The set-top box turns any TV into an oscilloscope with a large screen. You can observe low-frequency oscillations on it, and with the help of a sweep frequency generator (MSG) you can visually tune the IF amplifiers of radio receivers. The set-top box can be considered as a miniature television transmitter. Despite the relatively simple circuit, this transmitter produces a complete television signal, which differs from the standard one only in the absence of equalizing pulses.
Frame sync pulses are generated from the reference sinusoidal voltage by the limiting amplifier VT1, the differentiating circuit R8C4 and the threshold amplifier on VT4. Their duration is about 1.9 ms. The blocking generator (on transistor VT5) generates horizontal sync pulses. These are not the main pulses of the blocking generator, but surges of the collector voltage that occur immediately after the main ones. A diode VD3 is connected between the collectors of transistors VT4 and VT5. At the moment the main pulse is generated, the collector of transistor VT4 is closed to the chassis through an open transistor VT5 and diode VD3. As a result, insets appear in the vertical sync pulses, which, as required, precede the horizontal sync pulses. The windings of the blocking generator transformer VT1 are wound on a toroidal core made of oxypherite (F-1000). The outer diameter of the core is 10 mm, thickness 2 mm. Windings I and III each contain 100 turns, and winding II contains 30 turns of PELSHO o0.1 wire. At the beginning of the horizontal scanning period, the voltage pulse of the blocking generator quickly charges capacitor C6 through the diode VD2. During the rest of the period it is slowly discharged through resistor R6. The resulting sawtooth voltage is supplied to the base of transistor VT2. Here it is added to the input voltage. The three-stage amplifier, due to its high gain (50,000-100,000), operates practically in relay mode, characterized by a certain response threshold. The attachment parameters are chosen such that in the absence of the voltage being tested, the center line is in the center of the screen. If necessary, the image on the screen can be shifted in one direction or another by changing the resistance of resistor R3. To improve the clarity of the line image on the TV screen, the amplifier (VT2, VT3, VT6) is covered by positive feedback from the collector of transistor VT3 to the base of transistor VT2 through capacitor C5. This significantly increases the gain in the high frequency region and therefore increases the slope of the output pulses. Visually, this manifests itself in an increased sharpness of the transition from white to black. Frame, line and video pulses are added at the input of the emitter follower VT7, which is the modulation amplifier of the VHF generator VT8. The latter is assembled according to a three-point capacitive circuit. The generation frequency must be chosen equal to the carrier frequency of the image of a free television channel. Otherwise, the set-top box may interfere with the operation of neighboring TVs. The required generation frequencies can be obtained by selecting the number of turns of coil L1.
When tuning to the second television channel (59.25 MHz), coil L1 contains 5 turns of PEV 0.6 wire, coil diameter 9 mm. The modulated RF voltage is supplied to the output of the set-top box through a divider R18-R19, which reduces the voltage to 3 mV to avoid overloading the RF path of the TV. The output of the set-top box is connected with a coaxial cable or twisted double wire to the antenna input of the TV.
Construction and setup. All parts of the set-top box, with the exception of the VHF generator, can be placed on the circuit board in any order. Parts related to the VHF generator (SP-S15, L1, VT8) must have short leads, they should be connected to each other using short conductors and grouped in one place. No shielding of the set-top box is required. If the pulse frequency of the block generator does not lie in the line frequency range of the TV, it is necessary to enter it into this range by changing the resistance of resistor R14 within small limits. It should be noted that the synchronization of TV scans from the set-top box is usually very stable, so poor synchronization when setting up the set-top box indicates some kind of installation error. To achieve precise tuning of the VHF generator of the set-top box to the selected television channel, you have to stretch or compress the turns of the winding of the L1 coil, i.e. change the winding pitch. When set correctly, the line on the screen is sharply defined. The parameters of the set-top box are selected so that the largest image size on the TV screen corresponds to an input voltage of about 0.3 V. The sensitivity of the set-top box can be adjusted by changing the resistance of resistor R2. To test sensitivity, an alternating voltage of a known magnitude or from a sound generator is supplied to the input.
The attachment (see picture) turns any TV into an oscilloscope with a large screen. You can observe low-frequency oscillations on it, and with the help of a sweep frequency generator (MSG) you can visually tune the IF amplifiers of radio receivers. The set-top box can be considered as a miniature television transmitter. Despite the relatively simple circuit, this transmitter produces a complete television signal, which differs from the standard one only in the absence of equalizing pulses. Frame sync pulses are generated from the reference sinusoidal voltage by the limiting amplifier VT1, the differentiating circuit R8C4 and the threshold amplifier on VT4. Their duration is about 1.9 ms. The blocking generator (on transistor VT5) generates horizontal sync pulses. These are not the main pulses of the blocking generator, but surges of the collector voltage that occur immediately after the main ones. A diode VD3 is connected between the collectors of transistors VT4 and VT5. At the moment the main pulse is generated, the collector of transistor VT4 is closed to the chassis through an open transistor VT5 and diode VD3. As a result, insets appear in the vertical sync pulses, which, as required, precede the horizontal sync pulses. The windings of the blocking generator transformer VT1 are wound on a toroidal core made of oxypherite (F-1000). The outer diameter of the core is 10 mm, thickness 2 mm. Windings I and III each contain 100 turns, and winding II contains 30 turns of PELSHO o0.1 wire. At the beginning of the horizontal scanning period, the voltage pulse of the blocking generator quickly charges capacitor C6 through the diode VD2. During the rest of the period it is slowly discharged through resistor R6. The resulting sawtooth voltage is supplied to the base of transistor VT2. Here it is added to the input voltage. The three-stage amplifier, due to its high gain (50,000-100,000), operates practically in relay mode, characterized by a certain response threshold. The attachment parameters are chosen such that in the absence of the voltage being tested, the center line is in the center of the screen. If necessary, the image on the screen can be shifted in one direction or another by changing the resistance of resistor R3. To improve the clarity of the line image on the TV screen, the amplifier (VT2, VT3, VT6) is covered by positive feedback from the collector of transistor VT3 to the base of transistor VT2 through capacitor C5. This significantly increases the gain in the high frequency region and therefore increases the slope of the output pulses. Visually, this manifests itself in an increased sharpness of the transition from white to black. Frame, line and video pulses are added at the input of the emitter repeater VT7, which is the modulation amplifier of the VHF generator VT8. The latter is assembled according to a three-point capacitive circuit. The generation frequency must be chosen equal to the carrier frequency of the image of a free television channel. Otherwise, the set-top box may interfere with the operation of neighboring TVs. The required generation frequencies can be obtained by selecting the number of turns of coil L1.
When tuning to the second television channel (59.25 MHz), coil L1 contains 5 turns of PEV 0.6 wire, coil diameter 9 mm. The modulated RF voltage is supplied to the output of the set-top box through a divider R18-R19, which reduces the voltage to 3 mV to avoid overloading the RF path of the TV. The output of the set-top box is connected with a coaxial cable or twisted double wire to the antenna input of the TV.
Construction and setup. All parts of the set-top box, with the exception of the VHF generator, can be placed on the circuit board in any order. Parts related to the VHF generator (SP-S15, L1, VT8) must have short leads, they should be connected to each other using short conductors and grouped in one place. No shielding of the set-top box is required. If the pulse frequency of the block generator does not lie in the line frequency range of the TV, it is necessary to enter it into this range by changing the resistance of resistor R14 within small limits. It should be noted that the synchronization of TV scans from the set-top box is usually very stable, so poor synchronization when setting up the set-top box indicates some kind of installation error. To achieve precise tuning of the VHF generator of the set-top box to the selected television channel, you have to stretch or compress the turns of the winding of the L1 coil, i.e. change the winding pitch. When set correctly, the line on the screen is sharply defined. The parameters of the set-top box are selected so that the largest image size on the TV screen corresponds to an input voltage of about 0.3 V. The sensitivity of the set-top box can be adjusted by changing the resistance of resistor R2. To test sensitivity, an alternating voltage of a known magnitude or from a sound generator is supplied to the input.
RADUAMATOR 6/99, Shronin, Kremenchug, Poltava region.
It remains to add that this connection diagram is not suitable for all TV models and is more useful for people who know how to handle circuitry and have experience in electronics. But the idea itself contains many interesting points.
To get it to draw an image on a very thin line vertically deflected by the input signal, you need to adjust the number of turns of the screen coils. It is also important to “work” with the inductor coil. Its impedance depends on frequency. The higher the frequency, the more difficult it will be to display it on the screen. With an outer diameter of the toroidal core of 10 mm and a thickness of 2 mm, windings I and III should each contain 100 turns of PELSHO 0.1 wire, and winding II should contain 30 turns.
It’s also worth remembering that the signal on a TV is mathematically integrated. This causes the input square wave to appear as a triangle wave on the screen, and the input triangle wave as a sine wave. This only applies to the image, not the sound. Sine waves will be displayed without distortion. The phenomenon will not be as noticeable on very old TVs that are capable of displaying white noise or a blue screen when there is no signal, rather than automatically turning off the image.
The horizontal coil connection wires are now removed from the main PCB. Do not forget that you will have to deal with a frequency of 30,000 Hz and a voltage of more than 15,000 volts. The future oscilloscope does not need them. Before touching, they must be short-circuited, then well insulated and placed inside the case so that they do not touch anything after turning on the device. So, the 60 Hz vertical marking line is ready. To obtain the same horizontal line of 60 Hz, we solder the two remaining wires going to the vertical coil to the horizontal one. And the vertical one will become the input of the oscilloscope for connecting the amplifier circuit.
To implement it we will need: a TL082 microassembly, including 2 op-amps, a pair of transistors (for example, 41NPN/42PNP), an LM317 power regulator, a Pole rotary switch, a 1 mOhm potentiometer, two 10 kOhm trimers, 4 1A diodes, a transformer for 30 VAC, 1000 µF 50 V electrolyte, two 470 µF 16 V electrolytes and 5 resistors (10 Ohm, 220 Ohm, 1 kOhm, 100 kOhm and 10 mOhm).
The finished circuit requires mandatory calibration. The power regulator is designed for a difference of 30 V, so the op amp will typically output +15/-15 V, but for good filtering its output should be a few volts lower than the voltage across the 1000 uF capacitor. For this purpose, there is trimmer 1. The output of the circuit is connected to the horizontal deflection coil. Music passed through the circuit begins to be “cut off” at the top/bottom. To avoid this, trimmer 2 is adjusted until the tops of the clips touch the edges of the screen. This will lower the voltage and prevent the transistors from overloading the RF path of the device (burning the deflection coil).
Now you can connect the built-in speaker system to the TV output. If the volume is excessive, a large load resistance is added (for example, 10 Ohm 1 W); if there is insufficient sound, the load resistance is placed on the deflection coil, after which the latter is recalibrated. To protect yourself from unnecessary annoying beeps while scanning for the desired input signal, you can install a switch on the speaker.
After connecting the transformer to the circuit, connect S1 and S2 respectively, run the input wires through the hole in the body of the television receiver, connect the output of the circuit to the speaker and deflection coil. A minimum wire length should be used in all connections made to reduce leaky loop inductance. All that remains is to find a convenient place to install S1 and S2, close the back cover and start the test drive.
The ability to amplify the incoming signal is a great feature if you don't need it to be absolutely precise. The 60 Hz noise amplified by the circuit can still be detected with reasonable accuracy. But this phenomenon is also caused by the stray inductance of the input wire. Only shielded grounding of all parts of the circuit can reduce interference.
You can clearly view the operation of the Internet channel on the device. A dedicated home line (120 VAC) was used as an input signal for this, and, having shown its “picture”, the device still works.
Descriptions of oscillographic TV attachments have already been published on the pages of the magazine ("Radio", 1959, No. 1; 1965, No. 8, etc.). However, unlike them, the proposed set-top box does not require intervention in the TV circuit (it is connected to the TV antenna socket). Together with a sweep frequency generator, it can be used to set up IF amplifiers for radio receivers.
The set-top box (Fig. 1 and 2) can be considered as a miniature television transmitter. Despite the relative simplicity of the circuit, this transmitter generates a complete television signal, which differs from the standard signal only in the absence of equalizing pulses.
Frame sync pulses are generated from an alternating sinusoidal voltage by a limiting amplifier (T1), a differentiating circuit R8C4 and a threshold amplifier (T1). Their duration is about 1.9 ms.
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The blocking generator on the transistor Гз generates horizontal sync pulses. These are not the main pulses of the blocking generator, but surges of the collector voltage that occur immediately after the main ones. A diode D3 is connected between the collectors of transistors T4 and T5. At the moment the main pulse is generated, the collector of transistor T4 is closed to the chassis through the open transistor T5 and diode D3. As a result, insets appear in the vertical sync pulses, which, as required, precede the horizontal sync pulses. The windings of the transformer Tr1 of the blocking generator are wound on a toroidal core made of oxyphere (H=1000). The outer diameter of the core is 10 mm, a. thickness 2 mm. Windings I and III contain 100 turns each, ll - 30 turns of PELSHO 0.1 wire.
At the beginning of the horizontal scan period, the voltage pulse of the blocking generator quickly charges capacitor C5 through diode D2. During the rest of the period it is slowly discharged through resistor R6. The resulting sawtooth voltage is supplied to the base of transistor T2. Here it is added to the oscilloscope voltage.
The three-stage amplifier (T2, T3, T6), due to its high gain (50000-100000), operates practically in relay mode, characterized by a certain response threshold. Since an oscillograph voltage is superimposed on the input sawtooth voltage of the amplifier, the switching of the output voltage coincides in time with the moment when the total voltage passes through the response threshold of the amplifier. The rise time of the front of the voltage pulses at the output of the amplifier is small, again due to the high gain. The moment of appearance of these pulses during the horizontal scan period is determined by the instantaneous value of the oscilloscope voltage. The attachment parameters are chosen such that in the absence of the voltage being tested, the center line is in the center of the screen. If necessary, the image on the screen can be shifted in one direction or another by changing the resistance of resistor R3.
To improve the clarity of the line image on the TV screen, the amplifier (T2, T3, T6) is covered by positive feedback from the collector of transistor T3 to the base of transistor T2 through capacitor C6. This significantly increases the gain in the high frequency region and therefore increases the slope of the output pulses. Visually, this manifests itself in an increased sharpness of the transition from white to black.
Frame, line and video pulses are added at the input of the emitter follower (T7), which is the modulation amplifier of the VHF generator (T8). The latter is assembled according to a three-point capacitive circuit. The generation frequency must be chosen equal to the carrier frequency of the image of a free television channel. Otherwise, the set-top box may interfere with the operation of neighboring TVs. The required generation frequencies can be achieved by selecting the number of turns of coil L1. When tuning to the second television channel (59.25 MHz), coil L1 contains 5 turns of PEV 0.6 wire, coil diameter 9 mm.
The modulated RF voltage is supplied to the output of the set-top box through a divider R18-R19, which reduces the voltage to 3 mV to avoid overloading the RF path of the TV.
The output of the set-top box is connected with a coaxial cable or twisted double wire to the antenna input of the TV.
Construction and setup. All parts of the set-top box, with the exception of the VHF generator, can be placed on the circuit board in any order. Parts related to the VHF generator (C11-C15, L1, T8) must have short leads, be connected to each other by short conductors, and in addition, they should be grouped in one place.
No shielding of the console is required. After turning it on, you need to adjust the TV as usual using the adjustment knobs (frame rate, line frequency, contrast). If the frequency of the pulses of the blocking generator of the set-top box does not lie in the range of adjusting the line frequency of the TV, you need to enter it into this range by changing the resistance of resistor R14 within small limits. It should be noted that the synchronization of the TV scans from the set-top box is usually very stable, so poor synchronization when setting up the set-top box indicates some kind of installation error. To achieve precise tuning of the VHF generator of the set-top box to the selected television channel, you have to stretch or compress the turns of the winding of coil L1 (that is, change the winding pitch). When set correctly, the line on the screen is sharply defined.
The parameters of the set-top box are selected so that the largest range of images on the TV screen corresponds to an input voltage of about 0.3 V. The sensitivity of the set-top box can be adjusted by changing the resistance of resistor R2.
To check the sensitivity of the set-top box, an alternating voltage of a known magnitude is supplied to its input either from a power source with a voltage of 6 V, a frequency of 50 Hz through a divider, or from a sound generator.
If desired, the input impedance and sensitivity of the set-top box can be significantly increased by connecting to it a conventional low-frequency amplifier with an emitter follower at the input.
Assemble an oscilloscope Only the most experienced can do it 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 available in stores.
We offer a simple attachment with which you can turn your TV into a simple oscilloscope. In this case, you do not have to make any changes to the TV circuit; you just need to connect the output of the set-top box to the antenna input of the TV, and an image of the signal being studied will appear on the screen.
Let's now get acquainted with the basic principles of operation of the oscilloscope attachment. Using a blocking generator and a pulse shaper, the set-top box produces vertical and horizontal sync pulses. When added together, they form a complete television image signal. When the signal under study is supplied to the output of the set-top box, 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 a television set tuned to the same channel, then an image of the signal being studied will appear on the screen.
As you have already noticed, two voltages are supplied to the input of the set-top box - the test signal Usign and an alternating voltage of 6.3 V synchronizing frame scan with a frequency of 50 Hz. It can be removed from the filament winding of any network transformer or from a special additional winding of the transformer of the set-top box power supply.
An alternating voltage with a frequency of 50 Hz is supplied to a pulse shaper made on transistors VT6 and VT7. Transistor VT6 forms a voltage amplification stage. 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 - amplification and switching. Then, through a differentiating chain of capacitor C11 and resistor R13, the synchronization voltage is supplied to the base of transistor VT7, which generates frame sync pulses according to the television standard.
Horizontal sync pulses are generated by a transistor blocking generator based on a VT8 transistor with inductive positive feedback. The sawtooth shape of the horizontal sync pulses is obtained due to the periodic process of charge-discharge of capacitor C13, connected to the circuit of winding II of the blocking transformer T1. From it, horizontal sync pulses through resistor R19 and capacitor C15 are supplied 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 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. Transistor VT4 is connected according to the emitter follower circuit and works as a current amplifier.
The complete television image signal generated by the set-top box is fed to the input of a VHF generator assembled on a VT5 transistor, which models it by frequency. The output signal of the set-top box is removed from the voltage divider from resistors R9 and R10. With the component ratings indicated in the diagram, this VHF generator operates in the frequency range of the second television channel of meter waves.
The set-top box is powered by a stabilized 12 V voltage source, which can be used as the power supply described in Appendix No. 2 for 1987. However, it can be assembled according to a simplified scheme (see Fig. 4), using a TVK series transformer. Zener diode VD1 sets the stabilization voltage, which is supplied to the base of the powerful transistor VT1, operating in current amplifier mode. Resistor R1 sets the base current, and capacitor C2 “white” filters the output voltage.
Instead of the D814D zener diode, you can use D813 or KS512 with any letter index. The transistor can be replaced with any other n-p-n with a rated power dissipation of at least 1 W. The power supply is mounted on a printed circuit board or breadboard. Mount transistor VT1 on a radiator with a total area of 15-20 cm 2.
The circuit of the set-top box itself is mounted on a printed circuit board with PCB foil on one side or getinax. The location of printed conductors is shown in Figure 2, and the radio components on the board are shown in Figure 3.
Wind 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 procedure for winding a transformer can be simplified if the ferrite core is first carefully split into two parts, windings are wound on them, and then glued with BF-2 or Moment glue. Coil L1 of the oscillating circuit of the VHF generator contains only 6 turns of copper wire in an enamel shell 0.6-0.8 mm thick and is wound on a plastic frame with a ferrite core, for example, from the circuits 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 the 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 case, place the terminals for connecting the synchronization voltage and the signal under study. They can only be connected to the board with a shielded wire.
The capabilities of the console will expand significantly if you carry out the following modification. For example, if you replace the resistor with another one with a resistance of 50 Ohms, and connect 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 image size vertically and horizontally.
The output of the set-top box is connected to the antenna socket of the TV only with a coaxial cable of the RK-75 type. Solder its braids to the common wire bus. After soldering, the cable itself must be secured to the board using clamps made of tin or aluminum. For ease of connection, you can solder an antenna plug to the coaxial cable.
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 bridges from solder drips have formed between them, they must be carefully removed using rosin flux or simply scratched with a sharp awl. And if everything is in order, you can start testing.
The sensitivity of the set-top box is such that the maximum range of the image on the screen is obtained when the amplitude of the signal under study is about 0.3 V. And in order to study signals of larger amplitude, you will have to make an attenurator (attenuator) based on a simple voltage divider. The formulas and diagram in Figure 5 will help you calculate it correctly. To study weak signals, you can connect a sensitive ULF with an emitter follower to the input.
Yours will come in handy homemade oscilloscope and to measure the voltage of the signal under study. In order to turn the set-top box into a voltmeter, just attach a scale grid to the screen. It can be made from a sheet of plexiglass, and the lines can be drawn with a compass needle. For clarity, color the scratched grooves with a black or brown felt-tip pen. Remains of paint from the surface of plexiglass can be 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 console and fix its value on the scale grid. This is how calibration is carried out.
Young Technician For skilled hands 1988 No. 9
