دارة مكبر التردد المتوسط للمرئيات في جهاز سيرونكس .. كتاب التلفزيون الملون والعادي

6-9 The mid-frequency amplifier circuit for visuals in the Seronix device:

This circuit contains a transistor (107811 TEA), an integrated circuit TDA 420B, and a filter circuit (F170 (SW174) - Figure (6) - (24) and (6 - (25)) and the work of these
circuit is:

1- IF amplification.

2- AGC voter automatic gain control.

3 - Automatic gain control gate.

4 - AFC automatic frequency control detector.

e - photo detector

6 - A preamplifier for the visual signal.

The circuit work is as follows

The IF signal coming from the VHF and UHF elector output enters and reaches the base of transistor V155 via capacitor C155 - this signal is amplified and enters the filter circuit (SW174) (F170) via coil L161 and capacitor C161. This filter determines the response curve and takes the place of several LC tuned circuits and wave traps (according to the previously described SAW system). The circuit, which is represented by two triangles, and the amplification of the first stage is controlled by the AGC voltage connected to it from the automatic gain control gate AGC - square 2, and the signal is taken after amplification to the image detector (the square within the integrated circuit, and the resonance circuit consisting of 174 173 174 175 tuned at the medium frequency is connected for the picture, which is 38.9 MHz with circuit poles (14-18) Each triangle means an amplification stage.

The complete visual signal, including the image and the colors, is magnified after leaving the detector. A part of the visual signal from the primary amplifier reaches the AGC circuit to generate the voltage whose value changes with the change in the amplitude of the synchronization pulses in the visual signal. Negative horizontal pulses also reach pole (13) of the integrated circuit. High voltage transformer to cover the AGC voltage.

The AGC voltage that we obtained works by controlling the amplification of the first stage of the medium frequency amplifier of the image, which is within the integrated circuit. This voltage is also taken to the voter to control the amplification of the high frequency RF amplifier. The capacitors (C89-C90) filter the AGC voltage, and the resistance (165) controls the value of this voltage, and the resistance R164 ensures that this voltage does not change with the change of temperature.

The medium frequency amplifier and the integrated circuit are fed from a voltage (12 volts) (U72) to reach the pole (16) of the high and medium frequency board, and this voltage reaches the transistor collector (155) passing through the resistance (R204) and the coil (L160). As for the integrated circuit It is connected by a voltage (12) (I said to pole (1) via coil (L165).

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