1. INTRODUCTION
11AK37 is a 110° chassis capable of driving 25?, 28?, 29?, 33? 4:3 and 28?, 32? 16:9 tubes at the
appropriate currents. The chassis is capable of operating in PAL, SECAM and NTSC standards and
multiple transmission standards as B/G, D/K, I/I?, and L/L´. standards. The sound system is capable
of giving 12 watts RMS output into a load of 8 ohms. One page, 7 page SIMPLETEXT,
TOPTEXT, FASTTEXT and US Closed Caption is also provided. The chassis is equipped with a
double-deck 42 pin scart connector for AV input/output, front -AV input, one back-AV output, one
SVHS, one headphone and one subwoofer.
2. SMALL SIGNAL PART WITH STV2248:
STV2248 video processor is essential for realizing all small signal functions for a color TV
receiver.
2.1 Vision IF amplifier
The vision IF amplifie r can demodulate signals with positive and negative modulation. The PLL
demodulator is completely alignment -free. Although the VCO (Toko -coil) of the PLL circuit is
external, yet the frequency is fixed to the required value by the original manufacturer thus the
Toko -coil does not need to be adjusted manually. The setting of the various frequencies (38.9 or
45.75 MHz) can be made via changing the coil itself.
2.2 QSS Sound circuit (QSS versions)
The sound IF amplifier is similar to the vision IF amplifier and has an external AGC de -coupling
capacitor. The single reference QSS mixer is realised by a multiplier. In this multiplier the SIF signal
is converted to the inter -carrier frequency by mixing it with the regenerated picture carrier from the
VCO. The mixer output signal is supplied to the output via a high -pass filter for attenuation of the
residual video signals. With this system a high performance hi -fi stereo sound processing can be
achieved. The AM sound demodulator is realised by a multiplier. The modulated sound IF signal is
multiplied in phase with the limited SIF signal. The demodulator output signal is supplied to the
output via a low -pass filter for attenuation of the carrier harmonics. The AM signa l is supplied to
the output via the volume control.
The AM demodulated signal results from multiplying the input signal by itself, it is available on
AM/FM output.
2.3 FM demodulator and audio amplifier (mono versions):
The FM demodulator is realized as narrow -band PLL with external loop filter, which provides the
necessary selectivity without using an external band -pass filter. To obtain a good selectivity a linear
phase detector and con stant input signal amplitude are required. For this reason the inter -carrier
signal is internally supplied to the demodulator via a gain controlled amplifier and AGC circuit. The
nominal frequency of the demodulator is tuned to the required frequency (4.5/ 5.5/6.0/6.5 MHz) by
means of a calibration circuit that uses the clock frequency of the ? -controller/Teletext decoder as a
reference. The setting to the wanted frequency is realized by means of the software. It can be read
whether the PLL frequency is insi de or outside the window and whether the PLL is in lock or not.
With this information it is possible to make an automatic search system for the incoming sound
frequency. This is realized by means of a software loop that alternate the demodulator to various
frequencies, then select the frequency on which a lock condition has been found. De-emphasis
output signal amplitude is independent of the TV standard and has the same value for a frequency
deviation of ?25 kHz at the 4.5 MHz standard and for a deviation of ?50 kHz for the other
standards. When the IF circuit is switched to positive modulation the internal signal on de -emphasis
pin is automatically muted. The audio control circuit contains an audio switch and volume control.
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ACEC
