Hitachi CP2896TA, Руководство по обслуживанию

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2
POWER SUPPLY
General
The power supply is a mains isolated Switched Mode Power
Supply (SMPS). Mains isolation is provided by transformer
Mo2. The mains voltage is full wave rectified by diodes
Do1...Do4 and filtered by capacitor Co10. This filtered volt-
age is fed to the switching transistor To1 (MOSFET) via the
primary winding 11 and 5 of the mains transformer. Dur-
ing the conduct period of To1, energy is stored in the pri-
mary winding 11 and 5. When transistor To1 is switched
off, energy flows to the secondary windings. These pulses
are rectified by secondary diodes Do11...14 and Do16.
The following supply voltages are available from the sec-
ondary diodes:
+130 V horizontal output stage
+28 V audio amplifier, subwoofer and audio feature
module
+17 V +12 V regulator IC, horizontal driver and +8 Vp
regulator transistor
+7 Vfb feature box
+7 V +5 Vr regulator IC, +5 Vstb regulator IC and +7 V
supply voltage
Note! The voltage levels may vary depending on the pic-
ture tube. More detailed values are given in the schematic
diagrams.
The power supply is designed to operate with a master-
slave structure, where the power supply controller ICo1
operates as a slave and the secondary controller ICo2 as a
master. The power supply operates in the following ways
in different operation states:
Start up phase:
Power supply is in the primary regulation mode (burst
mode). Power supply controller ICo1 generates independ-
ent drive pulses for switching transistor.
Normal on mode:
Power supply is in the secondary regulation mode (mas-
ter-slave mode). Secondary controller ICo2 generates drive
pulses for power supply controller ICo1. The secondary
controller is synchronized to the line flyback pulses.
Recording mode:
Power supply is in the secondary regulation mode (mas-
ter-slave mode). Secondary controller ICo2 generates drive
pulses for power supply controller ICo1. The secondary
controller is synchronized to the free running frequency of
an internal oscillator.
Switching off to standby phase:
Power supply is in the primary regulation mode (burst
mode). Power supply controller ICo1 generates independ-
ent drive pulses for switching transistor.
Standby mode:
Due to the micropower control, the power supply is com-
pletely without voltage.
The power supply also has a so-called Service standby
mode. The receiver is in service standby mode when it is
set to the service mode by pressing the buttons -vol / menu,
TV and i, but has not yet been switched on by pushing the
TV button twice. In this mode, the power supply operates
(burst mode), but the Vr and Vp voltages are not available.
Start up
After switching on with the mains switch and when triac
Dfc11 (on the micropower control) conducts, capacitor Co15
is charged via resistors Ro9, Ro11, Ro70 and thyristor To15.
When the start up voltage on pin 16 of ICo1 reaches the
switch-on threshold level, that is typ11.8 V, the IC
starts to operate.
The supply voltage of ICo1 is then taken from the second-
ary winding pin 3 via half wave rectifier diode Do8. The
same winding pin 3 supplies pulses to diode Do9. This
voltage drives transistor To9 to conduct putting the gate of
thyristor To15 to ground, and thus switching off the start
up voltage.
The same DC voltage that is taken from the cathode of di-
ode Do9 is used for power supply regulation. The DC volt-
age is fed via the filter network Ro26, Co28 and Ro24 to the
error amplifier input pin 6. The error amplifier compares
the input voltage with the internal reference (+2.5 V) and
varies the burst time. Resistors ro4 and ro10 set the volt-
age to the proper level.
Furthermore, a possible magnetization state of the trans-
former can be checked by sensing the voltage across the
winding pins 3 and 2. This information is fed via resistor
Ro15 to pin 1 of ICo1, and if the specified level is exceeded,
the output pulses can not be generated.
To avoid magnetization during the start up phase, the op-
eration will be started with the internal oscillator’s operat-
ing frequency divided by four, until voltage on soft start
pin 9 reaches a level of +2.5 V. The operating frequency of
the oscillator is set to 27 kHz by capacitor Co16 on pin 10
and resistor Ro3 on pin 11.
Drive of the switching transistor
Pin 14 outputs square wave pulses to the gate of switching
transistor To1. Resistors Ro2 and Ro13 limit the gate cur-
rent. To1 conducts during the positive going pulse and drain
current flows through the primary winding pins 11 and 5.
The clamping circuit Do6, Co11 and Ro16 limits the volt-
age spikes, when To1 is switched off. The source of To1 is
connected to ground via current limiting resistors Ro18,
Ro19, Ro21 and Ro22. Information about the current is fed
to pin 3 of the power supply controller.
After the start up phase, when supply voltages are gener-
ated, the power supply moves from primary regulation
mode to secondary regulation mode. The microcontroller
will be reset and it’s pins 48 (P_on) and 49 (R_on) will go
low. The R_on line allows, via transistor to6, regulator ICo3
to feed +12 Vr out. This voltage is fed to the secondary
controller ICo2 (pin 2) and the IC starts operation.
The capacitor co58 on pin 1 operates as a soft-start capaci-
tor causing the duration of soft-start to be around 20ms.
The free running frequency of an internal oscillator is set
to 32 kHz by capacitor co72 on pin 7 and resistor Ro37 on
pin 8. In normal operating mode, the oscillator is synchro-
nized using the line flyback pulses via differentiator Ck6,
resistor ro38 and diode Do18.
In recording mode, the Vp voltages are switched off and
therefore the line flyback signal is not available. In this case,
the oscillator is in free running mode.
The internal pulse width modulator is controlled by com-
paring the input voltage level on pin 5 with the oscillator’s
sawtooth pulses. Pin 5 is connected to +140 V via resistor