Miniature TV. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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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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This TV is designed for individual viewing of TV programs on the first, second or third channels within a radius of three to five kilometers from the television center. It uses an oscilloscope cathode ray tube. The size of the image is 30 mm diagonally, the clarity of the image is 100-150 lines, the color of the screen glow is green.

Despite such a small image size and its low definition, the TV allows you to follow the main course of the shown action. The soundtrack is listened to on a miniature telephone TM-2A, and the wire connecting the telephone to the TV is used simultaneously as a receiving antenna. TV dimensions 135X100x45 mm, weight with power supplies - 500 g. The TV is powered by three STS-1,5 silver-zinc batteries connected in series, placed in its case. These batteries provide normal operation of the TV for 1-1,5 hours. The power consumed by the TV is about 4 watts (of which almost 3 watts is spent on heating the cathode ray tube). The TV is assembled on 15 transistors, 14 semiconductor diodes and a ZL01I cathode ray tube.

The TV is mounted on three printed circuit boards. To reduce the size, and also because of the small margin for image brightness and sensitivity, the TV does not contain any controls, except for the power switch and the knobs for adjusting the line and frame rates, which are brought out under the slot. The appearance of the TV is shown in fig. 1.

Miniature TV

The schematic diagram is shown in Fig. 2

The receiving part is assembled according to the direct amplification circuit (the use of a superheterodyne circuit would require the introduction of an additional control element - the local oscillator tuning knob), and the main amplification is performed after detection.

The RF amplifier is two-stage on T1T2 transistors, connected according to a common base circuit. It is intended primarily to provide . TV selectivity. After detection, the video signal is amplified by a three-stage video amplifier based on T6-T8 transistors. All three stages of the video amplifier are galvanically interconnected and covered by a deep negative DC feedback (resistors R13, R15), which stabilizes the transistor modes.

The video amplifier provides a maximum output voltage of 50 V in the frequency band of 1 Hz - 32 MHz at a noise level of 20 dB. It is easy to set up, stable in operation and takes up little space. In this case, it does not make sense to expand the bandwidth of the video amplifier by more than 1 MHz, since, firstly, due to the imperfection of the ZL01I cathode ray tube focusing and its rather long afterglow, it will still not be possible to realize a higher image clarity, and, secondly, this has led to would lead to an unacceptably high energy consumption for powering the TV. The video amplifier draws 3,5mA at 24V.

The frequency-modulated carrier frequency of the sound accompaniment enters the detuned circuit L3C7, where the frequency modulation is converted into amplitude modulation, which is detected by the amplitude detector on diode D2. The detected audio signal is amplified in a three-stage low-frequency amplifier based on T3-T5 transistors, assembled according to a scheme similar to a video amplifier. From its output, the voltage of the sound accompaniment is fed to a miniature telephone.

Since the conductors connecting the phone to the TV are simultaneously used as an antenna, they are decoupled in LF and HF using chokes Dr1 and Dr2, as well as capacitors C1, C2 and C10.

The amplitude sync pulse selector is assembled on a T9 transistor, the vertical sync pulses are separated from the horizontal ones by a simple integrating circuit R23C15.

The line-scanning oscillators on the T10 transistor and the vertical-scanning oscillators on the T12 transistor are assembled according to similar schemes and are conventional blocking oscillators. The horizontal output stage has no features. In the vertical scan output stage, the capacitor C21 is charged during the forward stroke through the resistors R28 and R29 from a constant voltage source of 550. V and is discharged during the reverse stroke through the transistor T13, which is periodically opened by the master oscillator pulses. At the same time, on the voltage across capacitor C21 and transistor T13 has a swing of about 100 V.

To obtain all the constant voltages necessary to power the TV, including high voltage for the cathode ray tube, a push-pull converter is assembled on T14T15 transistors with voltage multiplication. The constant voltages at the cathode and the tube modulator are chosen so as to provide acceptable contrast and brightness of the image.

The main components of the TV are mounted on two printed circuit boards with dimensions of 120x67 mm. On one board there is a voltage converter, a high-frequency amplifier and an audio bass amplifier, on the other - a video amplifier and sweep generators. These boards are screwed onto a metal cassette that houses the batteries. In the space between the scanning units and the voltage converter, a printed circuit board of the sync signal selector with dimensions of 45X25 mm is placed.

The body of the TV is made up of two halves, extruded hot from vinyl sheet 1,5 mm thick.

When assembling the TV, parts were used as small as possible. Capacitors C8, C9, C11-C13 type EMI, C14, C25, C26-K50-3, C29-C33 - MBM for 160 V, C34, C35 - BMT-1 for 400 V. Resistors R24 and R26 - SPO-0,15, 13. The T100 transistor is specially selected - it must withstand a reverse voltage of at least 26 V. It is not recommended to use the P403 germanium transistor here, as it quickly fails. The remaining transistors do not require any selection. In the RF amplifier, transistors P423, P416, P3 can be used (except for those indicated in the diagram), in sweep generators (except for T39) and in the MP41-MPXNUMX converter.

The inductors Dr1 and Dr2 of the TV are wound on ferrite rods used in the IF circuits of the Neiva radio receiver (Jupiter, Signal) and contain 50 turns of PELSHO 0,16 wire each. The data of the coils L1, L2, L3 are summarized in Table. 1, and transformers - in table. 2.

Table 1
NN TV channels Coil L1 Coil L2 Coil L3,
number of turns the wire number of turns the wire number of turns the wire
1 12 PELSHO 0,41 13 PELSHO 0,31 11 PELSHO 0.41
2 9 " 12 PELSHO 0,41 9 "
3 6 " 8 " 6 "

The coils are wound in one layer turn to turn on plastic frames 3 mm in diameter and 9 mm long. They are placed in rectangular screens 9x9x14 mm in size and are tuned with 600НН ferrite cores with a diameter of 2,3 mm and a length of 9 mm.

Table 2
NN transformers
according to the scheme Core N windings Wire: Brand
and diameter, mm Number of turns
Tr1 Permalloy 50N ZX6 I
II 350 + 350
110 PEL 0,12
Tr2 Too I
II 350
2500 PEV 0,06
Tr3 Also I
II 1200 + 1200
390 "
tp4 ferrite ring
1000NN K20x10x5 I
II
III
IV 15 + 15
20
100
200 PELSHO 0,41
"
PELSHO 0,12
"

Author: Yu.Reutov; Publication: N. Bolshakov, rf.atnn.ru

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