Pin assignments on the Chinese fm modulator board. Foreign circuits of FM transmitters. Video of the transmitter working

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Modulator repair Basically it comes down to repairing the power supply, which is done in most car FM modulators, according to a simple scheme: a stabilizer on a chip and capacitors. Even the diode shown in the diagram (see below) and capable of protecting against polarity reversal is not present in all modulators.

Several reasons can lead to the failure and repair of the FM modulator, one of which I have already indicated is polarity reversal, then a voltage surge in the car network (malfunction of the charging relay, disconnection of the battery while the car is running) and a break in the wires from the power supply to the circuit. I already wrote about these malfunctions in the article “FM modulator malfunctions”. When repairing a modulator, these faults occur most often. Often there is a failure in the firmware (not in all models and I do not repair such a malfunction, for the simple reason that the repair will be comparable to the purchase of a new car FM modulator and not all clients agree to this).

Let's focus on repairing the fm modulator power supply.

As already mentioned, the power supply is quite simple - it is a stabilizer installed on the board on an IC type SP7805V2-L/TR (5 volt stabilizer) and capacitors. At modulator repair You need to start with a visual inspection. We check: the capacitors are not swollen, the wires are broken, and there is no damage to the stabilizer.

There are quite a few FM transmitter circuits on the network, but most of them are micro-power (100-300 mW).
Using a modern element base, it is easy to assemble a power amplifier of minimal dimensions and quite decent power...

The proposed transmitter, the diagram of which you see below, is powered by 12 V, is small in size, and operates in stereo. Only 9-12 mW is supplied to the input of the power amplifier and this is enough to drive it up to 1.3...1.7 W. In this case, I did not have to assemble a modulator and an FM generator; I was quite satisfied with a car FM transmitter powered by a cigarette lighter. Its power output is just right. Since the weak point in these transmitters is the voltage stabilizer, made on a 5-volt bank, we had to lower the voltage to 9 V, adding another stabilizer to the circuit.
Now let's take a closer look at the power amplifier itself. It is assembled on two transistors of the BFG-591 type (VT1 can be replaced with BFG-135), as mentioned above, it is powered by a voltage of 12 V. A correctly assembled transmitter begins to work immediately, all that remains is to adjust the tuning capacitors (their capacitance with the given coil data within 22-30 pF, capacitor C12, if necessary, can be excluded from the circuit).
The gain bandwidth of the PA is quite wide, so it is quite suitable not only for amplifying signals in the FM range, but also on almost all television channels of meter waves, however, in this case its output power will drop to 0.7...0.9 W at the same input level (90-120 mW).

Coils L1, L3, L5 - 5 turns of PEL-2 wire with a diameter of 1.0 mm, on a mandrel with a diameter of 5.5 mm
L2, L4 - contain 7 turns of the same wire, on a mandrel of the same diameter. (all coils are wound turn to turn)

The amplifier is assembled using SMD parts on one-sided foil fiberglass. All coils and tuning capacitors are installed on one side of the board, and the rest of the installation of parts is on the “printing” side.
Although this amplifier, assembled in SMD design, is not prone to self-excitation, just in case I separated the cascades with shielding partitions. Coils L2-L3 and L4-L5 are located perpendicular to each other.

At the transmitter output it would not hurt to install a P-circuit tuned in the desired frequency range, although you can use an amplifier without it.

PS. This amplifier was tested with modulators for cable television and showed good results when transmitting a TV signal on the air.

http://radio-device.ru/radio.php?p=prostoy_peredatchik_iz_car_fm_transmittera

In general, the modeler can be powered based on the power supply conditions of the stabilizer - at least from 24 volts, but the stabilizer without a radiator gets quite hot.

I decided to make a transmitter out of it to transmit radio from a satellite receiver to a receiver. The reception, by the way, is quite long - 20-25 meters, even taking into account the walls.

First of all, I found an old 5-volt power supply, from which I used only a board with a rectifier and a housing. We replace the dried Soviet electrolytic capacitor with a 470 microfarad 10 volt capacitor and a 25 volt 2200 microfarad capacitor for better filtration. We solder in a Chinese transformer, cut off the tap, because we don’t need it, and as a result the output is 13.8 volts under load.

Just in case, I put a resistor at 35 ohms so that the stabilizer does not heat up - it is better to let the resistor heat up, since it is in the power supply and there is ventilation in it, and the housing of the FM stereo modulator is closed.

In total, the output of the unit under load is about 10.8 volts - this is what is needed. The stabilizer, in turn, produces the same 4.9 volts. This means everything is fine and he was satisfied with the food!

Instead of plugs (we disconnect them at the connector), we solder a piece of wire in rigid insulation about a meter long, now the transmitting properties of the FM modulator reach about 35 meters to a regular Chinese receiver! The transmission is naturally in stereo mode, there is a minimum of noise and the quality, I would say, is excellent.

Finally, some characteristics of the Videovox AXFM modulator:

Master RF oscillator with digital frequency synthesizer;
- Selection of carrier frequency 87.5 - 98.5 MHz from 110 possible values;
- Large LED display to indicate the tuning frequency;
- Memory of the tuning frequency when the power is turned off (very convenient, I set it to 97.7 and I still have it);
- Encoding of a stereo signal with a pilot tone;

In practice, stereo works well; the receiver does not start to hiss even at a considerable distance.

We assemble the case, hang it under the table with double-sided tape next to the receiver and enjoy the radio in any FM receiver). Sometimes my neighbors listen across the street and even car enthusiasts in nearby garages, since a good receiver can cover the whole 40-50 meters! Comrade was with you. vanesex.

Typical malfunctions of FM modulators (transmitters) can be divided into 3 types. I will talk in detail about only one, since the other two in 50% of cases lead to the complete unsuitability of the device!

The first type of malfunction is the failure of the stabilizer chip. Figure 1 shows the modulator itself in disassembled form.

FIGURE 1 (click to view)

Figure 2 shows a stabilizer board with a 7805 chip (the letters may be different) in a maintenance package.

FIG.2 (click to view)

This microcircuit is the reason for the device failure.

The microcircuit lowers the battery voltage to 4.5 - 5v and stabilizes it. From its output, this voltage is supplied to the main board to power the modulator circuits. For various reasons, the microcircuit may fail. If a break occurs inside it, then power does not flow to the main board. But this is half the trouble. You just need to replace the chip and everything is fine.

It is much worse when a breakdown occurs - then this is an almost 50% guarantee of complete repairability of the entire device. So, the main board will receive voltage from the battery directly. And this will most likely kill the device completely!

In Figure 2, this microcircuit is marked with an arrow. There is noticeable swelling on its body, which indicates 100% failure of the microcircuit.

If necessary, the microcircuit can be replaced with any one that has suitable parameters. The main thing is that it has a stabilized output voltage of about 4.5 - 5v! I even had to install 7805 in a regular case instead of 7805 in the TO case! You just need to sharpen the metal part a little.

If, after replacing the microcircuit, the device still refuses to show signs of life, then there is still a chance to revive it. On the main board, not far from the soldering point of the conductor through which the +5v supply voltage is supplied, there is an SMD transistor (not in all models). Through it, voltage is supplied to the board. Most likely he is in a “cliff”. It is necessary to short-circuit the terminals of its collector and emitter.

If the device works normally after this, then you can either replace the transistor, or simply leave the jumper. Everything will work fine even with a jumper, it has been tested on more than a dozen devices!

If there is no such transistor on the board or the device does not turn on after installing the jumper, leave this idea, more serious components are most likely damaged!

There were cases when the device turned on after the indicated replacements, but the number 9 was displayed on the screen, which slowly began to go out. In this situation, further attempts to restore the device’s functionality were unsuccessful. Unfortunately, the reason for this behavior could not be determined exactly.

FM MODULATOR FOR CAR

An FM modulator is a small device that allows you to listen to music from any audio source or flash drive to any FM radio receiver. The modulator is connected to the car's cigarette lighter socket, from which 12V power is supplied. USB flash drives serve as storage media for audio files. Just record MP3 files onto a flash drive, insert it into the modulator and listen to it in the car. After all, it’s easier to record the desired list of songs on a flash drive than on an MP3 CD.We leave the modulator in the car, and take home only a flash drive to download new music or audiobooks from the computer onto it. If you have an audio output on your mobile phone, you can connect it to the modulator via the headphone jack and listen to music from your car speakers or a conversation as if over a wireless channel. FM modulators with memory can be used as a regular USB flash memory.

There are also such interesting varieties on the market as Car Handsfree Bluetooth Adapter and Car FM Transmitter. CharacteristicsFM modulatorfor car:

• - LCD screen 1.4".
• - Connection via: Bluetooth, USB, SD card, linear input for PDAs, MP3 players, laptops.
• - Display on the screen: FM wave number, track number, artist name, song name, equalizer, caller number.
• - FM frequency range: 87.5-108FM.
• - Support for USB, SD cards up to 16GB.
• - Display backlight.

This device can be used as a headset in the car. On your mobile phone, in the search mode for Bluetooth devices, find this modulator and connect it as a headset. When a call comes in on your mobile phone, press the off-hook button on the FM modulator control panel and talk without picking up the phone. When the conversation is over, press the end call button on the FM modulator remote control.When you turn on the ignition in the car, the device automatically connects to the mobile phone if the Bluetooth function is turned on on the mobile phone.

Technical characteristics of a standard PM modulator:

• The FM modulator is powered from a car cigarette lighter 9 - 24 V. Rated current: up to 100 mA. There is a USB connector for reading files from a flash drive.
• Signal-to-noise ratio: 90 dB.
• Output frequency range: 20 Hz - 20 kHz.
• Equalizer modes: Normal, Rock, Pop, Classic, Soft, Bass.
• Audio input - reading files from the audio output of any player. Supports MP3 and WMA files.
• Display to indicate volume, number, song title.
• About two hundred fixed FM frequencies are used to communicate the modulator with the receiver.
• The optimal operating distance is up to 10 m.

FM modulator operating instructions. Using a USB cable, connect the MP3 FM modulator to your computer. Copy the necessary music files to the root folder of the modulator.Place the MP3 FM modulator in the cigarette lighter socket, first making sure that the voltage is 9-24 volts.Press the "Play/Stop" button (bottom center button on the device or green top left button on the remote control), the melody will start playing.By pressing the “CH” button on the modulator or the “CH-” and “CH+” buttons on the remote control, you can change the FM frequency.Turn on your radio and switch to the FM band, tune in to the frequency you selected on your auto MP3 modulator. Make sure the frequencies match.

Initially, music files play in order. To switch melodies, use the “Back” and “Forward” buttons (the extreme left and right buttons of the modulator). To stop playing a melody, press the Play/Stop button. If you want to listen to melodies in a chaotic order, press the number buttons on the remote control.By pressing the EQ button on the remote, you can control the built-in equalizer.Before using the remote control, remove the protective film from the battery compartment.To improve the sound of the music being played, set the volume on the modulator to less than 50 units. The volume level is adjusted using the Vol + and Vol - buttons (on the remote control) or “Back” and “Forward” (on the modulator panel (press and hold the button, you will find the optimal ratio of sound volume and sound quality)).Advanced models of FM modulators have an equalizer function, balance function, quick song search, display brightness adjustment, voice recorder, and language menu settings.

A typical PM modulator circuit is shown in the figure below:

The basis of the device is the BH1417 chip. It is powered by a voltage of 5 V and consumes 5-10 mA. Despite the small output power, it seems interesting to connect to it a simple RF power amplifier using two transistors KT368 and KT610, in order to increase the range to a kilometer or more. The sound quality of this BH1417 microcircuit is much better than that of the well-known BA1404 stereo modulator. According to some information received from radio amateurs, with an antenna installation height of 10 meters - then when using a 2-watt amplifier - the radius was not about two kilometers. If you raise the antenna higher, a radius of up to 7 km is covered with reliable reception. In general, this device, at a relatively low price of 15-25 euros, is of interest in terms of its use in various amateur radio devices.

Suggestions and discussions on the device write to