Do-it-yourself oscilloscope from a TV. Oscilloscope from TV

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.

Puc.2

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.

The Y (VERTICAL) channel block contains input connectors CH1 and CH2, AC/DC input switches (closed/open input), GND buttons - input grounding. The deviation coefficient is set by calibrated attenuators (VOLTS/DIV), as well as by an uncalibrated variable VAR control. The vertical offset of the oscillogram is adjusted smoothly in each channel using the POSITION knob. The oscilloscope provides the following modes of operation of the ALT/CHOP/ADD channel switch - alternate (for each sweep stroke) or intermittent channel switching (with a frequency of 250 kHz). ADD mode provides the addition of signals from channels CH1 + CH2.

Rice. 2.2. Designations of the GOS-6200 oscilloscope controls

Channel X of the oscilloscope (HORIZONTAL) contains two generators: the main one (MAIN) and the delayed sweep (DELAY). The sweep factor is set discretely (TIME/DIV). If necessary, use uncalibrated smooth adjustment with VAR mode enabled. The sweep stretch is activated by the 10 MAG button. The horizontal position of the oscillogram is adjusted with the POSITION knob. The operating mode of channel X is switched by the MAIN/ALT/DELAY button. In this case, the following operating modes of channel X are implemented:

1. MAIN sweep only.

2. Combining sweep oscillograms with highlighting the area of action of the delayed sweep.

11 -

3. Only delayed sweep, launched from the main sweep with a continuously adjustable delay (DELAY TIME knob).

The scan is turned off using the X–Y mode button.

Synchronization and trigger block(TRIGGER) allows you to select the source of the synchronization signal (SOURCE), the operating mode of the scan generator (MODE) - self-oscillating (ATO), standby - NORM and triggered by a video signal (TV). The COUPLING switch is used to set the synchronization signal processing mode.

The function of the SLOPE switch is to select the polarity of the synchronization signal: (+) – synchronization on an increasing signal (triggering on an edge), (–) – on a decreasing one (triggering on a pulse cutoff). The trigger level of the synchronization and triggering device is manually adjusted using the LEVEL knob.

The oscilloscope has a trigger and trigger delay mode. Using the HO knob (combined with the DELAY TIME adjustment), you can manually increase the sweep voltage blocking time tbl. This makes it possible to increase the stability of the synchronization unit in the case when more than one trigger signal can be generated during a signal period. The normal setting for this adjustment is 0%.

The measuring unit (MEAS’MT) turns on and off the cursor measurement mode and switches the type of cursors. In normal mode, the FUNC button is used to switch functions for measuring signal parameters - frequency, period, duration and duty cycle.

The settings block (SETUPS) allows you to remember the state of the oscilloscope’s controls in memory and, if necessary, restore the previous state of the device.

Amplitude and time parameters of a standard TV video signal

In laboratory work, a standard television video signal is used as the object of study. The parameters of this signal for broadcast television systems - the period and duration of synchronization pulses, amplitude and shape - are strictly standardized in GOST 7845-92. In table 2.2 shows the standard parameters of the video signal of domestic television.

A television video signal consists of image signals, as well as horizontal and vertical blanking (blanking) and synchronizing pulses. In the video signal there are:

active interval during which the image is transmitted;

passive interval in which blanking and synchronizing pulses, color recognition signals, teletext signals, image test signals, etc. are transmitted.

	Table 2.2
Standard video parameters

Magnitude	Meaning
Number of lines
Field frequency, Hz
Line frequency, Hz
Line duration, µs
Synchronization pulse duration, μs
Duration of the front of the quenching pulse of the lines, μs
Duration of the blanking horizontal pulse, μs
Duration of a full frame, ms
Interval between the edge of the horizontal and blanking pulses, µs
Vertical blanking pulse duration (line duration)

The image signal is a voltage whose value changes continuously as the beam moves along the line in accordance with the nature of the transmission. This voltage reaches 75% of the maximum value when transmitting white and decreases to 10–15% when transmitting dark areas of the image. In Fig. Figure 2.3 shows the shape of the complete video signal of two adjacent image fields for the domestic television standard.

The amplitude values of the image signal correspond to the instantaneous brightness of the transmitted image element. The zero level in the video signal is considered to be the blanking level. In the active part of the video signal (above the blanking level) there are levels of “white” (about 70% of the signal amplitude) and “black” (about 5%). The interval between the blanking level and the zero level is called the protective one. The amplitude of the sync pulse is 30% of the swing of the entire video signal.

The complete video signal contains horizontal and vertical sync pulses. They are transmitted during the reverse motion of horizontal and vertical scans, respectively. To prevent line synchronization from being disturbed during vertical scanning reverse, the vertical sync pulse has horizontal pulse inserts with a duration of 4.7 μs. With this arrangement of the transmitted sync pulses, a slight shift in the phase of the frame sync pulses of two adjacent fields is possible. This leads to a violation of the relative position of the raster lines, resulting in a deterioration in the vertical clarity of the image on the TV screen. To eliminate this phenomenon, equalizing pulses with a duration of 2.35 μs are transmitted before and after the frame pulse. The repetition rate of equalizing pulses and insertions is 2 times higher than the horizontal frequency. With them

presence, the dedicated frame sync pulses of two adjacent fields are identical
by phase and shape.
Even field				Odd field of current frame
previous frame				Odd field of current frame
previous frame
			Line numbers


Lowercase

	Front
equalizing				equalizing

Odd field of current frame				Even field of the current frame
Odd field of current frame
			Frame damping pulse
			Line numbers

Lowercase			Personnel
sync pulse		sync pulse

		2.3. Composite Video

For video signals with a simplified sync mix without inserts and equalizing pulses (for example, signals from game consoles, simple video cameras, video testers - generators of test television signals), the vertical clarity of the image noticeably deteriorates.

Thus, on the vertical blanking pulse of a standard video signal, the synchronization signals are placed in the following order: first there are six equalizing pulses with a repetition rate of 31,250 Hz, followed by six wide pulses representing the frame synchronization signal, then again six equalizing pulses, followed by ordinary horizontal clock pulses. Due to the use of interlaced scanning, vertical scanning must be reversed 2 times during the transmission of a full frame (first after transmission of odd and then even lines). First, the beam is thrown upward after the end of the transmission of the whole line, then after the transmission of half the line. This sequence is provided by two half-frame pulses, differing from each other by various time shifts relative to the transmission of the last horizontal synchronizing pulse. In the first of them, this time corresponds to the development of one

line, and in the second - half a line. Accordingly, all other synchronizing pulses placed on the second half-frame blanking pulse are shifted by half a line. This form of signal makes it possible to obtain stable interlaced scanning, ensure the continuity of horizontal synchronizing pulses during the transmission of a vertical blanking signal, and easily separate synchronization signals from the full television signal.

The duration of pulse transmission is determined by the standard. The transmission time for one line is 64 μs. Accordingly, the transmission duration of the horizontal blanking pulse is 10...11 µs, the horizontal synchronizing pulse - 4.4...5.1 µs, the vertical blanking pulse - 1500...1600 µs, the vertical synchronizing pulse - 192 µs and, finally, the equalizing pulses - 2.56 µs. Line blanking pulses are sent after the end of each line transmission. Their value is fixed at 75% (black level) of maximum amplitude. The horizontal synchronizing pulses are placed on the horizontal blanking pulse, occupying the remaining 25% of the amplitude. They regulate the accuracy of the start of scanning of each subsequent line.

Vertical blanking pulses are sent at the end of scanning of the last line (bottom of the image). They block the beam during the return stroke as it moves from bottom to top, and serve as a “stand” for the frame sync pulses, lowering them above the signal level into the “blacker than black” region. The frame synchronizing pulse causes the beam to reverse from bottom to top in exact accordance with the movement of the beam in the transmitting tube of the television center.

The laboratory prototype consists of a GOS-6200 analog television oscilloscope, a television camera mounted on a frame along with a tablet with a test black and white image.

Assignment and instructions for performing the work

Preparing the oscilloscope for use

Before operation, study the purpose of the oscilloscope controls. Otherwise, many of the work assignments will be difficult to complete.

Check the oscilloscope calibration for the second channel CH2. To do this, use an oscilloscope probe to connect the 1:1 CAL 2V 1 kHz terminal of the oscilloscope calibrator to the input of the selected channel. Turn on the oscilloscope.

Set the CH2 channel input switch to the AC position - “closed input”, the GND button must be turned off. Select Channel Deflection Ratio

0.5 V/div, MTB MAIN = 0.5 ms/div. Let us remind you that the indication of the installed parameters and modes is carried out in the service areas of the screen. Turn on the self-oscillating scanning mode (ATO), the clock source (SOURCE) is CH2, the synchronization filter (COUPLING) is AC, the synchronization polarity SLOPE is positive. An image of a square wave (calibrator sample signal) should appear on the screen. Get a fine scan line by adjusting the brightness (INTEN) and focusing (FOCUS) of the beam.

The calibrator signal amplitude is 2 V, so with a properly calibrated Y channel, the waveform should take 4 divisions. vertically. Use the HORIZONTAL POSITION knob to align the start of the first pulse with the left vertical scale line. The coincidence of the end of the fifth period with the last right line of the scale indicates that the oscilloscope is calibrated by duration.

If the vertical and/or horizontal calibration is broken, the oscilloscope requires maintenance from a metrological service.

Measuring horizontal TV video signal parameters

Apply a video signal from a television camera to the input of channel CH1. Turn on CH1 and turn off the second channel by briefly pressing the button

Set the following control positions on the oscilloscope: channel input switch CH1 - to the DC position - “open input”,

the GND button must be disabled;

scan mode – main (MAIN);

scan start mode (MODE) - TV, sync source (SOURCE)

Using the TV-V/TV-H button, set the synchronization mode from the television video signal according to the TV-H line frequency. SLOPE synchronization polarity is negative. Select the deflection and sweep factors to display the waveforms in one or more lines on the screen. Due to the presence of vertical sync pulses in the video signal, jittery horizontal lines may appear on the screen. Sketch the appearance of the video signal of one line of the image.

Turn on the cursor measurement mode (long press the CURSOR ON/OFF button). By briefly pressing the FUNC button, select the duration measurement mode D T D. Press the CURSOR POS button and, moving the cursors with knobs C1 and C2, measure the repetition period of the horizontal sync pulses. Switch the cursors to the 1 D T D frequency measurement mode (by briefly pressing the FUNC button) and record the frequency of the horizontal pulses. Record it like this:

the same result of frequency measurement in automatic mode, which is displayed in the lower right corner of the screen. Enter the results in a table in the form of the table. 2.3.

			Table 2.3
Measured parameters of horizontal video signal

Parameter	Standard	Measured	Error,
Parameter	meaning	meaning
	meaning	meaning

Period of horizontal sync pulses, μs

Horizontal pulse frequency, Hz

Horizontal pulse frequency
(automatic measurement), kHz
(automatic measurement), kHz

Duration of line quenching pulse, µs

Duration of horizontal sync pulse,



Clock shift duration
relative to the beginning of the damping pulse, μs
relative to the beginning of the damping pulse, μs

To measure horizontal sync pulse parameters, use delayed sweep. Set the oscilloscope to dual sweep (ALT) mode first. An image of the complete signal and a fragment of the signal created by the delayed sweep will appear on the screen (its area of action is highlighted by two dotted lines - not to be confused with the cursors!). Set the delayed scan area to horizontal sync using the DELAY TIME and TIME/DIV knobs. Switch the oscilloscope to delayed sweep mode (DELAY). A large-scale image of the sync pulse will appear on the screen. Using the cursors in the D T D mode (with the CURSOR POS mode turned on), measure the duration of the blanking pulse and horizontal sync pulse, as well as the shift of the sync pulse relative to the beginning of the blanking pulse (see Fig. 2.3). Compare them with standard values. Enter the measurement results into a table in the form of the table. 2.3.

Return to the MAIN sweep mode. Measure the signal duration of the black and white fields. It looks like a step, reflecting the levels of white (maximum) and black (minimum). Turn on the voltage difference measurement cursors V 1 (FUNC button) and measure the video signal voltage levels: white level (maximum voltage value), black level (step level) and level of blanking pulses relative to the minimum voltage value - the level of horizontal sync pulses. Tabulate the results in the form of a table. 2.4. Draw the appearance of the synchronization pulse and plot the measured parameters on it.

Table 2.4

Measured parameters of the image signal of black and white fields

Duration	Duration			Amplitude	U si U max,
	steps			lowercase
Images,	white level,			impulse
Images,	white level,	Umax, V	Umin, V	impulse
				U si, V

Measuring frame TV video signal parameters

Examine the shape of the frame sync pulse. It contains a vertical blanking pulse with a vertical sync pulse at its beginning (see Fig. 2.3). The frame sync pulse is filled with double line frequency insertion pulses. Before and after the vertical sync pulse, there are equalizing pulses of double horizontal frequency and duration, 2 times less than the duration of the horizontal sync pulses and insertion pulses.

To observe frame pulses, use the main sweep (MAIN). Use the TV-V/TV-H button to set the synchronization mode to TV-V frames. SLOPE synchronization polarity is negative. Select the main sweep factor (MTB) so as to obtain several periods of fields (half-frames) of the signal on the screen. Set the cursor measurement mode by long pressing the CURSOR ON/OFF button. Select the duration measurement mode D T D with the FUNC. button. Using cursors, measure the period and frequency of the vertical sync pulses using a method similar to that described earlier for the horizontal sync pulses. Record the result of the automatic frequency measurement displayed in the lower corner of the screen. Enter the results in a table in the form of a table.

Table 2.5

Measured frame video signal parameters

Parameter	Standard	Measured	Error,
Parameter	meaning	meaning
	meaning	meaning
Frame sync period, ms
Frame pulse frequency, Hz
Vertical pulse rate
(automatic measurement), Hz
Frame blanking pulse duration, µs
Frame sync pulse duration,

Turn on ALT mode and set the delayed sweep area to the second vertical blanking pulse. Switch the oscilloscope to delayed sweep mode and image the vertical blanking pulse from the start of the sync pulse to the next line image signal. Sketch its appearance.

By moving the cursors with knobs C1 and C2, measure the duration of the vertical blanking pulse and the duration of the vertical sync pulse. Compare them with standard values. Enter the measurement results into a table in the form of the table. 2.5.

Measuring the signal-to-noise ratio of a video signal from a television camera

Apply a video signal from a television camera to the input of channel CH1. Set the following control parameters on the oscilloscope: channel input switch - to the DC position - “open input”, button

GND – disable;

main scan mode – MAIN;

startup mode (MODE) – TV, clock source (SOURCE) – CH1; synchronization polarity (SLOPE) – negative;

using the TV-V/TV-H button, set the mode for selecting a given line in the system

Select the deflection and sweep factors to obtain a single line image at a convenient scale. Use the TV LINE SELECT knob to select a line within the center of the field (with a number in the range of 100–200).

Use the video camera at maximum gain by covering the lens with a light-proof cap. The camera's automatic gain control (AGC) system will set the maximum gain, and the oscillogram will show a trace of the camera's internal noise at the black signal level. Draw the resulting oscillogram of the video signal.

Place it on the top edge of the noise track (according to the largest emissions), the other on the bottom. Assuming normal noise distribution, we assume that the width of the noise track corresponds to the deviation of the random signal within 3y. Then we define y (root mean square value of noise) as

V w 6.

Measure the amplitude of the desired signal as the peak-to-peak between the signals from the black and white fields of the image. The oscillogram of such an image shows a stepped video signal. Measure its span Vc from the black level to the white level. Calculate the signal-to-noise ratio, dB, using the following formula:

Record the results of the measurement and calculation of the signal-to-noise ratio.

Lab report should contain a block diagram of the oscilloscope, measurement results, and brief conclusions.

The attachment, the diagram of which is shown in Fig. 1, turns any TV into a large-screen oscilloscope. You can observe low-frequency oscillations on it, and with the help of a sweep frequency generator (SWG) you can visually tune the IF amplifiers of radio receivers.

Rice. 1. An attachment that turns your TV into an oscilloscope:

a - block diagram:

A - block for generating frame synchronization pulses;

B — pulse generator synchronization along lines;

C - blocking generator;

D - block that converts voltage into video pulses;

E - VHF generator with amplitude modulation;

“Input” - terminals to which the voltage being tested is supplied:

b - electrical circuit diagram.

The set-top box 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 the limiting amplifier T1, the differentiating circuit R8C4 and the threshold amplifier T4. Their duration is about 1.9 ms.

The blocking generator on transistor T5 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 blocking generator transformer Tp1 are wound on a toroidal core made of oxyfer (= 1000). The outer diameter of the core is 10 mm, thickness 2 mm. Windings I and III contain up to 100 turns, and Winding II - 30 turns of PELSHO 0.1 wire.

The three-stage amplifier (T2, T3, T6), 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.

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 L2 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. The 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 must, as usual, adjust the TV using the adjustment knobs (frame rate, line frequency, contrast). If the pulse frequency of the blocking generator of the set-top box does not lie in the range of adjusting the line frequency 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 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, 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 scope 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 resistor R2.

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.

The TV turns into an oscilloscope by turning the switch knob

Turn 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 to turning a TV into an oscilloscope is in a small attachment, which is mounted on the back wall of the TV and is a switch that is used to switch the power supply circuit of the deflection system (Fig. 2).

Rice. 2. Electrical diagram of the console-switch.

In switch position 1, the TV operates as usual. In position 2 P1, the voltage from the horizontal scan generator is turned off. If you now connect a signal to terminals Y, 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” control, the oscillogram 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, by applying voltage to terminals X and Y, Lissajous figures are observed.

So, using 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 with inductive and capacitive loads, damped oscillations, beating, etc. The device is designed for TVs “Record”, “Volkhov” , “Yenisei”, but it is not difficult to configure it to work with any other television receiver.

An oscilloscope is a whole measuring laboratory for input control

During the manufacture and repair of radio-electronic equipment, various radio elements are installed. To ensure their serviceability, a preliminary (input) test 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 oscilloscope attachment.

The oscilloscope attachment 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 evaluate the quality of potentiometers, inductors, serviceability of switches, relays, transformers, etc.

Thus, one oscilloscope can replace almost the entire incoming inspection laboratory.

It must be borne in mind that the oscilloscope is used not only for observing various processes associated with changes in the voltage shape. The oscilloscope can be used as an electronic voltmeter, ohmmeter, and by using the attachment to the oscilloscope, you can observe the characteristics of transistors on the oscilloscope screen, which expands the possibilities of using 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 reduces 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 the attachment. The terminals of the attachment 1, 2, 3 are connected to the corresponding terminals of the oscilloscope (Fig. 4).

Rice. 4. Connecting the set-top box with an oscilloscope.

The oscilloscope is switched to operating mode with external synchronization or 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 KH1 button, the line on the oscilloscope screen should deviate by a certain angle. Use the “Horizontal Gain”, “Vertical Gain” and “Vertical Gain” knobs to ensure 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 testing electrical radio elements.

The element being tested is always connected to terminals 3 and 2 of the attachment. A vertical line on the screen (see Fig. 5) indicates a short circuit, a horizontal line indicates an open circuit in the circuit or in the element. The nature of the image on the oscilloscope screen is determined by the dependence of the resistance of the element under test on the magnitude and polarity of the sinusoidal voltage supplied to it.

Let's show what can be seen on the oscilloscope screen when examining the following elements.

Semiconductor diodes. The switching polarity and the appearance of the curves on the screen are shown in Fig. 5, a, b. When the diode is turned on in reverse, the curve shown in Fig. is obtained. 5, c. This way you can determine the anode and cathode terminals of diodes whose markings have been 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 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 kink will appear on the horizontal line (Fig. 5, d). The distance from the break to the vertical line will correspond to the stabilization voltage (in our case, 10 V).

Selenium valves. If the element is in working order, 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 oscillogram will be very short or with a large slope. Such a curve indicates a large voltage drop across the valve when current flows in the forward direction. The voltage drop on selenium rectifiers is much greater than on germanium or silicon rectifiers.

Tunnel diodes. The connection method is shown in Fig. 5, e. The characteristics of a working diode are shown in the figure (curve 1). Sometimes, by increasing the horizontal gain, it is possible to obtain the pattern 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 moved to the position found during calibration.

Controlled valves (thyristors) (Fig. 5, g). The type of current-voltage characteristic for a serviceable element (with the control output disabled - CE) is shown in Fig. 5, f, 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. Their connection to the console 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 on the screen will draw a horizontal line, it may be slightly curved. Then connect the base to clamp 2 and obtain the characteristic shown in Fig. 5, h (1 - for p-p-p type transistor, 2 - for p-p-p type). This is another way to determine the electrode terminals of unknown transistors. When switching the base output to terminal 3, the first waveform shown in Fig. 5, h, will correspond to the p-p-p transistor.

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 oscillogram segments (letter L) is bent, this means that one of the pn junctions of the transistor is faulty.

The bend of the vertical line indicates a high resistance in the forward direction, the slope of the horizontal line indicates a low reverse resistance of the junction (large reverse collector current). Deviation of the sides of the angle from the horizontal and vertical indicates poor quality of transitions.

Typically, 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 use them as standards to check others. The phenomena indicating a short circuit or open circuit in a transistor are the same for all types of transistors.

Unijunction transistors. The connection diagram is shown in Fig. 5, k. First, the measurement should be carried out with the emitter turned off. A straight line with a slope of 30° relative to the horizontal axis should appear on the oscilloscope screen (Fig. 5, j, 1). Then connect the emitter to clamp 2, while part of the straight line on the screen should bend upward (Fig. 5, j, 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, j, 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 resistance values of various resistors. To do this, use the diagram in Fig. 5, l and the graph shown in Fig. 6. For resistors with a resistance of up to 100 ohms, the beam on the screen will draw a vertical axis, over 100 kOhm - a horizontal axis.

These two straight lines determine the measurement range of the oscilloscope. Before measurement, the resistor should be connected to terminals 3 and 2. One of the outer terminals and the middle terminal of the adjustable resistor (potentiometer) is connected to the attachment. When you rotate the axis of the variable resistor under study, the slope of the straight line on the screen should change. An unclear line image on the screen indicates contamination of the moving contact of the resistor.

Photoresistors are connected to terminals 3 and 2. If the input hole of the photocontroller is covered, a straight line with a small angle of inclination will appear on the screen. If the device is illuminated, a vertical line will appear. Using the graph shown in Fig. 6, you can determine the resistance of the device when illuminated with different intensities. This is how photoresistors with similar characteristics are selected, and photo exposure meters are also calibrated.

Rice. 6. Graph for determining the resistance value of fixed and variable resistors.

Capacitors of any type are 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, the screen will form a circle, 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, you can use 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 capacitors being tested.

Coils, relays and transformers. The terminals of the coils, relays and transformer windings are connected to terminals 3 and 2 of the console and the ellipse is observed on the oscilloscope screen. With a coil inductance of less than 5 G, the screen will display an ellipse, the major axis of which is slightly inclined relative to the vertical; with an inductance of 5 G, the screen will appear as a circle, and above 5 G, an ellipse will appear, the major axis of which is slightly deviated from the horizontal axis. Naturally, the accuracy of such measurements is not high, since the appearance of the oscillogram is affected not only by the inductance, but also by the capacitance of the windings. A waveform shape 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, installation wires and electrical circuits.

Attachment to an oscilloscope for monitoring the characteristics of transistors

(characterist)

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 traced. A typical collector junction characteristic 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 determines the saturation region. On the horizontal part of the curve, the operating point for the class A amplifier is selected. In Fig. 8, in addition to horizontal scan line 1, shows the characteristic of the reverse collector current at base current 1 equal to zero (curve 2), as well as output characteristics at base currents of 0.2 ... 1 mA. The characteristics obtained using an oscilloscope can be compared with those given in reference books.

Rice. 8. Attachment to an oscilloscope for monitoring the characteristics of transistors:

a - circuit diagram for transistors p-p-p, and for p-p-p the polarity of switching on elements B and D1 should be changed; b - main characteristic; c - family of characteristics.

Transistors designed to operate in push-pull cascades must have similar parameters. Our example shows a p-p-p structure transistor connected according to an OE circuit. You can also examine the pnp transistor by appropriately connecting it to the attachment (in the OE, OB or OK circuits).

The article is compiled based on publications by V.G. Bastanova

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 alternating 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 of generation of the main pulse, the collector of transistor VT4 is closed to the chassis through the 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. 30 turns of PELSHO 0.1 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 VT7 emitter follower, which is the modulation amplifier of the VT8 VHF generator. 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 tuned 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, VT8) must have short leads, they should be connected to each other with short conductors and grouped in one place.
No shielding of the console is required. If the pulse frequency of the blocking generator does not lie in the range of the TV line frequency, 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 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. 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 turns of the winding of coil L1, i.e. change the winding pitch. When set correctly, the line on the screen is sharply defined. The betting parameters 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.

Literature RADIOAMATOR 5.99

Frame sync pulses are generated from alternating sinusoidal voltage by the limiting amplifier T1, the differentiating circuit R8C4 and the threshold amplifier T4. Their duration is about 1.9 ms.

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 oxyfer (ts = 1000).

The outer diameter of the core is 10 mm, thickness 2 mm. Windings I and III contain up to 100 turns, and Winding II - 30 turns of PELSHO 0.1 wire.

Here it is added to the oscilloscope voltage.

Rice. 1. An attachment that turns a TV into an oscilloscope: a - block diagram: A - block for generating frame synchronization pulses; B - line synchronization pulse generator; C - blocking generator; o-block, converting voltage into video pulses; E - VHF generator with amplitude modulation; “Input” - terminals to which the voltage being tested is supplied: b - circuit diagram.

The three-stage amplifier (T2, T3, T6), due to its large gain coefficient (50,000-100,000), operates practically in relay mode, characterized by a certain response threshold.

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 front of the output pulses. Visually, this manifests itself in an increased sharpness of the transition from white to black.

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.

No shielding of the console is required. After turning it on, you must, as usual, adjust the TV using the adjustment knobs (frame rate, line frequency, contrast).

If the pulse frequency of the blocking generator of the set-top box does not lie in the range of adjusting the line frequency 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 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, i.e. change the winding pitch. When set correctly, the line on the screen is sharply defined.

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.

Literature: V.G. Bastanov. 300 practical tips, 1986