Hitachi CP2896TA, Service Manual

Page: 13 / 112

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network Ro46, Ro44, Ro50, Ro45 and Ro42. By adjusting
the trimmer potentiometer Ro45, the output voltages of
the power supply can be controlled.
In order to avoid excessive decreasing of +7 V during re-
cording mode, diode Do20 is connected from the above
mentioned resistor network to +7 V. If the voltage level
drops too far below +7V, the diode conducts causing a lower
voltage level on pin 5. The controller then generates wider
pulses from the output pin 3 and the supply voltages of
the power supply will be increased.
The width modulated drive pulses are output from pin 3
via the pulse transformer Mo3 to power supply controller
pin 2. The rising edge of the drive pulse causes the switch-
ing transistor to conduct and the falling edge, which is syn-
chronized to the line flyback pulse, switches off the tran-
sistor.
This arrangement prevents disturbances caused by the
switching-off time of the transistor from upsetting the
screen display.
Under / over voltage detection
The power supply controller has an internal monitor for
both under and over supply voltage on pin 16. The under
voltage threshold level is typ8.5 V. Lower voltage
levels disable the output pulses.
The over voltage threshold is typ15.7 V. Higher volt-
age levels disable the output pulses.
Restarting requires that the voltage level on pin 16 is first
decreased below +8.5 V and then increased to +11.8 V,
unless the voltage level across capacitor Co22 has reached
+2.5 V. In this case, circuit operation is completely stopped.
Current limitation
As mentioned above, the source of the switching transis-
tor is connected to ground via resistor network Ro18/19
and Ro21/22. The measured result is fed to input pin 3 of
ICo1. A double threshold system is used, first limitation
level (+0.6 V) against momentary overloads and a second
limitation level (+0.8 V) against very strong overloads.
When the first threshold level is reached, the switching tran-
sistor stops conducting until the end of the period, and a
new pulse is needed to start it conducting again. During
the first threshold period, capacitor Co22 is charged. If the
voltage level across Co22 r2.5 V, the output will
be disabled. This system is called “repetitive overload pro-
tection”. However, if the overload subsides 2.5 V
is reached, capacitor Co22 will be discharged and normal
operation will continue.
If a very powerful overload causes the second threshold
level to be reached, the output will immediately be disa-
bled.
If the power supply stops because the first threshold has
been exceeded, it can be restarted by decreasing the sup-
ply voltage on pin 16 below +8.5 V and then increasing it
to +11.5 V.
If, however, the power supply stops due to exceeding the
second threshold, the circuit is stopped completely and can
only be restarted with mains switch.
Regulators / voltage switches
ICo4 regulates the +5 Vstby, which is always available when
the power supply is operating . Due to the micropower
system, there are no voltages available in standby mode.
ICo3 regulates the +12 Vr and +12Vp supply voltages. The
+12 Vr is available in normal and recording modes, but not
in service standby mode.
The +12 Vp can be switched off in recording mode by the
microcontroller. In this case the P_on line is high, transis-
tor to7 conducts and transistor to4 is switched off.
The +12 Vp and +8 Vp voltages are absent, and therefore
the horizontal deflection stage will not operate.
ICo6 reg5 Vr, which is available in normal and re-
cording modes, but not in service standby mode.
The tuning v30 V for the tuner is regulated from
+130 V. The circuitry is located in the horizontal output stage
consisting of resistors Rk11...Rk114 and zener diode ZDk1.