BELINEA 10 55 70, Руководство по обслуживанию

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R339. When the feedback pulse voltage exceeds
set voltage, the
output from Pin 7 of
after passing through D37 and input to Pin8
of Because of this, is OFF
so there is no vertical or horizontal sync output from
pin
of and the monitor shuts down.
3.7.8. Horizontal linearity and CS Switching
Switching CS is necessary to ensure the lines are in
accordance with the specifications in multi-sync
monitors.
For frequencies up, is ON
For frequencies is ON and
For frequencies is and
is and CS is and C324 i n par-
allel.
For frequencies RI.301 is and
is ON and CS is C3 13 and C324 in
O n Pulses Pulses 1
1
Standby No Pulses
Pulses 0 1
Pulses No
Pulses 0 0
Off 0 0
I
H L H
SCAP2 H
I
L
I
H L L
I
3.8. Video Amplifier
The RGB video and sync signals are supplied through a video
cable directly to the Video Board at connector RGB
signals are terminated in 75 ohms by and
and R506.
The RGB signals then enter an video pre-amplifier
IC, providing synchronous black level clamping, variable
picture contrast (gain) and RGB gain balance for color align-
ment. Separate gain control voltages for the three
channels are provided via R557 and from
the TDA8444 DAC which is loaded by the microcontroller
via the
bus. These inputs enable the individual gains of
each channel to be varied to allow channel gain balance. In
addition, a common signal is applied on pin12 to adjust all
three channels by the same amount, to allow for overall gain
or contrast control.
A synchronous clamping signal is derived from the horizontal
sync pulse by one half of
This takes the trailing edge
of horizontal sync pulse, differentiates it through C531,
then squares it via the monostable feedback action of
and to provide a precise length digital clamping pulse
which is applied via pin14. The timing is shown in
Figure 3-24, below.
Signal
clamp
Pin 14 of ICB
signal after
horizontal blanking
N O T E :
B
.
3-24 Timing of Pin 14 Clamp Signal
The outputs of the video pre-amplifier are fed to a
hybrid power amplifier IC type LM2419, through resistors
R526 and In addition, on screen display video
information generated by can be injected via diodes
D514 and
amplifies the video signals to around The
outputs are AC coupled to the CRT cathodes via C523, 12
and C513. In order to bias the DC level of the cathodes
correctly, the AC coupled signal is DC restored by clamping
to a DC voltage which can be varied under microprocessor
control. Considering Red channel output on
as an
example, the signal is clamped by 17 to the voltage set by
the two transistor amplifiers formed by Q502 and
which amplify the adjustable voltage
output of the DAC.
A similar stage can be seen for the green and blue channel
outputs.
When RC video signal amplification circuit is added for
amplification, this waveform will change as shown in Figure
3-25 (a). Without the DC component, as shown in Figure 3-25
(b). the DC level of darker and brighter displays will be
different, so when this kind of signal without a DC component
is sent to the CRT, it will cause the contrast of the image to
change as the signal changes. Therefore,
and
17 serve as a DC clamp and the CRT’s anode DC voltage
can be adjusted by the DAC.
C o r r e c t w a v e f o r m w i t h D C c o m p o n e n t
( D a r k i m a g e ) (Bright image)
A v e r a g e
Layer
Figure 3-25 The Post Output Amplifier Circuit