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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Only the most experienced can assemble an oscilloscope in their home workshop. There are many reasons for this: the complexity of the electronic circuit, scarce parts, a large amount of work ... The industry, however, produces two or three models for radio amateurs, but they are quite expensive, and they are rarely found in stores.

We offer a simple set-top box with which you can turn any industrial or TV set assembled from the Republic of Kazakhstan into a simple oscilloscope. In this case, you will not have to make any changes to the TV circuit, it is enough to connect the output of the set-top box to the antenna input of the TV, and an image of the signal under study will appear on the screen.

Let's now get acquainted with the basic principles of operation of the oscilloscope attachment. With the help of a blocking generator and a pulse shaper, the set-top box generates vertical and horizontal sync pulses. When added together, they form a complete television picture signal. When the signal under study is applied to the output of the attachment, its periodically changing voltage controls the illumination of individual segments of the raster lines. Thus, the set-top box generates a complete television video signal with a picture, which is then fed to the input of the VHF generator and modulates its radiation in frequency. The generator itself operates in the range of the second television channel, so if the output of the set-top box is connected to the antenna input of the TV tuned to the same channel, then an image of the signal under study will appear on the screen.

As you have already noticed, two voltages are applied to the input of the set-top box - the investigated signal Usign and an alternating voltage of 6,3 V synchronization with a vertical scan with a frequency of 50 Hz. It can be removed from the filament winding of any mains transformer or from a special additional winding of the power supply transformer of the set-top box.

Fig. 1

Variable voltage with a frequency of 50 Hz is supplied to the pulse shaper, made on transistors VT6 and VT7. Transistor VT6 forms a voltage amplification cascade. As soon as the amplitude of the synchronizing voltage exceeds a certain level, the transistor enters saturation mode and turns off, i.e., it operates simultaneously in two modes - amplifying and switching. Then, through a differentiating chain of capacitor C11 and resistor R13, the synchronization voltage is supplied to the base of the VT7 transistor, which generates frame sync pulses according to the television standard.

Horizontal sync pulses are generated by a transistor blocking oscillator based on a VT8 transistor with inductive positive feedback. The sawtooth form of horizontal sync pulses is obtained due to the periodic charge-discharge process of the capacitor C13, which is included in the winding circuit II of the blocking transformer T1. From it, horizontal sync pulses through resistor R19 and capacitor C15 are fed to the base of transistor VT3.

The signal under study is amplified by cascades on transistors VT1, VT2 and VT3. The high gain of these stages is determined by the values ​​of the resistor R3 and capacitor C3, which are included in the positive feedback circuit. The periodically changing voltage of the signal under study controls the brightness of the illuminated lines - as if simulating horizontal sync pulses. The VT4 transistor is connected according to the emitter follower circuit and works as a current amplifier.

The full signal of the television image, formed by the set-top box, is fed to the input of the VHF generator, assembled on the transistor VT5, which models it in frequency. The output signal of the set-top box is taken from the voltage divider from resistors R9 and R10. With the ratings of the parts indicated on the diagram, this VHF generator operates in the frequency range of the second television channel of meter waves.

The scheme of the prefix itself is mounted on a printed circuit board of textolite or getinaks foiled on one side. The location of the printed conductors is shown in Figure 2, and the radio components on the board are shown in Figure 3.

Fig. 2

Fig. 3

The set-top box is powered by a stabilized voltage source of 12 V, which can be used as a power supply (see Fig. 4), using a TVK series transformer. The zener diode VD1 sets the stabilization voltage, which is fed to the base of a powerful transistor VT1 operating in the current amplifier mode. Resistor R1 sets the base current, and capacitor C2 "clean" filters the output voltage.

Fig. 4

Instead of the D814D zener diode, you can use D813 or KS512 with any letter index. The transistor can be replaced with any other npn with a power dissipation rating of at least 1W. The power supply is mounted on a printed circuit or breadboard. Mount the transistor VT1 on a radiator with a total area of ​​15-20 cm2.

Wind the transformer T1 on a ring ferrite core measuring 10x14x2 mm. Winding I contains 100 turns, II - 35, and III - 90 turns of PEL-0,1 wire. The transformer winding procedure can be simplified if the ferrite core is first carefully split into two parts, winding windings on them, and then glued with BF-2 or Moment glue. Coil L1 of the oscillatory circuit of the VHF generator contains only 6 turns of copper wire in an enamel sheath 0,6-0,8 mm thick and is wound on a plastic frame with a ferrite core, for example, from the contours of an old TV.

Transistors VT1-VT8 - KT315, diodes VD1-VD6 - KD522.

The printed circuit board of the set-top box must be placed in a housing made of shielding material - brass or aluminum, connecting a common wire to the housing.

If the case is made of wood or plastic, glue its inner surface with copper or aluminum foil and connect it to the common wire of the circuit.

On the front panel of the housing, place the terminals for connecting the synchronization voltage and the signal under test. They can only be connected to the board with a shielded wire.

The capabilities of the set-top box will expand significantly if you carry out the next revision. For example, if you replace the resistor with another one, with a resistance of 50 ohms, and turn on a variable resistance of 100 ohms in series with it, you can adjust the amplitude of the output television signal of the set-top box. By changing the resistance of resistors R15 and R8, you can control the size of the image vertically and horizontally.

The output of the set-top box is connected to the antenna jack of the TV only with a coaxial cable of the PK-75 type. Solder its braids with the common wire bus. After soldering, the cable itself must be fixed to the board using tin or aluminum clamps. For the convenience of connecting to a coaxial cable, you can solder the antenna plug.

When all the parts are installed on the board and soldered, carefully check the correct installation, paying special attention to the gaps between the current-carrying tracks of the board. If jumpers formed between them from solder streaks, they must be carefully removed with a rosin flux or simply scratched with a sharp awl. And if everything is in order, you can start testing.

First of all, disconnect the TV from the antenna and connect it to the set-top box. Set the TV program selector to the second channel. Then set the vertical and horizontal scan rates. A raster should appear on the TV screen. Synchronization of the TV from a properly assembled set-top box, as a rule, turns out to be very stable, so if lines or frame frames suddenly run across the screen, then the error must be sought in the installation. You may need to more accurately select the resistor values ​​in the sweep generator circuit or rewind the blocking transformer. It may also happen that when the set-top box is connected, there will be no image on the TV screen at all. In this case, it is necessary to check the transistor of the VHF generator again. You can fine-tune it to the frequency of the second television channel by rotating the ferrite core of the L1 coil or simply by changing the distance between the turns (winding pitch). Finally, the setting of the VHF generator is checked by the clarity of the center line on the TV screen in the absence of the signal under study at the input of the set-top box. If the line remains fuzzy all the time, then stray interference is most likely to blame, which will disappear as soon as you ground the set-top box.

The sensitivity of the set-top box is such that the maximum range of the image on the screen is obtained with an amplitude of the signal under study of about 0,3 V. And in order to investigate signals of greater amplitude, you will have to make an attenuator (attenuator) based on a simple voltage divider. The formulas and the diagram in Figure 5 will help you calculate it correctly. To study weak signals, you can connect a sensitive VLF with an emitter follower to the input.

Fig. 5

Your homemade oscilloscope will also come in handy for measuring the voltage of the signal under study. In order to turn the console into a voltmeter, it is enough to fix a scale grid on the screen. It can be made from a sheet of plexiglass, and draw lines with a compass needle. For clarity, paint over the scratched grooves with a black or brown felt-tip pen. Residues of paint from the surface of plexiglass are easily removed with a cotton swab dipped in cologne. When the grid is ready, apply a voltage with a known amplitude to the input of the attachment and fix its value on the scale grid. This is how the calibration is done.

And now let's talk about another device that should be in every home radio laboratory - a functional generator. It can be assembled from a special RK "Start 7218" "Function generator". It generates sinusoidal, triangular and rectangular waveforms. With it, you can adjust and take characteristics from ULF of all types and purposes, tape recorders, TVs, radios - in a word, from almost any industrial and home-made household radio equipment. Such a generator is also used to set up devices assembled on digital microcircuits - rectangular pulses will come in handy here.

The PK kit includes a printed circuit board, switches and a complete set of parts. You have to make the case and the power supply for it yourself. The technical characteristics of the RK are as follows: Operating frequency range - 20-135000 Hz.

Number of frequency bands - 4.

The output voltage of the sinusoidal signal is 0,3 V (effective value).

The output voltage of the triangular waveform is 1,8 V (peak value). Kg - no more than 6%. Output impedance - 600 Ohm. Supply voltage - 8-12 mV. Current consumption - 60 mA.

RK "Function Generator" is assembled on a K155LA chip, which contains four 2I-NOT open-collector elements. It differs from the similar K155LAZ only in that it is designed for a higher load or, in other words, power. All four elements of the microcircuit in the RK are included as inverters. The main components of the generator are made on them: a master oscillator, an integrator, a comparator and a linear amplifier. Between them, RC chains are included that set the frequency and shape of the pulses. Low current consumption and standard voltage allow you to connect any power supply unit from portable radio equipment to the RC.

It must be said that the possibilities of this RK are far from being fully used. If you add another capacitor to the group of frequency-setting capacitors C1-C4, then the operating range of the generator will increase from megahertz. True, you will have to take a capacitor with a capacity of 50-100 pF and turn it on so that when the SA1 switch is off, it determines the frequency of the generator.

Author: S. Mukhamedov

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