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

BLANKING, MASTER GAIN, AND FAULT CIRCUIT, FUNCTION, DESCRIPTION.

SIMPLIFIED GAIN CONTROL CIRCUIT:
+12V
1K
MASTER GAIN
485
HORIZONTAL BLANKING
FLYBACK PULSE
0VDC Hs
56Vpp 62,D6
BIAS ACTIVE
+2.5V
HIGH Z
3
Vertical Bias O/S
1/2
LM393
2
+2.5V
Blanking in this monitor is accomplished by
reducing the video gain to zero during the
vertical and horizontal blank time. During
video time, the gain is set by the master gain
control which is located on the remote control
PCB. If the overall beam current exceeds
.75mA or 1.5mA (depending on model) for
more then ten frames, the beam current
limiter circuit will reduce the video gain to
protect the FBT. A high temperature sensor,
close to the FBT, will also reduce the beam
current if the high temperature limit (70°C) is
exceeded.
The fault circuit senses the temperature
or beam current line and will turn off the
monitor if either of these signals exceeds the
beam current shut off value. If an abnormal
condition exists in the monitor or the cooling
system of the enclosure fails, the high
temperature sensor will activate the fault
circuit at 80°C. The fault circuit is also
turned on when the beam current becomes
large enough to damage the FBT. This
condition will happen if the video bias supply
(V+ –9V) on the video board fails. An OP
Amp. is used to sense the fault condition and
a transistor is used to transmit the fault
signal down to the power supply.
GAIN SELECT
RESISTORS
1K
VIDEO GAIN LINE
062
PN2222
SIGNAL
CONDITIONING
CIRCUIT
VERTICAL BLANKING
6
1/2
1N4148
LM393
253
1
5
210
.047uF
207
VIDEO INTERFACE
C5346
241
3.6K
One of three input circuits.
FAULT CIRCUIT
072 To P/S OVP
BEAM CURRENT LIMITER
PN2222
7
210
071
The video P-P voltage amplitude at the
cathodes, is the video input signal
amplitude times the master gain control
setting times the video amplifier gain.
The gain select resistors set the maximum
video gain via the master gain line. For a
greater range of brightness, (highlighting)
the video system is allowed to supply high
peak video currents which could damage
the FBT if sustained. The beam current
limiter circuit insures that the long term
maximum beam current is not exceeded.
Horizontal blanking is achieved by
amplifying the flyback pulse (FBP) with
transistor
as soon as the LA7851 starts the vertical
retrace sequence and is terminated by the
auto bias, bias active signal. A comparator
is used to sense the vertical bias O/S, at pin
16 of the LA7851, which goes low when
vertical retrace starts. Capacitor
holds the vertical blanking active, between
the vertical bias O/S pulse, and the bias
active pulse. When the bias active line
goes high, the capacitor
vertical blanking ends, after the bias active
line returns to it's high impedance state.
+12V
200Ω
+7.5V
3
+3.4V
1/4
LM324
2
033
beam current
+6V
High Temp.
Beam Limit
MPSA64
1N4148
D
018
+
10uF
036
014
. Vertical blanking starts
072 072
is reset and
207 207
To
Video
Amp.
CRT
Total
From FBT
+12V
28.0K
020
100K
T @ 25°C
180
207 207
75