Today we will discuss radio receivers. A video about an old car radio from 1960 on from the Volga, look at YouTube, modern foreign semiconductor equivalents differ in their element base only. Lamp technology is good, giving a person an idea of the principle of operation of the device. Repairing the radio with your own hands turns into a useless, hopeless exercise if the master is unable to understand the actions. A person is not so surprised that dental crowns serve as a strong radio signal detector with an in-ear column in the ear; if you are familiar with the concept of amplitude modulation, serve as the basis for supplying information to an analog broadcast channel of a station. Without penetration into the scheme of a typical radio receiver, the text would have turned into a fiction of a narrow focus, without representing interest to a wide circle of readers.
The device of a typical radio receiver
Receiver catches a wave, amplifies. Retrieves useful information, gives to the speaker. Create designs according to the criteria:
- economic feasibility;
- quality;
- reliability.
The resonant circuit of the radio receiver
The radio receiver begins with an input stage tuned to the desired wave. The antenna is considered a relatively broadband device that catches a large number of channels. In order to find the right one among the mash, some gates are needed that let the useful signal through. The portal will serve as resonant circuits. The theory is not important, it is useful for readers to know the following facts:
- The resonant circuit transmits a narrow section of the mass of the spectrum, the width of which is tuned to the band occupied by the channel. For example, with amplitude modulation of 10 kHz, about that. The level of the characteristics at the level of 0.7 of the normalized graph shows the specified size along the horizontal axis. The shape of the amplitude-frequency response is determined by the type of circuit.
- In the simplest case, the resonant circuit is formed by inductance connected in parallel, capacitance. Not the only option. Tuning the circuit to the frequency is varicapami( capacitor with variable capacitance).Coarse channel selection is performed by a mechanical switch, transistor keys. The resonant circuits of the DW, SV, VHF are different in physical terms, no one can adapt to all ranges by changing the capacitance of the varicap.
- Resonant circuit is considered a passive element that does not carry a large electrical load, rarely breaks. Let's follow the breakdown simply:
- only one band stopped working, it is here, to the mixer( read below about the high-frequency amplifier);
- if, on the contrary, only one range works, the switch is broken: mechanical, transistor switch.
The difficulty is the same: the high-frequency voltage of the output of the resonant circuits can hardly be measured, a typical multimeter is not designed for such an application.
Radio Frequency( High Frequency) Amplifier worn on the screen, reducing lossesThe initial frequency goes along the path, the wave differs by an order of magnitude for LW and VHF, it is impossible to perform the electronic circuit of the radio receiver on one transistor, microcircuit. It is customary to divide the input stages for FM, other frequencies. However, for older models and modern ones. A high frequency amplifier is not recognized as a selective circuit - a broadband device. Easy to explain. Having a section of the radio receiver path filters, the cascades would need to be rebuilt parallel to the input resonant circuits. Complicates the design of the electrical circuit.
Mixer, radio frequency intermediate frequency amplifier
For normal detector operation, a fixed frequency signal is required. For FM - 10.9 MHz( frequency modulation), for LW, CB - 450 kHz( amplitude modulation).The input wave is mixed with the frequency of the local oscillator( high-frequency reference oscillator), the output gives the difference, the values are indicated above. The local oscillator and the mixer will in fact become amplifiers on a transistor or a microcircuit, the first one has a generation mode configured, the second one works in a linear mode. The receiver is built on cascades of this type. These include the considered high frequency amplifiers, intermediate frequency amplifiers, which we turn to below.
Radio detector
Following the stabilization of the frequency, the radio station receives useful information from the broadcasting station. Implemented in detectors. Both cascades are built on diodes, transistors, microcircuits, the difference in the use of oscillations. With amplitude modulation, useful information is laid down by a voltage swing. Consequently, the simplest diode cuts the negative part, the envelope is obtained after filtering the RC-chain. So works the simplest amplitude detector. The frequency variant is organized, for example, by the discriminator. The device, whose peak of the amplitude-frequency characteristic falls on the resonance( 10.9 MHz), goes to the edges of the decline. The result is a useful signal.
To avoid distortions, signal distortion, it must be 100% symmetrical with respect to the carrier. In fact, the transport is moving, the Doppler effect, other nuances shift the signal. An automatic frequency control comes into play. The cascade acts on the resonant circuits, heterodyne, keeping reception normal. The principle of operation is based on an assessment of the symmetry of the incoming signal. The spectrum is mirrored from the carrier( in both directions).There are exceptions with one sideband; it is rarely used in household radio receivers.
To save energy, the transmitter is often cut off the carrier, leaving the pilot signal, for peaceful purposes usually do not, the design of the receiver is complicated. The progressive method indicates the future. In the receiver, the carrier, the missing part of the spectrum is restored according to the rule specified above.
Low Frequency Amplifier Radio
Low Frequency Amplifier is the critical part, quiet speech and music are not needed by customers. The cascade of the radio receiver is easy to find; powerful chips, transistors equipped with hefty aluminum radiators are placed here. Whatever the elemental base, you can achieve a screaming radio receiver by spending power, a certain part is dissipated by heat. Overheating is blocked by radiators.
Important! Germanium is afraid of temperatures above 80 degrees Celsius. Semiconductor pn junctions have advantageous characteristics. It is necessary to cool the power elements of the radiators.
In radio receivers two channels or more. In case of receiving stereo. Channel separation on the right and left is adopted in the broadcast with frequency modulation, VHF band, including FM.The method of encrypting information is different, it does not matter when independent repair of radio receivers matures. A low-frequency amplifier is a common cascade where information is fed directly from an amplitude detector, and from a frequency amplifier through a stereo presence detection circuit.
Repair of radio receivers
In general, it is necessary to split the radio receiver into cascades. The purpose of the schemes described. Forgot the power supply for a reason, discussed the topic of reviews. In lamp radios, a greater number of ratings are required. The cathodes of the lamps are heated by an alternating voltage of 6.3 V. By the way, the efficiency of the cascades can be estimated by the glow in the dark of the electrodes. You must wait until the radio warms up, then check for reddish reflections by turning off the light. You can simply understand the location of the breakdown. Bulbs of burnt lamps turn black. Glow can in a completely normal style. Repair of a lamp radio is easier than modern.
The device is visually divided into logical parts, it is possible to approximately localize the fault. A radio receiver often contains control contacts, another thing is where to find information. We believe that, if desired, information will be found on a specialized forum in the technical library. Now it is not accepted, remembering the good old days, to supply the radio with a detailed electrical circuit, everyone who is ready for that. In the case of hybrid electronics, the device can be a single chip, the low-frequency amplifier stands alone. We'll have to find a new radio.
In other cases, you can repair transistor radios, repair tube radios. Take the last bills off. Musicians still prefer tube amplifiers.
So, the independent repair of the radio receiver is performed according to the indicated scheme:
- Disassembly of the device for the assessment of the internal state, inspection.
- Breaking the electrical circuit into logical parts.
- Search for radio documentation on available channels.
- Survey radio amateurs on the forums on the subject.
Speech concerns old devices - first of all we clean off dust, we look at installation, we check paths. If a light tapping on the device responds by the crash of the radio receiver columns, the matter is in the broken contact. Solder cracks, peeling of the tracks, breaks - to be fixed, take the trouble to re-check the performance. In Soviet-era car stereos, an inverter is used, the noise of which will be heard after switching on. Repair of old radios is useful for beginners, allowing you to learn how to handle equipment. Masters are engaged daily. Learn the types of radios, repair methods.
