Linear Regulator; Switching Regulator - Tatung OMNISCAN SERIES Service Manual

voltage will shut down.
For the single power supply PCB test only, a dummy
load should be connected to P802. (see alignment.)
DEGAUSSING CIRCUIT
(Refer to circuit diagram Fig-3A)
AUTOMATIC DEGAUSSING CIRCUIT
Posistor R802 and degaussing coil compose an
automatic degaussing circuit to demagnetic the
shadow mask and internal shield in the CRT.
Posistor shows low resistance value (8 ohms) in
normal temperature, when apply AC line voltage, a
big current flows into degaussing coil, a few seconds,
the resistance of posistor increase rapidly due to
rising in heat, degaussing current thus decrease to
less than 100 rnA rapidly, degaussing function is
completed.
The degaussing coil has a resistance of 20 ohms
(approx.).
MANUAL DEGAUSSING CIRCUIT
Switch S803 and posistor R804 are used as a manual
degaussing.
controlled by the resistor divider R203 and R205.
Pin I of 0202 is the voltage input, pin A is the voltage
adjust Pin, pin 0 is the voltage output pin.
R201 is a fusible protection resistor.
16V REGULATOR
0<:02
lM317
Fig-4
OUTPUT
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24V
R203 A
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3.5 12V SWITCHING REGULATOR
R201
4.7
112W
INPUT
(Refer to circuit diagram Fig-5)
The 12V switching regulator is mounted on main PC
board, network 0203a-0204a-0205-0206-T201 and
some discrete components achieve a high efficient
12V switching regulator.
DEGAUSSING CIRCUIT
DEGAUSSING COIL
5803
MANUAL SW.
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Fig-3A PWM (PULSE WIDTH MODULATION) GENERATOR
0204a is the Pulse Modulation Comparator, saw-tooth
signal is fed to pin 3 via the collector of 0602, the
error voltage is fed to pin 2 via pin 1 of 0203a, the
PWM (square waveform) pulses is generated from pin
1 of 0204a.
R804
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AC INPUT
LOW VOLTAGE REGULATOR
R803
220
On the main board, there are four different dc
voltages (16V, 12V, 6V, 5V) converted from the 24V
of the switching power supply.
3.4 16V LINEAR REGULATOR
(Refer to circuit diagram Fig-4)
The 16V voltage supply is derived from network
0202-R203-R205-C202-R201.
0202 (LM317) is a three-terminal adjustable output
positive voltage regulator, the output voltage is
0203a (LM358) is the error amplifier,the reference
voltage on pin 3 of 0203a is provided by R214 and
zener 0202.
The voltage on pin 2 is the feedback signal from
output voltage (12V). If have any voltage difference
on the output will be detected by the error amplifier.
Oivider R210-R211-R212 with R211 enabling fine
adjustment of the output voltage.
C203-R216-R215 (on pins 1, 2) optimize the stability
of the control system by defining the loop gain of the
Error Amplifier.
OUTPUT STAGE
The drive signal is fed to the output stage via buffer
stage 0205.
Network 0204-C205 is the voltage pump-up circuit to
provided 40Vp-p signal on the collector of 0206, the
signal is clipped by 0208-0205.
Power MOSFETs 0206 switches the termination of
T201 primary (pin 2) between OV and 24V with a
=
3-5
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