The combined signals of picture video and the second sound intermediate frequency are
output from Pin (47) and sent into the sound intermediate frequency circuit SIF unit to be
processed via N201 Pin (53) after being filtered and switched over by sound intermediate
frequency band-pass filter Z233 or Z234. The sound audio signals obtained are then sent into
the audio switch circuit of the TV/AV electronic switch circuit. After the electronic volume
attenuation by ATT circuit, they are output to the sound amplification circuit N701 via the
input terminal Pin (6). The sound power amplification is carried out by LA4275. Finally, the
signals are output to loudspeaker SP901 and SP902.
(5)
Horizontal scanning signal processing
Switch on the set and 9V voltage is sent to N201. The horizontal start voltage is provided by
the internal regulated voltage circuit to make VCO generate 32-fold horizontal frequency
oscillation. With 1/32 frequency distributor and AFC1 and AFC2 phase-shifting circuit, the
horizontal drive pulse generated is output via Pin (32) to drive horizontal driver V444 to make
it work in switch status. The switch pulse is coupled by the horizontal drive transformer T401
and make the horizontal output tube V411 work in switch status. The output signals are sent to
the horizontal deflection coil via S correction capacitance C412 and horizontal linearity
adjustment L414. One linearly varying magnetic field is generated to make the electron beam
of the picture tube move horizontally until the horizontal scanning completes.
About 22KV anode high voltage, 8KV focusing voltage and 6.3 Vrms filament voltage is
generated via horizontal output transformer T402 by making use of the pulse high voltage
generated on the collector electrode of the horizontal output tube V411 during the horizontal
scanning return period. Apart from this, one part of the return pulse voltage output from Pin 5
of T402 is sent to Pin 26 of CPU after phase reverse to be used as synchronizing signal; and
the other part is sent to Pin 30 of N201 via phase shifting network to implement AFC 2
control.
(6)
Vertical scanning signal processing
The vertical oscillation signals output from Pin 24 of TB1238N are added to the vertical
scanning saw tooth wave signal output terminal Pin 4 of vertical scanning module LA7830
via R908. Vertical scanning saw tooth wave signals are formed with the differentiation by
driving the internal differential amplifier. Finally, saw tooth wave signals are output from Pin
2 and added to deflection coil. Vertical deflection of the electron beam is generated to
implement the vertical scanning.
(7)
Power supply circuit
The power supply circuit of HT-3718 is a typical free-running pulse width switch power
supply. The structure of the main circuit belongs to step-down type. It is a cold mechanism
power supply. The circuit is mainly composed of switch tube V513, exciter tube V512, switch
transformer T501 and rectifying filter circuit. The AC220V power voltage is rectified by
diode D503 to D506 and smooth filtered by C507 and voltage about DC 300V is got. The
voltage is added to the collector electrode of switch tube V513. V513 and the primary group
3-7 and the feedback winding 1-2 of the switch transformer T501 form a free-running
blocking oscillator. About 560VP-P intermittent oscillation pulse is generated at the collector
