Blanking, Master Gain, And Fault Circuit Description - Ceronix 1493 Service Manual

BLANKING, MASTER GAIN, AND FAULT CIRCUIT DESCRIPTION.

The master gain control
to the video gain line through a 1K resistor
. The voltage range of the video gain
062
062
line is programmable via resistors
and solder bridges at
The solder bridges may connect resistors
, , , and
244 244 245 245 258 258
line. This arrangement permits a variety of
input signals and picture tubes to be used
with the same monitor PCB.
Horizontal blanking (
to the gain line by transistors
transistor pulls down on the gain line when
the flyback pulse is high. Capacitor
charged by diode
080
080
that, as soon as the flyback pulse starts going
positive the NPN transistor
and horizontal blanking starts.
constant of capacitor
and is chosen such that the capacitor
093
093
will lead the FBP on the downward slope and
turn the horizontal blanking transistor off
just at the end of the FBP. This advanced
timing compensates for the turn off delay of
transistor
072
072 .
Vertical blank time is started when a low
going pulse from the LA7851 pin 16 causes
the output, pin 1, of the dual comparator
to go low. Capacitor
210
210
through resistor
208 208
end of the LA7851 pulse, the capacitor
holds the output, pin 7 of the
207 207
comparator, low until the bias active pulse
recharges the capacitor
. During the high time of the bias
253 253
active pulse, the comparator output pin 7 is
still low, because of the voltage drop across
the diode . The end of vertical blank
253 253
time occurs when the bias active line
returns to it's high impedance state. The
capacitor holds the charge from the
207 207
bias active pulse until the next vertical
blank time. The advantage of this type of
vertical blanking circuit is that, if the CRT
auto bias IC fails to produce a bias active
pulse, the screen stays blanked. This action
protects the CRT when the vertical
deflection system fails since the output of the
LA7838 boost pulse is needed for the CRT
auto bias vertical sync (CA3224E pin 8).
76
is connected
485
485
,
064
064 076 076
, , & .
S S T U
T U
to the video gain
260 260
) is added
H
B
. This
072
072
081 081
and resistor such
093
093
turns on
072
072
The time
and resistors
081 081 078 078
is discharged
207
207
at this time. After the
through diode
207 207
The video gain line will source up to 32mA
during blank time, which is the reason for
buffering the vertical blank comparator with
a PNP transistor
Resistors
251
251
that is midrange relative to the LA7851 pulse
for maximum noise immunity.
Resistors
248
248
midrange voltage for the bias active pulse
and the, vertical blanking, hold capacitor
to work against.
207 207
are used as jumpers.
368 368
The beam current limiter circuit uses the
is
base to emitter voltage of a darlington
transistor
current. To sense the beam current,
capacitor
010 010
produced by rectifying the high voltage
flyback pulses. The beam current is
converted to a voltage across resistor
This voltage is applied to a long time
constant RC circuit, resistor
capacitor
014 014
darlington transistor. The sharpness of the
limiting response is set by resistors
and
065 065 066 066
reduces the video gain by pulling down on
the master gain line upon excessive beam
current. The beam current is also reduced if
the FBT temperature sensor exceeds 74°C.
Resistor
020 020
this circuit becomes active. The resistance
of thermistor
temperature until the voltage at the cathode
of diode
018 018
transistor
036
036
and darkens the screen.
071
071
The fault circuit senses the temperature
or beam current line with a, comparator
connected, OP Amp.
The + input of the OP Amp. is biased to
3 volts by a voltage divider, resistors
and . The output of the, OP Amp. is
037
037
connected to a low pass filter, resistor
and capacitor
circuit does not become active on power up.
Transistor
the over voltage protect input of the power
supply IC. Resistor
translator transistor
supply controller IC.
.
212 212
and supply a voltage
252
252
and also supply another
250
250
Resistors
to set the maximum beam
036
036
integrates the current pulses
011
011
, before it is sensed by the
. Transistor
071 071
sets the temperature at which
decreases with increasing
180
180
is low enough to turn on
which turns on transistor
at pin 2 (– input).
033 033
to insure that the fault
035 035
conducts the fault signal to
008
008
protects the voltage
005 005
and the power
008 008
and
211 211
.
009 009
and
012 , 012
then,
034
034
017 017