakage is compensated in order
to get a reference voltage on the cut-off measurement in-
put pin 19. This compensation value is stored in the exter-
nal capacitor on pin 17. During cut-off current measure-
ment times for the R, G and B channels, the voltage on this
pin is compared with the reference voltage, which is indi-
vidually adjustable via the IIC-bus for each colour channel.
The control voltages so derived are stored in the external
feedback capacitors on pins 21, 23 and 25. Shift stages add
these voltages to the corresponding output signals. Finally,
the RGB signals are amplified to the nominal value of 2
Vpp and the signals are output on pins 24 (R), 22 (G) and
20 (B).
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-
ing generator includes a line counter which controls the
blanking and measurement pulse insertion stages.
Switch-on delay circuit
After switch on, all signals are blanked and a warm up test
pulse is fed to the outputs during the cut-off measurement
lines. If the voltage on the cut-off measurement input ex-
ceeds an internal level, the cut-off control is enabled but
the signal still remains blanked. Signal blanking is stopped
when the cut-off control has stabilized.
CRT module
Video output amplifiers ICh1, ICh2 and ICh3
The video output stage consists of three separate output
amplifiers, TDA6111. The circuit has a high slew rate and a
large 16 MHz bandwidth and is thus suitable for 100 Hz
applications. The circuit is protected against CRT flashovers
and electrostatic discharges (ESD).
The RGB signals from the RGB video processor are fed via
low pass filters to the inverting input pin 3 of the amplifi-
ers. The non-inverting input pins 1 are connected to a volt-
age level of +3 V. The amplifiers have two outputs for the
picture tube cathodes, pin 8 for the DC currents and pin 7
for the transient currents. After these output pins the am-
plified RGB signals are fed to the picture tube cathodes.
The feedback information is output from pin 9 and con-
nected via a resistor to input pin 3. Monitoring of the black
level of the cathodes is done via output pins 5. These out-
puts are connected together via resistors and the final re-
sult is fed to the RGB video processor, pins 19 (cut-off con-
trol) and 15 (beam current). The required supply voltages
are fed to pins 2 (+12 Vp) and 6 (+200 V).
A negative flyback pulse from the diode split transformer
Mk1 (DST) is fed to the control grid G1 via connector Xh1-
2, capacitor Ch19, and resistor Rh28. This intensifies the
horizontal blanking during the line flyback.
The blanking of the screen after switching off is done by
transistors Th1 and th2. During normal operation, capaci-
tor Ch27 is charged to +12 V and capacitor Ch18 to +200 V.
When the set is switched off, Ch27 discharges to the base
of Th1. The transistor conducts and discharges Ch18 as
negative charge on to the control grid G1. Also, capacitor
Ch28 is charged to +12 V and when the set is switched off,
the base of th2 goes low and the transistor conducts. Ch28
discharges to the non-inverting input pin 1 momentarily
causing about +200 V to be fed to the outputs. This pre-
vents the increase of the beam current until Th1 has had
time to block the picture tube.