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The Load consists primarily of the horizontal flyback circuit. The power supply will not
operate without the load since the voltage that sustains the power supply comes from a
secondary in the power transformer and depends on some primary current to generate
secondary current.
V
W
X
The Over Voltage Protect circuit is built into the power supply and monitors the flyback
transformer peak pulse voltage. This circuit will turn off the power supply and hold it off if the
EHT exceeds its maximum rated value. Since excessive X-ray output occurs with excessive
EHT, this circuit provides X-ray protection.
A +12V regulator is used to supply current, to all the control circuits in the monitor, with the
exception of the power supply. Many of the control circuits are decoupled from the +12 volt line
with a resistor or diode to minimize noise from common current loops.
The transformer has two taps on the main winding which are used to generate the +16 volt
and +24 volt supplies. It also has a secondary which is referenced to V- and supplies the power
supply. Since the power supply is generating its own power, a special start up circuit is built
into the power supply controller IC that delays start up until the capacitor which supplies the
IC is charged up enough to furnish the current to start the power supply. This capacitor is
charged with current through a high value resistor from the raw dc supply. This self sustaining
action is why the power supply chirps when an overload or underload occurs. Additional
secondaries to drive the horizontal raster shift circuit and the video amplifiers are also included
in the power transformer.
Y
The Fault Detector senses beam current and temperature. This circuit will activate the
power supply shutdown circuit if either the maximum temperature is sensed or if the beam
current becomes large enough to threaten the FBT.
Z
The Degaussing circuit is connected across the isolated AC line. A posistor is used to allow a
large current to flow, in the degaussing coil, on power up. This current is then gradually
reduced by the increased temperature of the positive temperature coefficient thermistor in the
posistor. A relay is used to short the degaussing coil after the degaussing operation. This
greatly reduces posistor residual current in the degaussing coil. When repairing a monitor, the
degaussing coil should be unplugged, to avoid possible damage to the degaussing coil shorting
relay.
+24V
14
15
XX93 Monitor Block Diagram.
SYNC
VIDEO
GAME
CRT
H
V
VERTICAL
CONTROL
OSCILLATOR
VERTICAL
CONTROL &
HORIZONTAL
H.H.
REMOTE
CONTROLS
HORIZONTAL
Size Control
DIODE
Modulator
FBT
LOAD
ISOLATION
Transformer DOUBLER
VOLTAGE
SWITCHING
REGULATOR
Raw DC
320V
+52V to +129V
-200V
Driver
Output
3
Interface
DY
DY
H
s
V
LA7851
I. V. Feedback
K
H
s
H. Pos.
V. Size &
V. Ras. Pos.
PINCUSHION
Sync delay
M
NO
(PCB)
P
QR
(IN GAME)
S
Beam Current
(VIDEO & DEFLECTION)
U
+16V
+24-27V
Regulator
+12V
T
Sync
OVER
VOLTAGE
PROTECT
Vertical Deflection
Supply
V
W
X
V-
VIDEO
VIDEO
AMPS.
Bright
Auto
CRT AUTO
SYNC
BIAS IC
Bias
BLANKING
Interface
F.B.P.
V retrace
Beam limit
M. gain
H. sync (FBP)
V. sync
Program pulse
Grid pulse
Beam current
buffer
3
3
3
3
3
3
AB
C
E
G
I
J
D
3
L
Beam
Current
Feedback
On Video Board.
VERTICAL
F
SYNC
DELAY
G2≈290V
G1≈–20V
EHT≈25KV
EHTG2
High temp. limit
2
OUTPUT
LA7838
LA7851
+12V
RGB
V. & H.
CA3224E
+
Shutdown
FAULT
DETECTOR
Y
DEGAUSSING
CIRCUIT
Z
FBP
The Switching Regulator is synchronized to the horizontal pulse and drives a power
MOSFET. Unlike most regulators that have a common GND, this power supply has a common
V+ and current is supplied from V- to GND. The MOSFET is connected to V– and signal
ground (GND) through a transformer which is used as an inductor for series switch mode
regulation. An operational amplifier, voltage reference, comparator, and oscillator in the power
supply controller IC are used to accomplished regulation by means of pulse width modulation.
U
Vertical
Dynamic
Focus
2 For Dual Focus
2
Horizontal
Dynamic
Focus
V+
Dynamic Focus
used only on Dual
Focus CRTs
H1
H2