Fm transmitter DIY repair

If you need to transmit audio sound over a relatively short distance, then you can assemble the transmitter circuit presented on this page. The circuit is based on two NPN transistors BC547... The range at best will be 70 meters. You can adjust the volume of sound transmission using a variable resistor of 100 kiloohms, as well as on the receiver itself. It is not necessary to install an LED with a 330 Ohm resistor, it serves as an indicator.

I used this device for broadcasting sound so that I could listen to the music I needed from a short distance from home, for example, in a garage, and receive a signal on an ordinary FM radio. Layout printed circuit board is available - download.

Analogue of imported silicon bipolar n-p-n transistor bc547 is domestic kt3102... The higher the gain of the transistors, the more powerful the audio transmitter will be. If you want to make the device miniature, use transistors in the sot-23 package: BC847... The picture below shows the location of the base, collector and emitter.

The best, in my opinion, the power supply for the circuit will be two batteries AA 1.5 V connected in series. Together they will give a voltage of three volts. The operating time depends on the current consumption, as well as on the capacity of the batteries. Usually, the higher the cost, the better they are. For example, if you use fairly expensive batteries GP Ultra Alkaline, with the manufacturer's declared capacity of 3.1 A at a current in the circuit of 8 mA, this device can work without interruption, roughly 387 hours. The problem is that it is very difficult to “drain” the entire battery charge. Therefore, in reality, the circuit will work without shutdown and with a stable signal transmission for about 150 hours, or almost 7 days.

The coil has six turns of insulated copper wire with a cross section of 0.3-0.5 mm. We wind this coil on the paste from the handle.

When testing the device, the current in the circuit was almost 10 mA.

It is very easy to catch the frequency of the transmitter by twisting the trimmer capacitor and “playing” with the coil, shifting and expanding its turns. I caught my transceiver at 89.90 MHz.

I assembled this circuit on smd parts, only I took the transistors in the TO92 package. An antenna is a piece of copper wire, the more the better. If you just touch the antenna wire, then the frequency does not go away, and if you pick it up, noise starts in the receiver's headphones.

I tried to transmit sound both from the computer and from the phone. Too loud a signal is transmitted with numerous noises and wheezing, the optimum sound strength is adjusted by a trimmer resistor. In general, the audio-sound quality is pretty good. Received on a black and white Nokia phone, and listened to the sound through headphones. There were no big reception problems.

A video of the operation of the sound transmitter is below. Song: bwb - my boys.

On this I say goodbye. I was with you EGOR.

Modulator repair basically boils down to repairing the power supply, which is made in most car FM modulators, according to a simple scheme: a stabilizer on a microcircuit 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 I have already indicated is a polarity reversal, then a voltage surge in the car's network (charging relay malfunction, battery disconnection while the car is running) and wire breakage from the power supply to the circuit. I already wrote about these malfunctions in the article "Malfunctions of the FM modulator". When repairing a modulator, these faults are most common.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 commensurate with the purchase of a new car FM modulator and not all clients go for it).

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 of the SP7805V2-L / TR type (5 volt stabilizer) and capacitors. When repairing a modulator, start with a visual inspection. We check: whether the capacitors are swollen, for a wire break and whether there is any damage to the stabilizer.

These stabilizers, unlike conventional ones, for example LM7805, have a smaller SMD D-PAK package, and a better ° C / W ratio.
Datasheet 78 M05

It is better to change the stabilizer to a similar one, but if there are difficulties in the radio components, then you can put the usual one by placing it in the FM modulator case and soldering it with wires, preferably providing it with a small radiator.

Recently I bought an MP-3 FM modulator: LCD display, 1 gig of memory + a slot for additional memory expansion - a traveling thing! BUT. did not work for a long time, the memory flew off and is not detected through the computer !! Does not keep the frequency settings, volume level and song number. When the ignition is turned off, it starts playing from the first song. Maybe someone has encountered this or a similar problem? These modulators are generally repaired.

My brother-in-law has already changed several of these. They don't work for more than six months. China, however.

Re: Re: Repair MP-3 FM modulator?

doing repairs. such a question how to repair MP-3

I bought it too. It is impossible to listen to music when you go along the trolleybus wires. Therefore, I decided to buy an inexpensive Elenberg tape recorder with a memory card slot and a flash drive for only 3000 rubles.

FM modulator brought to mind! there 3.5jack was this audio input! but due to its (transmitter) simplicity, by the method of scientific poking! - where there is an entrance, there may be an exit!)))) Now I listen to mp3 through the wire, bypassing the radio channel!

rubin4ik Please describe in more detail how to connect the ch.w. modulator. wire conduit

pysy: yes! I also did the modulator for my father! working

pypysy: yes! who has music with mini-disks, make an audio input like 2 fingers obos valt

I've done it, only the mini-disc drive could not be saved although he died without my help

with a modulator it is clear how to make an audio input I have a radio tape recorder with a TV, CD and mini disks?

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Look at the fuse, it's 99% his fault.

I bought just such probably 10 pieces already in a year, to my friends and myself. Nobody complains, everyone sounds without interference.

I bought just such probably 10 pieces already in a year, to my friends and myself. Nobody complains, everyone sounds without interference.

Ordering is simple - click Add to Cart and pay through PayPal, in which you pre-register and register your visa, with which you can pay online. It is completely safe.

Free shipping. The parcel takes two weeks if the local mail does not mess up. It happened that after three weeks a repeated notification came when I did not see the first one in my eyes =))

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 heats up decently.

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

First of all, I found an old 5-volt power supply unit, from it I used only a board with a rectifier and a case. We change the dried out Soviet electrolytic capacitor to 470 mkf 10 volts, to 25 volt 2200 microfarads, for better filtration. We solder the Chinese transformer, cut off the tap, because we do not need it, as a result, the output turns out to be 13.8 volts under load.

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

In total, at the output of the unit under a load of about 10.8 volts, this is what you need. The stabilizer, in turn, produces all the same 4.9 volts. So everything is in order and the food suits him!

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 an ordinary Chinese receiver! The transmission is naturally in stereo, there is a minimum of noise and the quality, I would say, is excellent.

Finally, some characteristics of the Videovox AXFM modulator:

- Master RF generator with digital frequency synthesizer;
- Selection of the 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 97.7 and I still have it);
- Stereo coding with pilot tone;

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

We assemble the case, hang it under the table on double-sided tape next to the receiver and enjoy the radio in any FM receiver). Sometimes my neighbors listen across the road and even motorists in the nearest garage, as it takes 40-50 meters for a good receiver! Comrade was with you. vanesex.

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Redesigning the transmitter for connecting to the CD / MP3 / DVD input of the car radio directly bypassing the FM modulator (Supplemented)

Why do you need to connect directly, you ask, but who has thought about this for a long time?

This is necessary so that there are no intermediaries in the transmission of signals, in this case, in order to bypass it, you need to intercept the signal before it enters the transmitter, and we get a clear sound without interference!

Plus, you get another 3.5 ”cord-in jack for connecting external devices!

To do this, if you have KT0803, and it is installed on all Chinese, cheap equipment. We solder it as drawn according to the wire diagram. And the resulting connector, we connect it to the car radio to the AUX in.

The microcircuit must be dismantled so as not to create unnecessary interference!

And voila, we get high-quality sound, and if we also supply 12 volts from the on-board network connector, we get an independent flash, Sd card, etc. reader!

5- Leg ground
6- Left analog signal input
7 - Input of the right analog signal

The output from the transmitter microcircuit of the kt series is connected to the AUX input of the tape recorder.

Why is this done - we remove the interference of the FM tuner, the sound becomes clearer without radio interference!

chip marking, pin, leg assignment, etc.

Microcircuit connection diagram. Or factory inclusion.

According to the circuit for switching on the microcircuit, you can determine which legs the linear signal is applied to. Next, we solder the microcircuit, and to those two inputs, we solder capacitors with a nominal value of 4.7 / 63 mF / v. And after the capacitors, we output the wires to the nests.

Wed 7 Jan 2014

Views: 14 260 Category: Do it yourself

Many users from different sites, as well as YouTube subscribers, after the video, asked for instructions on how to assemble a bug from the FM modulator. The FM modulator has become quite a popular device recently. It is intended for decoding audio tracks and transmitting over a radio channel. Operating range up to 10 meters.

In order to assemble a bug from a modulator of special skills and knowledge in radio electronics, basic knowledge of a novice radio amateur is enough. We disassemble the FM modulator and discard the case. The filling is powered by 5 Volts, the voltage from the car's on-board network goes to the linear voltage regulator, at the output of which we get the indicated 5 Volts, it is this voltage that feeds the digital part of the modulator.

Any modulator consists of a small processor (in our case it is not needed, but it cannot be separated from the main board) and a transmitter.The main advantage of such a bug is that the circuit has quartz stabilization, so the transmitted signal is very high-quality and there is no frequency slope observed here.

The circuit can even be powered by a single mobile phone battery with a voltage of 3.7 volts. On the main board, we can see the transmitter IC.

We turn on the radio transmitter and tune the modulator to any frequency; tune the receiver itself to this frequency. Then we take a thin and sharp metal object (necessarily bare), for example, a needle and alternately touch all the pins of the microcircuit with the tip of the needle. When we touch some of the conclusions, we observe distortions in the radio receiver.

We select the output that reacts to touches more than the others, that is, the distortion in the receiver is louder than in the case of the others. Directly to this output we connect (solder) a non-polar capacitor (ceramic or film does not matter) a capacitor with a capacity of 0.1 μF.

We send a sound signal to the free output, when an electret microphone is connected, we will receive a radio transmitter.

A week ago I bought a CarMp3Player radio modulator, but just yesterday it started to lag, or rather to hiss, and the song is almost inaudible (no matter what wavelength), the radio is working fine. I understand that China and so on and so forth, but can it turn out somehow to revive it for at least another month? I listen to music on a USB flash drive, songs on a flash drive somewhere around 300MB.

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... guarantee.
. /. /.
... no. Yes.
. /. /. /.
... throw it out, take a new one. pick it out, don't do it. change and do not take a steam bath
... and take a new one.

took on the market what a guarantee pffff

I hope that something has gone there and this problem can still be solved.

watch post # 2 carefully

No, I still need sensible advice

Drive to the market, find a seller, change the modulator, take the money.

Throw it away and take a new one (repairs will cost twice as much).,

xs, how these modulators are still alive, now a Chinese 1din radio tape recorder with yusb costs 500r. And on the topic, then of course throw it out and buy a new one, it will be cheaper.

Hello dear friends!

I want to tell you about a compact FM transmitter for connecting to audio devices via a 3.5 mm audio jack. The transmitter allows you to broadcast an audio signal over radio frequencies in the range from 87.5 to 108.0 Mhz. I ordered it for a friend, taking into account the price and attractive design (he asked for an inexpensive one to listen to audiobooks in the car). Before giving it to him, I disassembled it a little and tested it. The review contains detailed photos and impressions of the operation.

Delivery within 19 days, packed in a bubble wrap, contents intact.

V set the device itself and a micro USB cable for charging the battery, cable length 30 cm. The design of the transmitter is simple and concise, stylized as Apple products.

During charging, the state of charge is displayed (4 levels):

The transmitter can be connected to any 3.5mm headphone output: phone, laptop, mp3 player, computer, etc.

Connection and configuration are elementary:
1) Turn on the device with the onoff slider
2) Set the desired transmission frequency, for example 87.5 Mhz
3) Apply audio signal from source
4) Tune in the receiver (another phone, radio in the car, etc.) to the frequency 87.5 Mhz
5) Experimentally adjust the sound level at the receiver and source (to minimize radio interference)

Snap-on case, easily disassembled with a plastic card:

Microcircuit without designations, just in the form of a drop. The battery is also unnamed.

The backlight is bright enough, the numbers are easily distinguishable. After a few seconds, it goes out by itself.

The battery lasts for 5-6 hours of continuous operation. Estimated battery capacity 180 mAh.

The audio signal transmission quality is average for this class of devices.In a quiet room at a distance of 10 m from the source, weak radio interference can be discerned in the headphones. For testing, it transmitted an audio signal from the phone to the phone (the antenna in this case was the phone's headset).
In principle, this interference is barely perceptible, I removed them by adjusting the volume of the audio signal on the transmitter and receiver. In a car, this interference is not noticeable.

Purchase:
After buying and receiving the device, I found out that there are transmitters with a similar design, but with more solid characteristics.
For example, here is such a more expensive and better performance model BFM26
As far as I can tell, at the moment, this is the best option that can be ordered on the Internet, based on a specific design.

Conclusion: The universal FM transmitter has been tested and found to be serviceable, all functions are normal. During testing, I did not find any anomalous behavior.

Pros:
1. Nice laconic design
2. Versatility and ease of use
3. Autonomy (5-6 h)
4. Compact size
5. Decent quality of audio signal transmission
6. Display backlight
7. Transmitter can work while charging

Minuses:
1. There is no instruction
2. Microcircuit on the board without designations
3. No data on battery capacity

Thank you for your attention, all successful online shopping!

P.S.
About stereo:
The number of wires that go into the plastic seal of the audio plug and the designation on the board indirectly hints to us that there is stereo. Since the main microcircuit is not signed and it is impossible to look at the specification for it, it is difficult to say something definite about it.

It would be interesting if the owners of such devices, of which there are many judging by the comments, posted a photo of the disassembly and could be compared more objectively

I will add a useful comment from a Muskovo resident with the nickname lockeron:
“A good transmitter, I took one myself. You can take a look at my video reviewer on it: