Oscilloscope from an old TV

Oscilloscope from an old TV
Various instructions are available on the Internet for turning an old (sometimes partially inactive) TV into a widescreen oscilloscope. This article will also tell you how to create a decent electronic device using simple refinement worth about $ 20. In order for the input signal to be displayed on the screen and reproduced through the TV speaker, you will need to assemble a simple device that switches the power circuit of the deflecting system. Of course, you cannot stretch a large frequency spectrum on such a device (actually 20-20000 kHz), but it is quite accessible to monitor the low-frequency oscillations.

Watch the video

You can also install the main connectors and instrument controls in the TV box (good, it allows space). For example, the presence of an RCA connector would be an excellent opportunity to connect an iPod and at the same time allow the input of alternating voltage signals from millivolts tohundred volts. Nearby, you can place a trim resistance of 1 mΩ and a 6-section rotary switch. Using a small trimmer it will be convenient to control the horizontal scanning frequency, and the bright red button is suitable for turning on the device. in electronics. But the idea itself contains many interesting points.

Safety Requirements

The implementation of the described project involves working alongside an open television transformer and high-voltage capacitors. The voltage on the magnetron reaches 120 kV! To eliminate the risk of fatal electric shock, you must strictly follow proper safety precautions. The first step to perform any action should be a complete blackout of the device. There should not be forgotten about the high-voltage capacitors. Therefore, the protective casing of the high-voltage unit is removed very carefully. It is important not to damage the wires of the circuit board and nottouch its open contacts.
Oscilloscope from the old TV
Oscilloscope from the old TV
Oscilloscope from the old TV
Next, you need to forcibly discharge large capacities (50 V and more). This is done with a well-insulated screwdriver or tweezers. Their contacts are closed between themselves or on the case until full discharge. You should not do it on the printed circuit board, as the tracks may burn out. When doing work or testing the device, make sure that someone close to you is there who can call a doctor or give first aid.

Working Principle

Electron Beam Televisions (CRT) and oscilloscopes are considered the most interchangeable devices. Also, a television receiver is more complicated than a basic laboratory oscilloscope. To rework it, it is enough to get rid of some of the functions of TV embedded in it and add an uncomplicated amplifier. After all, each expanding line of the TV screen creates an electron beam,quickly scanned through the transparent material of the luminescent substrate of the tube.
Oscilloscope from the old TV
Oscilloscope from an old TV
Charged electrons are controlled by electric and magnetic fields created by coils located behind the tube. These cores with a wire deflect the beam in a horizontal and vertical plane, controlling the location of the image on the screen. To adjust it in the center of the oscilloscope line, you need to make a certain refinement with them.
Oscilloscope from the old TV
Oscilloscope from the old TV
Oscilloscope from an old TV
Recall that the video signal produces 32 frames per second, each of which consists of two "interlaced" images (that is, 64 frames are scanned). The NTSC standard defines 525 lines in screen format, other standards are slightly different values. So, for reproduction on the screen of the filled image, the vertical beam deflection of the electron beam is required every 1/64 second (frequency 64 Hz),and horizontally 1 / (64x525) seconds (frequency 32000 Hz). To ensure such values, the line voltage transformer exceeds 15,000 volts. In this case, the device works like a TV, and creates an expanded image on the screen. To make it draw an image on a very thin line, vertically rejected by the input signal, you need to adjust the number of turns of the screen coils. It is also important to “work” with the inductor coil. Its impedance impedance is frequency dependent. The higher the frequency, the harder it will be to display it on the screen. With an outer diameter of 10 mm toroidal core and a thickness of 2 mm, windings I and III should contain 100 turns of wire PELSCHO 0.1, and winding II - 30 turns. Another thing to remember is that the signal in the TV is mathematically integrated. This leads to the fact that the input rectangular wave will be displayed on the screen as a triangular wave, and a triangular wave will be displayed by a sinusoid. This applies only to the image, but not the sound. Sinusoidal waves will be displayed without distortion. The phenomenon will not be so noticeable in very old TVs capable of displaying white noise or a blue screen with no signal,rather than automatically disconnecting images.

Removing unnecessary nodes

In our case, we used an old TV receiver with a 15-inch screen and a classic UHF / VHF tuner. To create an oscilloscope, it is not required, so you can immediately remove the tuner and forget about its existence. Also, you can gradually disable one extra module after another, checking that the TV can still function. It will take only the main board and all that is connected to the kinescope. It is necessary that it only displays white noise or a blue screen. From the rest of the details, you can simply release the box.
Oscilloscope from an old TV
Oscilloscope from an old TV
Oscilloscope from an old TVOscilloscope from an old TV There was two potentiometers on the TV in front of the TV. One of them served to turn on and adjust the volume, and the other controlled the brightness. Both were removed: the first one was replaced by a power switch (big red button),The second had to be set to the maximum brightness and fixed by soldering additional resistances into the circuit. Just pay attention to the fact that the device with a built-in volume control for rework is not good. It amplifies the signal attached to the television and look for the amplifier will be on the main board, and this will cause additional problems. The speakers can also be turned off at this stage.

Deflecting system preparation

To achieve the oscilloscope images on the CRT screen, you will need to send the generated amplified frame and horizontal sync pulses to the deflection coils H and V . How to get it, will be disassembled a little later, and now you need to prepare a reject system. The coils are connected to the main board on four pins. It is necessary to disconnect the horizontal, to it go the red and blue wire. By connecting your iPod or computer directly to these pins, you can get a display of music on the screen of a kinescope. The vertical coil has a yellow and orange wire, but to receive a 64 Hz scan, they must be switched to a horizontal coil.
Oscilloscope from an old TV
Now you need to find where the coils are connected to a small circuit board on the tube of a kinescope. If the television receiver is not very new, there are only two coils and 4 wires to the main board go from them. Otherwise, the coils will be more and in this form, alteration will not work. But do not quit started, and you can experiment a little. In the meantime, we assume that the wires are still 4. It remains to deal with the wires leading to the kinescope. According to the right-hand rule (F = qVxB), we remove one of them in random order. If when turning on the device a horizontal line is displayed on the screen, the vertical coil is turned off, if vertical, then vice versa. The corresponding ends are testered and marked. The horizontal coil wires are now removed from the main circuit board. Do not forget that you will have to deal with a frequency of 30,000 Hz and a voltage of more than 15,000 volts. The future oscilloscope does not need them. Before touching, they must be short-circuited,then it is good to insulate and place inside the case so that they do not touch anything after switching on the device. So, the vertical marking line 60 Hz is ready. To obtain the same horizontal line of 60 Hz, the two remaining wires going to the vertical coil are soldered to the horizontal one. A vertical will become the input of an oscilloscope to connect an amplifier circuit.

Sweep Setup

The rest of the work is most dangerous, since it will be performed when the voltage is connected. Be especially careful! We try to connect the signal source to the vertical deflection coil (it can be an MP3 player or a computer headphone output). To display one frequency on the screen, try to generate a stable key. When the TV is turned on with an insulated screwdriver, gently touch the high-voltage wires one by one, finding out what changes on the screen this will result (this should be watched by your assistant or use a large mirror).
>img src="https://sdelaysam-svoimirukami.ru/uploads/posts/2018-02/medium/1519192640_17.jpg" style="max-width: 100%;" alt="Oscilloscope from the old TV">
Oscilloscope fromold TV
 Oscilloscope from an old TV
 Oscilloscope from an old TV
One of them will affect the frequency of scanning. On the board where it comes in, you need to solder the trimmer resistance (approximately 50-60 kOhm). Once you have verified that the node is working, you can pull the handle of the resistor involved from the instrument case. Even perfectly executed the horizontal frequency setting will not allow you to see the upper range, but just output the scroll waveform to the screen, and you can customize the existing ring tabs around the narrow part of the tube, usually black or dark gray, and also indirectly control the final image. >

Enhancing the input signal

Everything that has been done up to this point has allowed us to create a good visualizer of the input signal. It is enough to connect the iPod to the vertical deflection coil and the sounding music will be displayed on the screen. But in order to get a real oscilloscope, you will need an additional amplifier (you can assemble it where the tuned UHF / VHF tuner was located).His idea was borrowed from several thematic sites, in order to obtain the minimum cost and maximum efficiency. The basis was taken by Pavel Falstad’s development, and the presented printed circuit board was a modified two-stroke audio amplifier circuit. To implement it, we need: TL082 micro assembly, including 2 op amps, a pair of transistors (for example, 41 НПН / 42ПНП), power regulator LM317 , a Polyus rotary switch, a potentiometer of 1 mΩ, two trimers for 10 kΩ, 4 diodes for 1A, a transformer for 30 V of alternating voltage, an electrolyte of 1000 μF 50 V, two electrolytes of 470 μF 16 V and 5 resistors (10 Ohm, 220 Ohm , 1 kOhm, 100 kOhm and 10 mOhm).
Oscilloscope from the old TV
Oscilloscope from the old TV
Oscilloscope from an old TV
The first OU controls the gain of the input signal according to the formula R1 / R2, where R1 is the resistance selected by the rotary switch, R2 is the pot 1 mΩ. Theoretically, it is able to amplify the input signal up to 1 million times (with a minimum of 1 ohm present on the rotary switch).The second tracks the transistors to receive the necessary voltage to open the transitions and compensates for distortions. They need 0.7 V for opening and 1.4 V for switching. The finished circuit requires mandatory calibration. The power regulator is designed for a difference of 30 V, so the OS standard will give + 15 / -15 V, but for good filtering its output should be a few volts lower than the voltage on the capacitance of 1000 μF. There is trimmer 1 for this. The output of the circuit is connected to a horizontal deflection coil. The music passed through the circuit begins to be cut off at the top / bottom. To avoid this, trimmer 2 is adjusted until the tops of the clips touch the edges of the screen. This will lower the voltage and prevent the transistors from overloading the instrument's RF path (burn the deflection coil). Now you can connect the built-in speaker system to the TV output. With excessive loudness, a large load impedance is added (for example, 10 Ω 1 W), with a lack of sound, the load impedance is put on the deflection coil, after which the latter is recalibrated. To protect yourself from unnecessary annoying sound signals in the process of looking at the necessary input signal,You can install a switch on the speaker.

Build it all together

An additional amplifier can generate a strong magnetic field, so you should take care of its design. The board should be carried out as compactly as possible, with short leads and good grouping. It does not require special shielding, but in order to avoid interference with other TVs in your home, take care that it is located in the housing without creating interference to the main components. In extreme cases, you can use a wooden or plastic case, glued inside with foil.
Oscilloscope from the old TV
Oscilloscope from the old TV
Oscilloscope from an old TV
In the disassembled TV, when removing the analog tuner, there was enough space for installing a transformer with such a card and even a hole for the power switch came up. It is also advisable to shield the transformer so as not to interfere with the TV channels.Connect the terminals for the synchronization voltage and the signal under investigation; connect only shielded wire to the board. After connecting the transformer to the circuit, connect S1 and S2, respectively, run the input wires through the hole in the television receiver, connect the output circuit to the speaker and deflection coil. Use the minimum wire length in all conductive connections to reduce the dissipated inductance of the circuit. It remains to find a convenient installation location S1 and S2, close the back cover and proceed to a test drive.

Testing the device’s performance

In its functionality, the assembled oscilloscope is far from decent laboratory models, but indispensable for use in simple projects where you want to see the waveform. Also, a certain novelty has the ability to hear the signal under study, especially when receiving feedback resembling "signs". In this example, you can observe a change in the signal induced by a conventional wire coil when it is located in an arbitrary location, above the internal transformer of the device and when it is above the laptop processor. The ability to amplify the incoming signal is an excellent function,if you do not need its absolutely accurate parameters. The noise frequency of 60 Hz, amplified by the circuit, can still be determined with sufficient error. But this phenomenon also causes a stray inductance of the input wire. Only shielded grounding of all parts of the circuit can reduce interference.
Oscilloscope from the old TV
Oscilloscope from an old TV
The demonstrated coil with a wire connected to the input of the device allows you to use a large inductance with strong amplification. She can detect power sources for a few meters, directing the coil in the direction of the location of the transformers, then visually review their work. You can also find the location of the processor inside a complex device. You can use a coil as an inductive microphone, placing it near the speaker playing music. The magnetic field reproduced by the speaker’s coil will be detected and amplified by the created instrument, after which the played music will be reflected on the oscilloscope’s kinescope. The Internet channel can be clearly viewed on the instrument.A dedicated home line (120 VAC) was used as an input signal for this, and, having shown its “picture”, the device still works.

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