Samsung CX528ZSE Service Manual page 26

C. DC volume control circuitry :
D. AF pre—amplifier :
The output of the volume control is applied to the internal AF pre~amplifier and
Pin 4 is provided for
NFB from the sound output amplifier. The amplified AF signal from Pin 5 is coupled
through
C655 to
the output transistors (Q601, Q602), which compose a SRPP (shunt regulated push-pull)
amplifier.
The maximum
output voltage of AF pre—amplifier is 4V rms typical.
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9. Functions of TA7698AP (KA2154)
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(1) Power section and start—up circuit.
The TA7698AP (KA2154) combines a PAL/NTSC Video—chroma sub system and a defection
combination
on a single chip to provide a PAL or a PAL/NTSC color television. This device includes
a video amplifier,
PAL/NTSC color demodulators designed to provide color differential outputs, an
improved
sync—seperator,
a horizontal oscillator with sawtooth type AFC, @ horizontal pre—driver, a vertical
oscillator and a vertical
pre—driver in a 42 pins DIP.
So the Vcc line and the ground line are divided into two systems indivisually in
order to prevent the in—
terferences between each section. Refer to the Fig. 11,
Firstly, the Vcc lines are divided into two sources.
One of them is the 12V line of Pin 2 for the video—chroma subsystem, the sync sperator
and the vertical
deflection system.
The other is the 8V line of Pin 33 providing an internal zener regulator for the AFC
and the horizontal
section. This 8V line is driven by the 12V of the shart—up circuit.
When power is first applied to the set by the power switch ON, the starting voltage
of approximately 12V
is input to Pin 33 through R407 and horizontal drive stage. At this moment, FBT is
not producing 16.5
Volts. But once the horizontal oscillator and FBT has been operated, D401 is revers—biased
and cut off
because the cathode voltage of D401 is higher than the anode voltage by the 16.5V
back-up.
Thereafter the power source of horizontal deflection section is supplied totally from.
the 16.5V
line, so
the unnecessaried power loss is eliminated by this start-up circuit.
Secondly, the TA7698AP(KA2154) equipts seperated ground lines. To prevent inter—section
interferences
each section has it's own ground line. The lines are tied together at the IC ground
pad to reduce com—
mon impedances. Pin 11 is a GND line for the video section and chroma demodulator,
and Pin 31 is
another GND line for the sync seperator and the deflection system. These GND lines
are pulled out to
each side of the IC package to be minimize the common
impedance.
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