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Radio (a device for listening to broadcast audio signals)



Introduction

The radio is the receiver of radio broadcasting. The high-frequency modulated weak signal input by the receiving antenna is amplified, frequency-converted and detected, and then restored to an audio signal. According to different needs, the radio can be combined with different components and wiring methods. Early radios used electronic tubes, but now transistors and integrated circuits are commonly used. Rifle-type regenerative radios with simple circuits use only one transistor, while superheterodyne AM FM radios with complex circuits often use multiple integrated circuits.

Development History

In 1844, the telegraph machine was invented, which can communicate with each other in different places, but still must rely on "wires" to connect. The reception and transmission of radio signals are actually "radio communications."

In 1888, German scientist Hertz discovered the existence of radio waves.

In 1895, the Russian physicist Popov claimed to successfully send and receive radio signals in two places 600 yards apart.

Later in the same year, Marconi, the son of a wealthy Italian landlord, who was only 21 years old, successfully launched the first radio wave in his father’s estate.

In 1897, Popov used his wireless communication equipment to successfully communicate with platforms on land on naval cruisers.

In 1901, Marconi launched radio waves across the Atlantic Ocean.

In 1906, the Canadian inventor Fessenden first emitted "sound", and radio broadcasting began.

In the same year, the American De Forrest invented the vacuum tube, the ancestor of the vacuum tube radio.

From now on, improved semiconductor radios (atomic radios) and transistor radios have appeared.

Actually, there is a debate about the inventor of the radio: some people say it’s Popov, some say it’s Marconi. Popov, a Russian physicist, was born in Russia in 1859, the son of a priest. Since 1885, he began to study radio communications. And in a speech on May 7, 1895, he disclosed the results of his research on successfully transmitting and receiving radio signals after improving the receiver of Luozhi. Since 1901, he has served as a professor of physics at St. Petersburg University. Some people think that he is the one who really invented the radio, but perhaps because he was a scholar and was too dedicated to academic research, and did not make the invention of the radio widely known; or perhaps it was because Popov’s invention was lost. The Russian Navy considers it to be a great weapon in the military and is classified as a secret and will not be released to the public. On the contrary, Marconi is very business savvy. It is said that he established the world's first radio factory and obtained a patent. However, some people criticized the radio he made, which only combined other people's inventions-Hertz's coil antenna, Loj's tuner and receiver, and Nikola Tesla's sparkler. It is undeniable that he has made outstanding contributions to the practical application of radio equipment.

Receiving principle

It is to restore the high frequency signal received from the antenna to audio signal after detection (demodulation), and send it to earphone or speaker to become sound wave.

Radio (14 photos)

Due to advances in technology, there are many radio waves of different frequencies in the sky. If all these many radio waves are received, the audio signal will be like being in a busy city. Many sounds are mixed together, and as a result, nothing can be heard. In order to try to select the desired program, there is a selective circuit behind the receiving antenna. Its function is to select the required signal (station) and "filter out" the unwanted signal to avoid interference. This is When we listen to the radio, we use the "Select Channel" button. The output of the selective circuit is to select the high-frequency AM signal of a certain radio station. It is impossible to use it to directly push the earphone (electro-acoustic). It must be restored to the original audio signal. This restoration circuit is called demodulation. , Send the demodulated audio signal to the earphone, and then you can receive the broadcast.

Classification

Radio can be classified from different angles.

1. According to the different devices used, it can be divided into tube radios, transistor radios, and integrated circuit radios;

2, according to the different amplification methods, it can be divided into direct amplification radios And superheterodyne radio;

3. According to different broadcast formats, it can be divided into AM radio, FM radio and AM and FM radio;

4. According to the received band Different, can be divided into medium wave radio, short wave radio, medium and short wave radio, medium wave ultrashort wave radio, long medium and short wave radio, full band radio;

5. According to the different size, it can be divided into floor-standing radio , Desktop radio, portable radio, pocket radio, micro radio, etc.;

6. According to different power sources, it can be divided into AC radio, DC radio, AC and DC dual-purpose radio;

< p>7. According to different specifications and ministerial standards, it can be divided into Type A, Type B, and Type C. The radios are equivalent to three grades of high, medium, and low;

8. According to different uses, they can be divided into car radios, stereo radios, clock radios, dual-purpose radios, radios, and radios. Includes multi-purpose machines such as four-purpose machines for expanding and singing.

Overhaul

Inspection

1. Purpose: Before debugging the whole machine, ensure that the radio is working in a trouble-free state, so as to ensure smooth debugging.

2. Prerequisite: The installation is correct (such as the poles of the diode). There are no missing or wrong soldering of the components, no errors in connection, no false soldering, continuous soldering, etc. in the solder joints of the printed board.

3. Essentials: Be patient and careful, calm and orderly. The detection is carried out step by step, generally from the subsequent level to the previous level. First judge the fault location (signal injection method), and then find the fault point (potential method), step by step, remove the radio and eliminate the fault.

Avoid chaotic adjustments and dismantling, blindly hot soldering, which will lead to worse repairs.

4. Method:

(1) Signal injection method: The radio is a signal capture processing and amplification system, and the location of the fault can be determined by injecting signals.

①Use a multimeter with R×10 resistance, connect the red test lead to the negative electrode (ground) of the battery, and the black test lead touches the amplifier input terminal (usually the base of the transistor), and the speaker can hear the sound.

②Hold the metal part of the screwdriver to touch the input end of the amplifier. This method is simple and easy to see whether there is sound from the speaker, but the signal is relatively weak and cannot be heard without being amplified by the transistor.

(2) Potential method:

Using a multimeter to measure the working voltage of amplifiers or components at various levels can specifically determine the component that caused the failure.

Determine the location of the fault

1. Turn on the power switch and turn the volume potentiometer to the maximum. There is no sound in the speaker, and it can be determined that the low-amplification part must be faulty.

2. The method of judging whether the circuit before the low amplifier is working normally is as follows: turn down the volume, turn the multimeter to the DC 0.5V position, and connect the two test leads to the other two ends of the volume potentiometer. From the low-end to the high-end, dial the volume adjustment dial of the radio while watching the meter pointer. If you find that the pointer is swinging, and the number of pointer swings is about dozens of times when a sentence is normally broadcast. It can be judged that the circuit work before the low amplifier is normal. If there is no swing, it means that there is also a fault in the circuit before the low amplifier. At this time, the problem of the low amplifier circuit should be solved first, and then the problem in the circuit before the low amplifier should be solved.

Silent fault

Turn the volume up, use a multimeter with a DC voltage of 10V, ground the black test pen, and touch the red test pen to the center end and the non-grounded end of the potentiometer (equivalent to the input Interference signal), there may be three situations:

1. When touching the non-grounded terminal, there is no sound in the speaker, and when touching the center terminal, there is sound from the speaker. This is due to poor internal contact of the potentiometer. It can be replaced or repaired to eliminate the fault.

2. There is no sound when touching the non-grounding terminal and the center terminal. At this time, use the multimeter R×10 file, and the two test leads touch the lead of the horn. If there is a sound when touching, the horn is in good condition. Then touch the positive end of C9 with the resistance of a multimeter. If there is no sound in the speaker, it means that the headphone jack is not well connected, or the wire of the speaker is broken; if there is sound, check the push-pull amplifier Circuit:

(1) Check whether Q5 and Q6 work normally, and whether there is any disconnection in L5 Line.

(2) Measure the DC working state of Q4, if there is no collector voltage, then L5 primary disconnection, if there is no base voltage , Then R5 is open. If the red meter pen touches the center end of the potentiometer and there is no sound when it touches the base of Q4, it means that C7 is open or invalid.

3. Use the interference method to touch the center end and the non-grounded end of the potentiometer, and there is a sound in the speaker, which means that the low-amp is working normally.

No station failure

Silent means turning up the volume, there is a slight noise in the speaker, but the station cannot be received during tuning.

1. Measure the collector voltage of Q3: if not, then R4 is open or C5 is short-circuited; If the voltage is abnormal, check whether R4 is good. Measure the base voltage of Q3, if not, then R3 may be open circuit (there is no voltage at the base of Q3 at this time), or L4 secondary disconnection, or C4 short circuit.

2. Measure the collector voltage of Q2. There is no voltage, because the primary coil of the radio L4 has an open circuit. The horn will sound when the voltage is normal.

3. Measure the base voltage of Q2: no voltage, L3 secondary short wire or desoldering. The voltage is normal, but the interference signal is injected, and there is no sound in the speaker, which is the Q2 damage. The horn makes a sound when the voltage is normal.

4. Measure the collector voltage of Q1: no voltage, L2 secondary coil is broken, L3The primary coil is broken. The voltage is normal, there is no sound in the speaker, the L3 primary or secondary coil is short-circuited, or the tank circuit capacitor is short-circuited. If there is a short circuit fault in the inner coil of the middle circle, it is more difficult to detect because the number of turns is small, and it can be verified by an alternative method.

5. Measure the base voltage of Q1: no voltage, it may be R1 or L1 secondary Open circuit; or C1 short circuit. If the voltage is higher than the normal value, the Q1 transmitting junction is open. The voltage is normal, but there is no sound. Q1 is damaged.

At this point, if you still cannot receive the radio station, perform the following checks:

6. Set the multimeter pen to the DC voltage range, and connect the two test pens to R2< /sub>At both ends, use tweezers to short-circuit the primary of L2, as shown in Figure 1: The structure of a non-powered transistor radio, and see whether the needle indicator is reduced (usually reduced by about 0.2~0.3V). The voltage does not decrease, indicating that the local oscillator has not started to vibrate. Oscillation coupling capacitor C2 failed or opened. C1 is short-circuited (Q1 has no voltage at the base). L2 The internal primary coil is open or short-circuited, and the quality of the double connection is not good. The voltage decrease is very small, indicating that the oscillation of the machine is too weak, or the L2 is damp, the printed board is damp, or the double-connected leakage, or the trimming capacitor is not good, or Q1 The quality is not good. This method can also detect whether the Q1 bias current is appropriate.

The voltage decrease is normal, and it is determined that the fault is in the input circuit. Check whether there is a short circuit in the double connection, what is the quality of the capacitor, and whether there is any disconnection in the primary of the magnetic rod coil L1.

Maintenance

①Pay attention to the maintenance of the AC radio: It should not be used for too long in summer to prevent the power transformer from overheating and burning; pay attention to keeping the ventilation holes behind the radio for ventilation; Huangmei weather It is best to turn on the radio every day to let the various tubes and transformers of the radio emit heat to drive away the moisture, which can prevent parts from being moldy. You can put four paper-wrapped sanitary balls at the bottom four corners of the radio to prevent insects from eating the radio’s crate. (If it is a plastic case, it does not need to be).

②Pay attention to the maintenance of the semiconductor radio: use it frequently to prevent mildew and parts failure; when not in use, the battery should be removed to prevent leakage of slurry and damage the radio, and the battery should be replaced in time when it is almost used up , Because the internal resistance of the battery increases when the battery is almost used up, although it is audible, the sound is muffled and unpleasant, and it will leak if it is used barely.

③ Both AC and semiconductor radios should be shock-proof and heat-proof. If the antenna is installed, lightning protection measures should be taken (a isolating switch should be installed between the antenna and the radio, and the antenna should be disconnected from the radio when not in use or in a thunderstorm. , The antenna should be "grounded", that is, it is connected to a water pipe or iron or copper parts buried deep in the ground). The volume of the radio should not be turned on too loudly, as it will sometimes damage the speaker.

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