Image path for a miniature TV. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Image path for a miniature TV. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Телевидение

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The image path described here is designed specifically for miniature TVs. It is assembled according to the direct amplification scheme and allows you to receive signals in one of the first five television channels. Schematic diagram of the path for the first channel is shown in Fig.1. Its sensitivity is no worse than 250 microvolts. The selectivity for the upper adjacent channel is the same as in unified TVs. Clarity along the vertical wedge of the TIT 0249-300 line table, which is explained by poor beam focusing in small size kinescopes. Power consumption does not exceed 200 mW.

Fig.1 (click to enlarge)

The path consists of an RF amplifier (T1 and T2). video detector (D1) and video amplifier (TZ-T5).

The high-frequency signal from the antenna goes directly to the RF amplifier. In its first stage, with the help of capacitor C2, a partial neutralization of the pass capacitance of the transistor T1 is carried out, which significantly improves its operation. The selectivity of the amplifier is provided by the circuits L1C4 and L3C8, tuned to the carrier frequency of the image. A feature of the amplifier is that. that its bandwidth (about 2 MHz) is much less than the width of the spectrum of image signals (6,5 MHz). Moreover, since after the amplifier the signal goes directly to the video detector D1, as a result of the narrowing of the frequency band, noise-immune detection occurs. The suppression of the side components of the television spectrum in the RF amplifier is compensated by a corresponding increase in gain at these frequencies in the video amplifier.

Transistors T3 - T5 of the video amplifier are directly connected to each other. The entire video amplifier is covered by negative feedback through the resistor R9, which simultaneously stabilizes the operation mode of the T3-T5 transistors. In addition, each of the stages of the video amplifier is covered by negative feedback through a resistor in the emitter circuit.

Structurally, the path is made on a fiberglass board, foil-coated on both sides (see Fig. 2). The foil on the side of the parts acts as a screen, and when installing the board, the foil is soldered to the chassis in several places. The board has capacitors KD-16 (C4, C8, C9), K52-1 or K53 (C10) and KM (the rest), resistors MT-0,05 (R9) and MLT-0.125 or KIM-0,05 (the rest ). Coils L1 and L3 are wound on frames with a diameter of 5 mm and contain 6 turns of PEL 0,62 wire. Coil L2 is wound over L1 and contains 3 turns of PEL 0,31 wire. Winding of all coils - ordinary. They are tuned with M4 carbonyl cores.

Image path for miniature TV
Ris.2

When establishing the resistor R9 is selected as follows. so that the voltage at the collector of transistor T5 with a short-circuited resistor R7 is +13 V.

The reception quality is highly dependent on where the TV is installed. Antenna with a small distance from the television center (up to 6 km) can serve as a piece of installation wire. Its length (10-30 cm) is selected such that with increased screen brightness and vertical movement of frames, the image of the horizontal scanning quenching pulse between frames is slightly lighter than the image of its sync pulse.

Note

Given the details specified in the article, the image path ensures the reception of signals on the first channel with satisfactory quality. Its design is quite original. For reception on channels 2-5, it is necessary to select the number of turns of coils and capacitors C4 and C8. However, in this case, the sensitivity of the path will deteriorate (up to 1 mV on the fifth channel), which will lead to a significant decrease in image quality.

One of the drawbacks of the path is the lack of an automatic gain control device, which is especially necessary in a miniature TV set operating in changing reception conditions. Such an adjustment would be useful for improving the image quality, which is somewhat reduced due to the loss of the DC component of the detected signal at the input of the video amplifier.

The disadvantage of the path is also that it is designed to receive signals on only one television channel.

Despite these shortcomings, the editors hope that the described image path will be of interest to radio amateurs constructing simple miniature TVs.

Author: B. Pavlov, Lvov; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Телевидение.

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