Control Stages; I²C Bus Interface And; Picture In Picture - Loewe Q2500B Serviceanleitung

Technology of Q 2500 colour TV set
According to the operating mode selection is
either with H level on blank on pins 38 and 33
or controlled from the computer via the I²C
bus.
In the following section of the circuit Y and
colour difference signals are generated from
the RGB signals.
4.7.2

Control stages

In the Y path the first component is a circuit
for gamma correction and in the colour differ-
ence branch the colour saturation setting
stage.
After RGB has been generated again from the
difference signals and the Y signal, the setting
stages for contrast and brightness are
reached.
The control stages for contrast and colour
saturation in the other IC's are set to maxi-
mum and have no function. There is, in addi-
tion, in each channel, a setting stage for the
white value, and the black value is set with the
amplifier stages that follow.
The white value and the basic setting of the
black value are determined in the two corre-
sponding positions of the service mode. In
further a operation the black value is deter-
mined by the cut off control.
The RGB signals are output at a maximum of
3.5 Vss via pins 40, 41 and 42.
The measured values for cut off and leakage
current are fed to the IC on pin 44. With the
evaluation circuits for leakage current and cut
off, together with the computer, subsequent
adjustment is carried out via the output ampli-
fier, in order to keep the picture impression
stable independently of aging.
Pin 43 is connected to the base of the diode
split transformer. The circuit integrated into
the IC thereby receives information about the
beam current flowing into the c.r.t.
Document Q 2500
If the maximum permissible value is exceeded
there, the circuit reduces the contrast and the
brightness with the corresponding control
stages. (see basic board "Beam current limita-
tion").
4.7.3 I²C bus interface and raster correc-
tion
All control processes such as brightness, con-
trast, etc are controlled via pins 10 and 11 of
the I²C bus interface. In addition, a DAC can
also be controlled via the I²C bus. For this,
TDA 9332 on pin 25 outputs an adjustable
voltage. This is fed to the rotation panel on pin
3 of connector W 1021. At this point an addi-
tional coil on the tube turns the deflection
raster, thus providing compensation of the
earth's magnetic field at the installation site of
the device. The value of the raster correction
can set in both service mode and in the menu
for picture functions.
If the device is turned, the influence of the
earth's magnetic field changes. The planned
rotatable rack takes this into account. The
rack is controlled from the chassis, with the
rotation angle being determined by the TV
software. Raster correction can also be de-
termined, in that on rotation the voltage on pin
26 of TDA 9332 is changed.
The influence of the rotation angle on the
raster displacement is almost linear. On instal-
lation of the device, it is sufficient for correct
functioning of the correction to input a value
for raster correction into the EAROM for the
centre and the two end stops.

4.8 Picture in picture

On the signal board an optional picture in pic-
ture circuit is possible. This circuit consists of
the video processor
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