The set-top box (see picture) turns any TV into a large-screen oscilloscope. On it, you can observe low-frequency oscillations, and with the help of a sweeping frequency generator (GKCH), visually adjust the IF amplifiers of radio receivers. The set-top box can be thought of as a miniature TV transmitter. Despite the relatively simple circuit, this transmitter generates a complete television signal, which differs from the standard only in the absence of equalizing pulses.
Frame sync pulses are formed from an alternating sinusoidal voltage by an amplifier-limiter VT1, a differentiating circuit R8C4 and a threshold amplifier at VT4. Their duration is about 1.9 ms. The blocking generator (on the VT5 transistor) generates horizontal sync pulses. These are minor pulses of the blocking generator, but collector voltage surges that occur immediately after the main ones.
An isolation diode VD3 is connected between the collectors of transistors VT4 and VT5. At the moment of generation of the main pulse, the collector of the VT4 transistor is closed to the chassis through the open VT5 transistor and the VD3 diode. As a result, frames appear in the vertical sync pulses, which, as required, precede the horizontal sync pulses. The VT1 transformer windings of the blocking generator are wound on a toroidal oxyferite core (F-1000). The outer diameter of the core is 10 mm, the thickness is 2 mm. Windings I and III each contain 100 turns, and winding II. 30 turns of wire PELSHO 0.1.
At the beginning of the horizontal sweep period, a voltage pulse of the blocking generator quickly charges the capacitor C6 through the diode VD2. During the rest of the period, it slowly discharges through resistor R6. The resulting sawtooth voltage is fed to the base of the VT2 transistor. Here it is added to the input voltage. The three-stage amplifier, due to its high gain (50,000-100,000), operates practically in a relay mode, characterized by a certain response threshold.
The parameters of the attachment 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 the resistor R3.
To increase the clarity of the line image on the TV screen, the amplifier (VT2, VT3, VT6) is covered by positive feedback from the collector of the VT3 transistor to the base of the VT2 transistor through the capacitor C5. This significantly increases the gain in the high frequency region and therefore increases the slope of the output pulses. Visually, this is manifested in an increased sharpness of the transition from white to black. Frame, line and video pulses are added at the input of the VT7 emitter follower, which is a modulation amplifier of the VT8 VHF generator. The latter is assembled according to a three-point capacitive circuit. The generation frequency should 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 oscillation frequencies can be obtained by choosing the number of turns of the coil L1. When setting up a second television channel (59.25 MHz), the L1 coil contains 5 turns of 0.6 PEV wire, the coil diameter is 9 mm.
The modulated HF voltage is fed to the output of the set-top box through the R18.R19 divider, which reduces the voltage to 3 mV in order to avoid overloading the HF 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 adjustment. 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, VT8) must have short leads, they should be connected together with short conductors and grouped in one place.
No shielding is required for the set-top box. If the frequency of the pulses of the blocking generator does not lie in the frequency range of the lines of the TV, it is necessary to enter it in this range, changing the resistance of the resistor R14 within a small range.
It should be noted that the synchronization of the TV sweeps from the set-top box is usually very stable, therefore, poor synchronization when setting up the set-top box indicates some kind of error in editing. To achieve an accurate tuning of the VHF generator of the set-top box to the selected television channel, it is necessary to stretch or compress the turns of the coil winding L1, i.e. change the winding step. When properly configured, the line on the screen is sharply defined. The parameters at the rate 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 attachment can be adjusted by changing the resistance of the resistor R2.
To check the sensitivity, an alternating voltage of a known value is applied to the input or from a sound generator.
Literature RADIOAMATOR 5.99
The set-top box, the diagram of which is shown in Figure 1, turns any TV into a large-screen oscilloscope. On it, you can observe low-frequency oscillations, and with the help of a sweeping frequency generator (GKCH), visually adjust the IF amplifiers of radio receivers.
The set-top box can be thought of as a miniature TV transmitter. Despite the relative simplicity of the circuit, a complete television signal is formed in this transmitter, which differs from the standard signal only in the absence of equalizing pulses.
Municipal scheme
Frame sync pulses are formed from an alternating sinusoidal voltage by an amplifier-limiter T1, a differentiating circuit R8C4 and a threshold amplifier T4. Their duration is about 1.9 ms.
The blocking generator on the T5 transistor generates horizontal sync pulses. These are not the main pulses of the blocking generator, but collector voltage surges that occur immediately after the main ones. A separating diode DZ is connected between the collectors of transistors T4 and T5.
At the moment of generation of the main pulse, the collector of the transistor T4 is closed to the chassis through the open transistor T5 and the diode DZ. As a result of this, frames 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 an oxifer toroidal core (μ = 1000). The outer diameter of the core is 10 mm, the thickness is 2 mm. Windings I and III contain up to 100 turns, and II - 30 turns of PELSHO 0.1 wire.
At the beginning of the horizontal sweep period, the voltage pulse of the blocking generator quickly charges the capacitor C5 through the diode D2. During the rest of the period, it slowly discharges through resistor R6. The resulting sawtooth voltage is applied to the base of the transistor T2. Here it is added to the oscilloscope voltage.
The three-stage amplifier (T2, T3, Tb), due to the large gain factor (50,000-100,000), operates practically in a relay mode, characterized by a certain response threshold.
Rice. 1. Schematic diagram of a set-top box that turns a TV into an oscilloscope:
a - block diagram: L — block for shaping frame synchronization pulses; B - line synchronization pulse generator; С - blocking generator; D — block that converts voltage into video pulses; E - VHF generator with amplitude modulation; "Input" - terminals to which the investigated voltage is supplied: 6 - basic electrical circuit.
The parameters of the attachment 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 to one side or the other by changing the resistance of the resistor R3.
To improve the clarity of the line image on the TV screen, the amplifier (T2, TZ, Tb) is covered by positive feedback from the collector of the TZ transistor to the base of the T2 transistor through the Sb capacitor. This significantly increases the gain in the high frequency region and, therefore, increases the steepness of the leading edge of the output pulses. Visually, this is manifested 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 T1, which is a modulation amplifier for the T8 VHF generator.
The latter is assembled according to the capacitive three-point scheme. The generation frequency should 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 oscillation frequencies can be achieved by choosing the number of turns of the coil L1. When tuned to the second television channel (59, 25 MHz), the L1 coil contains 5 turns of the PEV 0.6 wire, the coil diameter is 9 mm.
The modulated RF voltage is fed to the output of the set-top box through the divider R18 - R19, which lowers the voltage to 3 mV in order to avoid overloading the RF channel 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.
Design and establishment
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 with short conductors, and, in addition, they should be grouped in one place.
No shielding is required for the set-top box. After turning it on, it is necessary, as usual, to tune the TV 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 frequency of the TV lines, it is necessary to enter it into this range, changing the resistance of the resistor R14 within small limits.
It should be noted that the synchronization of the TV sweeps from the set-top box is usually very stable, therefore, poor synchronization when setting up the set-top box indicates some kind of error in editing. In order to achieve precise tuning of the VHF generator of the set-top box to the selected television channel, it is necessary to stretch or compress the winding turns 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 maximum range of the image 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 the resistor R2.
To check the sensitivity of the attachment, an alternating voltage of a known value is applied to its input either from a power supply with a voltage of 6 V, a frequency of 50 Hz through a divider, or from a sound generator.
The input impedance and sensitivity of the set-top box, if desired, can be significantly increased by connecting to it a conventional LF amplifier with an emitter follower at the input.
The attachment, the diagram of which is shown in Fig. 76, turns any TV into a large-screen oscilloscope. On it, you can observe low-frequency oscillations, and with the help of a sweeping frequency generator (GKCH), visually adjust the IF amplifiers of radio receivers.
The set-top box can be thought of as a miniature TV transmitter. Despite the relative simplicity of the circuit, a complete television signal is formed in this transmitter, which differs from the standard signal only in the absence of equalizing pulses.
Frame sync pulses are formed from an alternating sinusoidal voltage by an amplifier-limiter 77, a differentiating circuit R8C4 and a threshold amplifier T4. Their duration is about 1.9 ms.
The blocking generator on the T5 transistor generates horizontal sync pulses. These are not the main pulses of the blocking generator, but collector voltage surges that occur immediately after the main ones. A separating diode DZ is connected between the collectors of transistors T4 and T5. At the moment of generation of the main pulse, the collector of the transistor T4 is closed to the chassis through the open transistor T5 and the diode DZ. As a result of this, frames appear in the vertical sync pulses, which, as required, precede the horizontal sync pulses. The transformer windings of the Trі blocking generator are wound on an oxifer toroidal core (c = 1000). The outer diameter of the core is 10 mm, the thickness is 2 mm. Windings I and III contain up to 100 turns, and II - 30 turns of PELSHO 0.1 wire.
At the beginning of the horizontal sweep period, the voltage pulse of the blocking generator quickly charges the capacitor C5 through the diode D2. During the rest of the period, it slowly discharges through resistor R6. The resulting sawtooth voltage is applied to the base of the transistor T2. Here it is added to the oscilloscope voltage.
The three-stage amplifier (T2, T3, Tb), due to the large gain factor (50,000-100,000), operates practically in a relay mode, characterized by a certain response threshold.
Rice. 76. A prefix that turns a TV into an oscilloscope:
a - block diagram: L — block for shaping frame synchronization pulses; B - line synchronization pulse generator; С - blocking generator; D — block that converts voltage into video pulses; E - VHF generator with amplitude modulation; "Input" - terminals to which the test voltage is supplied: 6 - electrical circuit diagram
The parameters of the attachment 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 to one side or the other by changing the resistance of the resistor R3.
To improve the clarity of the line image on the TV screen, the amplifier (T2, TZ, Tb) is covered by positive feedback from the collector of the TZ transistor to the base of the T2 transistor through the Sb capacitor. This significantly increases the gain in the high frequency region and, therefore, increases the steepness of the leading edge of the output pulses. Visually, this is manifested in an increased sharpness of the transition from white to black.
The frame, line and video pulses are added at the input of the emitter follower 77, which is a modulation amplifier for the VHF T8 generator. The latter is assembled according to the capacitive three-point scheme. The generation frequency should 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 oscillation frequencies can be achieved by choosing the number of turns of the coil L1. When tuned to the second television channel (59, 25 MHz), the L1 coil contains 5 turns of the PEV 0.6 wire, the coil diameter is 9 mm.
The modulated HF voltage is fed to the output of the set-top box through the divider R18 - R19, which lowers the voltage to 3 mV in order to avoid overloading the HF 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 adjustment. 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 with short conductors, and, in addition, they must be grouped in one place.
No shielding is required for the set-top box. After turning it on, it is necessary, as usual, to tune the TV 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 frequency of the TV lines, it is necessary to enter it into this range, changing the resistance of the resistor R14 within small limits. It should be noted that the synchronization of the TV sweeps from the set-top box is usually very stable, therefore, poor synchronization when setting up the set-top box indicates some kind of error in editing. In order to achieve precise tuning of the VHF generator of the set-top box to the selected television channel, it is necessary to stretch or compress the winding turns 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 maximum range of the image 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 the resistor R2.
To check the sensitivity of the attachment, an alternating voltage of a known value is applied to its input either from a power supply with a voltage of 6 V, a frequency of 50 Hz through a divider, or from a sound generator.
The input impedance and sensitivity of the set-top box, if desired, can be significantly increased by connecting to it a conventional LF amplifier with an emitter follower at the input.
An oscilloscope is a portable device designed for testing microcircuits. Additionally, many models are suitable for industrial control and can be used for various measurements. It is impossible to make an oscilloscope with your own hands without a zener diode, which is its main element. This part is installed in a device of various powers.
Additionally, devices, depending on the modification, may include capacitors, resistors and diodes. The main parameters of the model include the number of channels. Depending on this indicator, the limiting bandwidth changes. Also, when assembling an oscilloscope, consider the sampling rate and memory depth. In order to analyze the received data, the device is connected to a personal computer.
Simple oscilloscope circuit
A simple oscilloscope circuit includes a 5 V zener diode. Its bandwidth depends on the types of resistors that are installed on the microcircuit. Capacitors are used to increase the vibration amplitude. You can make a probe for an oscilloscope with your own hands from any conductor. In this case, the port is selected separately in the store. Resistors of the first group must withstand the minimum resistance in the circuit at a level of 2 ohms. Moreover, the elements of the second group should be more powerful. It should also be noted that there are diodes on the circuit. In some cases, they line up as bridges.
Single channel model
It is possible to make a single-channel digital oscilloscope with your own hands only using a 5 V zener diode. In this case, more powerful modifications are unacceptable in this case. This is due to the fact that an increased limit voltage in the circuit leads to an increase in the sampling rate. As a result, the resistors in the device fail. Capacitors for the system are selected only of the capacitive type.
The minimum resistor must keep the resistance at 4 ohms. If we consider the elements of the second group, then the transmission parameter in this case should be 10 Hz. In order to raise it to the desired level, various types of regulators are used. Some experts recommend using orthogonal resistors for single-channel oscilloscopes.
In this case, it should be noted that they raise the sampling rate indicator rather quickly. However, negative aspects in such a situation are still present, and they should be taken into account. First of all, it is important to note the sharp excitation of oscillations. As a result, the asymmetry of the signals increases. Additionally, there are problems with the sensitivity of the device. Ultimately, the accuracy of the readings may not be the best.
Dual channel devices
It is quite difficult to make a two-channel oscilloscope with your own hands (the diagram is shown below). First of all, it should be noted that zener diodes in this case are suitable for both 5 V and 10 V. In this case, capacitors for the system must be used only of a closed type.
Due to this, the bandwidth of the device can be increased to 9 Hz. Resistors for the model, as a rule, are of the orthogonal type. In this case, they stabilize the signal transmission process. To perform the functions of addition, microcircuits are selected mainly of the MMK20 series. You can make a divider for an oscilloscope with your own hands from a conventional modulator. It's not particularly difficult.
Multichannel modifications
In order to assemble a USB oscilloscope with your own hands (the diagram is shown below), a Zener diode will need a rather powerful one. The problem in this case is to increase the bandwidth of the circuit. In some situations, the resistors may malfunction due to a change in the limiting frequency. In order to solve this problem, many people use auxiliary divisors. These devices do a lot to help raise the voltage limit.
The divider can be made using a modulator. The capacitors in the system must be installed only near the zener diode. Analog resistors are used to increase the bandwidth. The negative resistance parameter on average fluctuates around 3 ohms. The blocking range depends solely on the power of the zener diode. If the limiting frequency drops sharply when the device is turned on, then the capacitors must be replaced with more powerful ones. Some experts in this case advise to install diode bridges. However, it is important to understand that the sensitivity of the system in this situation deteriorates significantly.
Additionally, you need to make a probe for the device. In order for the oscilloscope not to conflict with a personal computer, it is more expedient to use an MMP20 type microcircuit. You can make a probe from any conductor. In the end, a person will only have to acquire a port for him. Then, using a soldering iron, the above elements can be connected.
Assembling a 5V device
At 5 V, a do-it-yourself oscilloscope-set-top box is made only using an MMP20 type microcircuit. It is suitable for both conventional and high-power resistors. The maximum resistance in the circuit should be 7 ohms. In this case, the bandwidth depends on the signal transmission rate. Dividers for devices can be used in a variety of ways. Today, static analogs are considered to be more common. The bandwidth in such a situation will be at around 5 Hz. To increase it, it is necessary to use tetrodes.
They are selected in the store, based on the parameter of the limiting frequency. To increase the amplitude of the reverse voltage, many experts advise installing only self-regulating resistors. In this case, the signal transmission rate will be quite high. At the end of the work, you need to make a probe to connect the circuit to a personal computer.
10 V oscilloscopes
A do-it-yourself oscilloscope is made with a zener diode, as well as closed-type resistors. If we consider the parameters of the device, then the vertical sensitivity indicator should be at the level of 2 mV. Additionally, the bandwidth should be calculated. For this, the capacitance of the capacitors is taken and correlated with the limiting resistance of the system. The resistors for the device are most suitable for the field type. To minimize the sampling rate, many experts advise using only 2 V diodes. Due to this, a high signal transmission rate can be achieved. In order for the tracking function to be performed rather quickly, microcircuits are installed of the MMP20 type.
If you plan storage and playback modes, then you need to use a different type. Cursor measurements in this case will be unavailable. The main problem with these oscilloscopes can be considered a sharp drop in the limiting frequency. This is due, as a rule, to fast data scan. The task can be solved only with the use of a high-quality divider. That being said, many also rely on a zener diode. The divider can be made using a conventional modulator.
How to make a 15V model?
The oscilloscope is assembled with your own hands using linear resistors. They are able to withstand the ultimate resistance at the level of 5 mm. Due to this, there is not much pressure on the Zener diode. Additionally, you should take care of the selection of capacitors for the device. For this purpose, it is necessary to make measurements of the threshold voltage. Specialists use a tester for this.
If you use tuning resistors for the oscilloscope, you may encounter increased vertical sensitivity. Thus, the data obtained as a result of testing may be incorrect. Considering all of the above, only linear analogs should be used. Additionally, you should take care of installing a port that is connected to the microcircuit through a probe. In this case, it is more expedient to install the divider through the bus. So that the vibration amplitude is not too large, many advise using vacuum-type diodes.
Using resistors of the PPR1 series
Making a USB oscilloscope with your own hands with these resistors is not an easy task. In this case, it is necessary first of all to evaluate the capacitance of the capacitors. In order to keep the voltage limit below 3 V, it is important to use no more than two diodes. Additionally, remember the nominal frequency parameter. On average, this figure is 3 Hz. Orthogonal resistors are not suitable for such an oscilloscope unambiguously. Construction changes can only be made using the divider. At the end of the work, you need to deal with directly installing the port.
Models with PPR3 resistors
You can make a USB oscilloscope with your own hands using only grid capacitors. Their peculiarity lies in the fact that the level of negative resistance in the circuit can reach 4 ohms. A wide variety of microcircuits are suitable for such oscilloscopes. If we take the standard version of the MMP20 type, then it is necessary to provide at least three capacitors in the system.
Additionally, it is important to pay attention to the density of the diodes. In some cases, the bandwidth factor depends on it. To stabilize the fission process, experts advise to carefully check the conductivity of the resistors before turning on the device. Last of all, the regulator is connected directly to the system.
Oscillation suppression devices
Oscilloscopes with a vibration suppression unit are rarely used these days. They are most suitable for testing electrical appliances. Additionally, it should be noted their high vertical sensitivity. In this case, the parameter of the limiting frequency in the circuit should not exceed 4 Hz. Due to this, the Zener diode does not overheat much during operation.
A do-it-yourself oscilloscope is made using a grid-type microcircuit. In this case, it is necessary at the very beginning to determine the types of diodes. Many in this situation are advised to use only analog types. However, in this case, the signal transmission rate can be significantly reduced.
Oscilloscope - from an old TV
TV as an oscilloscope
The attachment, the diagram of which is shown in Fig. 1, turns any TV into a large screen oscilloscope. On it, you can observe low-frequency oscillations, and with the help of a sweeping frequency generator (GKCH), visually adjust the IF amplifiers of radio receivers.
Rice. 1. An add-on that turns a TV into an oscilloscope:
a - block diagram:
A - block for shaping frame synchronization pulses;
B - line synchronization pulse generator;
С - blocking generator;
D - block that converts voltage into video pulses;
E - VHF generator with amplitude modulation;
"Input" - terminals to which the investigated voltage is applied:
b - electrical circuit diagram.
The set-top box can be thought of as a miniature TV transmitter. Despite the relative simplicity of the circuit, this transmitter generates a complete television signal, which differs from the standard signal only by the absence of equalizing pulses.
Frame sync pulses are formed from an alternating sinusoidal voltage by an amplifier-limiter T1, a differentiating circuit R8C4 and a threshold amplifier T4. Their duration is about 1.9 ms.
The blocking generator on the T5 transistor generates horizontal sync pulses. These are not the main pulses of the blocking generator, but collector voltage surges that occur immediately after the main ones. An isolation diode D3 is connected between the collectors of transistors T4 and T5. At the time of generation of the main pulse, the collector of the transistor T4 is closed to the chassis through the open transistor T5 and diode D3. As a result, frames appear in the vertical sync pulses, which, as required, precede the horizontal sync pulses. The transformer windings Tp1 of the blocking generator are wound on an oxifer toroidal core (= 1000). The outer diameter of the core is 10 mm, the thickness is 2 mm. Windings I and III contain up to 100 turns, and II - 30 turns of PELSHO 0.1 wire.
At the beginning of the horizontal sweep period, the voltage pulse of the blocking generator quickly charges the capacitor C5 through the diode D2. During the rest of the period, it slowly discharges through resistor R6. The resulting sawtooth voltage is applied to the base of the transistor T2. Here it is added to the oscilloscope voltage.
The three-stage amplifier (T2, T3, T6), due to the high gain (50,000 - 100,000), operates practically in a relay mode, characterized by a certain response threshold.
The parameters of the attachment 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 to one side or the other by changing the resistance of the resistor R3.
To improve the clarity of the image of the line on the TV screen, the amplifier (T2, T3, T6) is covered by positive feedback from the collector of the transistor T3 to the base of the transistor T2 through the capacitor C6. This significantly increases the gain in the high frequency region and therefore increases the slope of the output pulses. Visually, this is manifested in an increased sharpness of the transition from white to black.
The frame, line and video pulses are added at the input of the T7 emitter follower, which is a modulation amplifier for the T8 VHF generator. The latter is assembled according to the capacitive three-point scheme. The generation frequency should 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 oscillation frequencies can be achieved by choosing the number of turns of the coil L1. When tuned to the second television channel (59, 25 MHz), the L2 coil contains 5 turns of 0.6 PEV wire, the coil diameter is 9 mm.
The modulated RF voltage is fed to the output of the set-top box through the divider R18 - R19, which lowers the voltage to 3 mV in order to avoid overloading the RF channel 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 adjustment. 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 with short conductors, and, in addition, they must be grouped in one place.
No shielding is required for the set-top box. After turning it on, it is necessary, as usual, to tune the TV 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 frequency of the TV lines, it is necessary to enter it into this range, changing the resistance of the resistor R14 within small limits. It should be noted that the synchronization of the TV sweeps from the set-top box is usually very stable, therefore, poor synchronization when setting up the set-top box indicates some kind of error in editing. In order to achieve precise tuning of the VHF generator of the set-top box to the selected television channel, it is necessary to stretch or compress the winding turns 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 maximum range of the image 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 the resistor R2.
To check the sensitivity of the attachment, an alternating voltage of a known value is applied to its input either from a power supply with a voltage of 6 V, a frequency of 50 Hz through a divider, or from a sound generator.
The input impedance and sensitivity of the set-top box, if desired, can be significantly increased by connecting to it a conventional LF amplifier with an emitter follower at the input.
The TV turns into an oscilloscope by turning the knob
Turning the switch knob - and the TV turns into an oscilloscope. It can be used in physics lessons at school, in the laboratory and in amateur radio practice. The secret of turning a TV into an oscilloscope is in a small set-top box, which is attached to the back of the TV and is a switch that switches the power supply circuit of the deflection system (Fig. 2).
Rice. 2. Electrical diagram of the switch box.
In position 1 of the switch, the TV operates as usual. In position 2 P1, the voltage from the line scan generator is disconnected. If now a signal is connected to the Y terminals, an oscillogram of the process under study will appear on the TV screen. Synchronization is set by rotating the "Frame rate" knob. Using the Vertical Size knob, the waveform can be compressed or stretched along the X-axis. In position 3 of the switch, the deflection system is completely disconnected from the power supply. Then, applying voltage to terminals X and Y, observe the Lissajous figures.
So, with the help of the set-top box, you can demonstrate and observe a wide variety of processes: rectification of alternating current, addition of mutually perpendicular oscillations, phase shift under inductive and capacitive loads, damped oscillations, beats, etc. The device is designed for TVs "Record", "Volkhov" , "Enisey", but it is not difficult to set it up to work with any other television receiver.
Oscilloscope - a whole measuring laboratory for incoming control
In the manufacture and repair of electronic equipment, various radio elements are installed. To make sure that they are in good working order, preliminary (input) control is carried out, which can be carried out using an attachment to any oscilloscope. The schematic diagram of the attachment is shown in Fig. 3.
Rice. 3. Schematic diagram of the attachment to the oscilloscope.
The attachment to the oscilloscope allows you to check almost all elements installed in electronic devices of household equipment: from resistors to controlled valves (thyristors), and also makes it possible to assess the quality of potentiometers, inductors, the health of switches, relays, transformers, etc.
Thus, one oscilloscope can replace almost the entire incoming inspection measurement laboratory.
It must be borne in mind that the oscilloscope serves not only for observing various processes associated with a change in the voltage waveform. The oscilloscope can be used as an electronic volt meter, ohmmeter, and by applying an attachment to the oscilloscope, you can observe the characteristics of the transistors on the oscilloscope screen, which expands the scope of the oscilloscope in repair and amateur practice.
The set-top box is assembled in a metal or plastic case with dimensions of 50x75x100 mm using a small-sized transformer that lowers the voltage from 220 to 6.3 V. The power of the transformer is small (20 mW), and the current consumption does not exceed 2-3 mA.
Working with a prefix. Conclusions of the attachment 1, 2, 3 are connected to the corresponding outputs of the oscilloscope (Fig. 4).
Rice. 4. Connecting the set-top box to the oscilloscope.
The oscilloscope is switched to the mode of operation with external synchronization or with a sweep from an external source. Connect the set-top box to the network. A horizontal line will appear on the screen (if pins 1 and 2 are not shorted). Then press the KN1 button, the line on the oscilloscope screen should deviate by a certain angle. Using the "Horizontal Gain", "Vertical Gain" and "Vertical Gain" knobs, make sure that the line is located in the center of the screen at an angle of 45 ° to the horizontal axis. The length of the image should be equal to half the diameter of the screen (Fig. 5).
Rice. 5. Oscillograms obtained when checking electrical radioelements.
The element under test is always connected to the terminals of the attachment 3 and 2. A vertical line on the screen (see Fig. 5) indicates a short circuit, a horizontal line indicates an open circuit or an element. The nature of the image on the oscilloscope screen is determined by the dependence of the resistance of the tested element on the magnitude and polarity of the sinusoidal voltage supplied to it.
Let's show what you can see on the oscilloscope screen when examining the following items.
Semiconductor diodes. The polarity of the switch-on and the appearance of the curves on the screen are shown in Fig. 5, a, b. When the diode is turned back on, the curve shown in Fig. 5, c. So you can determine the leads of the anode and cathode of diodes, which have the marking erased.
If the top of the corner on the screen is rounded or one of its sides is much larger than the other, or the direction of the straight lines is very different from the horizontal and vertical, then the diode should be rejected.
Zener diodes. If the stabilization voltage of the zener diode is less than 10 V, a break will appear on the horizontal line (Fig. 5, d). The distance from the bend to the vertical line will correspond to the stabilization voltage (in our case, 10 V).
Selenium valves. If the element is serviceable, then the beam on the screen will draw a horizontal line, which smoothly turns into a vertical one (Fig. 5, e).
For a faulty element, the vertical part of the waveform will be very short or with a large slope. This curve indicates a large voltage drop across the valve when current flows in the forward direction. The voltage drop across selenium rectifiers is much greater than on germanium or silicon ones.
Tunnel diodes. The way of switching on is shown in fig. 5, e. The characteristic of a working diode is shown in the figure (curve 1). Sometimes, by increasing the horizontal gain, it is possible to get the picture shown in the figure (curve 2), which is a typical characteristic of a tunnel diode. Before checking other parts, the Horizontal Gain knob must be set to the position found during calibration.
Controlled gates (thyristors)(Fig. 5, g). The type of volt-ampere characteristic for a serviceable element (with a disconnected control output - UE) is shown in Fig. 5, g, 1. When the control electrode is connected to terminal 2, the thyristor opens and the beam draws a curve on the screen similar to the characteristic of a conventional diode connected in the conducting direction (Fig. 5, g, 2).
Transistors. Connecting them to the attachment is shown in Fig. 5, h. If the emitter and collector pins are swapped, the pattern on the screen will not change (the base remains unconnected). The beam will draw a horizontal line on the screen, it may be slightly curved. Then attach the base to terminal 2 and obtain the characteristic shown in Fig. 5, h (1 - for the p-p-p type transistor, 2 - for the p-p-p type). This is another way to determine the electrode leads of unknown transistors. When switching the base output to terminal 3, the first oscillogram shown in Fig. 5, h, will correspond to the transistor p-p-p.
If, when testing transistors, a characteristic in the form of the letter L does not appear on the screen, this means that there is an open circuit in the transistor electrode circuit. When one of the segments of the oscillogram (letters L) is bent, this means that one of the pn junctions of the transistor is faulty.
The bending of the vertical line indicates a large resistance in the forward direction, the slope of the horizontal line indicates a low reverse resistance of the junction (large reverse collector current). The deviation of the sides of the corner from the horizontal and vertical indicates poor quality transitions.
Usually, high-power transistors (even the best ones) always have a large reverse collector current. Therefore, you first need to test several serviceable powerful transistors and then, using them, as by standards, check others. The phenomena indicating a short circuit or an open circuit in a transistor are the same for all types of transistors.
Unijunction transistors. The connection diagram is shown in Fig. 5, j. First, a measurement should be carried out with the emitter turned off. A straight line with an inclination of 30 ° relative to the horizontal axis should appear on the oscilloscope screen (Fig. 5, k, 1). Then connect the emitter to the clamp 2, while the part of the straight line on the screen should bend upward (Fig. 5, k, 2). If the emitter is connected to terminal 3 (to the base of the transistor), the lower end of the straight line will become vertical (Fig. 5, k, 3).
Resistors (fixed and variable). By measuring the angle of inclination of a straight line on the screen relative to the horizontal with a protractor, you can approximately determine the values of the resistances of various resistors. To do this, use the diagram in Fig. 5, l and the graph shown in Fig. 6. For resistors with resistance up to 100 Ohm, the beam on the screen will draw a vertical axis, over 100 kOhm - a horizontal one.
These two lines define the measurement range of the oscilloscope. Before measuring, the resistor should be connected to terminals 3 and 2. One of the extreme terminals and the middle terminal of the adjustable resistor (potentiometer) are connected to the attachment. When the axis of the variable resistor under study is rotated, the slope of the straight line on the screen should change. A fuzzy line on the screen indicates that the moving contact of the resistor is dirty.
Photoresistors connect to terminals 3 and 2. If the inlet of the photocontroller is covered, a straight line with a small angle of inclination will appear on the screen. If the fixture is illuminated, a vertical line will appear. Using the graph shown in Fig. 6, it is possible to determine the resistance of the device when illuminated with different intensities. This is how photoresistors with similar characteristics are selected, and photoexposure meters are calibrated.
Rice. 6. Graph for determining the resistance value of fixed and variable resistors.
Capacitors any type is also connected to terminals 3 and 2. For serviceable capacitors with a capacity of up to 0.85 μF, an ellipse with a horizontal major axis will appear on the screen (see Fig. 5, m). With a capacitance close to 0.85 μF, a circle will appear on the screen, and with a capacitance exceeding this value, an ellipse again, but with a large vertical axis. By measuring the ratio of the major and minor axes of the ellipse, it is possible according to the graph shown in Fig. 7, find the approximate capacitance of the capacitor. If the major axis of the ellipse is tilted, this indicates that the capacitor leakage current is too high.
Rice. 7. Graph for finding the capacitances of the tested capacitors.
Coils, relays and transformers. The conclusions of the coils, relays and transformer windings are connected to terminals 3 and 2 of the attachment and the ellipse is observed on the oscilloscope screen. With an inductance of the coil less than 5 G, an ellipse will appear on the screen, the major axis of which is slightly tilted relative to the vertical, with an inductance of 5 G, there will be a circle on the screen, and above 5 G, an ellipse, the major axis of which is slightly deviated from the horizontal axis. Naturally, the accuracy of such measurements is not high, since the shape of the oscillogram is influenced not only by the inductance, but also by the capacitance of the windings. A waveform that differs from that described indicates a short circuit in the coil.
Given coils whose inductance is known, the measured inductance can be determined by comparison.
Checking electrical circuits. Since the device allows you to evaluate very small resistance values between terminals 3 and 2, it can be used to test switches, light bulbs, fuses, field wires and electrical circuits.
Oscilloscope attachment for observing the characteristics of transistors
(characterograph)
In fig. 8, a shows a diagram of an attachment for observing the characteristics of transistors on the oscilloscope screen. Variable resistor R1 is designed to adjust the base current. A sheet of tracing paper is applied to the screen and the characteristic is circled. A typical characteristic of a collector junction is shown in fig. 8, b. The vertical axis is the collector current, the horizontal axis is the collector voltage. The slope of the curve defines the saturation region. On the horizontal part of the curve, the operating point for the class A amplifier is selected. 8, in addition to the horizontal scan line 1, the characteristic of the collector reverse current is shown at base current 1 equal to zero (curve 2), as well as the output characteristics at base currents of 0.2 ... 1 mA. The characteristics obtained with the oscilloscope can be compared with those given in the reference books.
Rice. 8. Attachment to an oscilloscope for observing the characteristics of transistors:
a - a schematic electrical diagram for transistors p-p-p, and for p-p-p, the polarity of the inclusion of elements B and D1 should be changed; b - the main characteristic; c - a family of characteristics.
Transistors intended for operation in push-pull stages must have similar parameters. In our example, a transistor of the p-p-p structure is shown, connected according to the OE circuit. You can also investigate the pnp transistor by connecting it to the attachment accordingly (in the OE, OB or OK circuits).
The article is based on the publications of V.G. Bastanova
Author-compiler. Patlakh V.V. 1999 year
© "Encyclopedia of Technologies and Techniques" Patlakh V.V. 1993-2007
