Hacking FM Transmitter

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In this instructable i am going to shed light on a FM transmitter i had purchased from ebay a month ago. After purchasing this gadget i was happy for a couple of days until i found out that there was some discrepancy in this gadget. Firstly i found out that the Gadget was powered by AAA batteries which didn't lasted longer. Secondly most important the range that it was covering was very much shameful. Like about 100 feet in open space and if you were inside a building about 50 feet. Therefore i thought of doing something. After some research and gaining some inputs from Google. I found out that no such description were given for the particular model, as it was a Chinese made. 
But indeed i got some information about the gadget. Therefore i would like to share with Instructable.

My primary Objectives would be as Follows:-

1. The power source should be compromised.
2. It should be capable of adapting with both the adapter as well as with the batteries.

3. The ease of access should be incorporated like it should have the ability to get along with any of the multimedia device.
4. Could be carried along while travelling.
5. Finally the most important objective, the range should well be increased to about more than the regular given specification.

Step 1: Opening the Shell

As i was shown various ways in instructable itself on how to open the transmitter, i was a bit surprised with my gadget. I had seen that in instructables posted to instructables it was shown that the FM Transmitter was hold up by only one screw and rest were just popping up the case.
But in my case i was surprised because there weren't any screws holding the Transmitter. I tried to pop it open but was unsuccessful. I tried and saw various opening to find if there were any hidden screws holding the unit. To my frustration at the end i found out none. Then by hook or by Crook i had to get inside therefore i made up my mind that i would open it, if also i had to break it.
Then i steady put up a wedge inside the shell and forced it to make a gap. I put a second wedge and again forced to make a more bigger gap. Still i was not able to see any screws from the gap. I continued till there was a breaking point. Then after a final push, there was large pop sound. Here i found out that this gadget was not held by any screws. 
The Company had made this product so that it was not serviceable, that no body would be able to open this product. I could see that the legs holding the gadget had broken, and in no way i wold be able to put it back like it would have been before. I had to think for that also. But on the other had i had successfully opened the shell. 
I guess this process of fixing it permanent was done deliberately so that no one like me could open it. But nothing was going to hold me back. I had successful opened the shell. But i would like to caution to the reader that this was process that i followed was done in a very slow and steady process, don't hurry up as you may end up damaging the PCB of the Transmitter.

Step 2: The Discovery Inside

After successfully opening the shell. I was able to see the PCB of the transmitter. I have given the photo's of both the back and Front side. It was totally different from the PCB that i had seen in other Instructables. I noted the parts No. that were printed on, especially the IC Nos. I took a look at the PCB with a much closer vision and kept note of the various inputs and outputs. Like i noted the audion in wires and the Battery connection and kept a note. I then removed all the wires from the board. Then i was back on to Internet finding the parts for details. 
I could distinctly see two IC on the board. 
1. 
2.

I was not able to find the datesheet for the first IC but luckly i was able to find out the details of the second IC, i have also attached the link for further clarification. I was able to find the classfied documents of the transmitter chip IC and i have attached it along here. Follow the link.
https://skydrive.live.com/#cid=1EA0AF6D5EE30BC0&id=1EA0AF6D5EE30BC0%216841

What i found out from the datesheet that the Second IC was the brains of the gadget. It was the transmitter IC. The date sheet is also not given in detailed but i found out what the pins are configured for. and particularly the pin for the antennae which was most important for me. It was pin No. 5. After following the track i was able to find out the RF pin out and also found out that the antennae was just a coil of PCB in built wound up tracks which was linked to the earth of the 3.5mm socket. Might be it may be helpful for the projection of the RF signal..

Step 3: Removing the Holdup

After exactly finding out the limiting resistor for RF, i decided to remove it from the board permanently. I would suggest to all the readers to do to remove it permantly as i think it may cause some RF loss. As if the resistor would be kept there itself, the RF would be splitted to go to the antennae as well as through the resistor, this would definately cause a loss in the RF outpur. Therefor if you would like put as much juce to the antennae, i would recommend to remove it fully.  I used a micro soldering Iron gifted by my sister on my Birthday to lift it off. Kindly take a look at the images. I started by dislocating the lower end of the resistor then going up. After successful removal of the resistor. I took an idea from one of the instructables for the Antennae part.

Step 4: Attaching the Antennae

Here come the delicate part of soldering the antennae part to the board. Try to get a thin wire, not as thin as a thread and solder on to where i have soldered. It should be perfectly soldered as when moved around it should not get disconnected. Then at one end attach a ring or a small washer of a kind. After successful attaching the antennae wire along with the holder, in the PCB fix it to the transmitter as if there should be minimum movement. I did it for myself by attaching a binding wire to the antennae and fixing it in the shell itself. 
Just take a look at the pictures.
It should be remembered that as Antennae is the important part and for the objective to be completed successfully you should follow the steps here very precisely as this is the primary hack for the project.
Now finally to solder the Audio jack to the FM transmitter.

Step 5: Attaching the Audio Input Jack

This is a stereo transmitter and it has the capabilities to transmit audio in two channels simultaniously. Therefore it is very much important to solder the audio jacks in their respectively places. In my case there color of the wires were WHITE, RED and BLACK. 
White = Left channel
Red = Right channel
Black = Ground & Antennae


I connected the White and Red terminal to the board but i didn't connect to the Internal antennae as i thought the RF would be splitted to the on-board antenna.

Step 6: Adding a Seperate Power Supply

I thought of adding a separate power supply to the board as i wanted to make it more handy as i didn't wanted my device to operated via a small batteries.
I had found out in the manual of the IC that it was not able to handle more that 5V, therefore i had to make my power supply as specified. Then with the help of a simple voltage regulator IC 7805 i made a power supply, along with some filter capacitors, which was also not needed but i thought of adding it.  I made it in such a way that i didn't needed a transformer in one end. 
I had a lot of switching adapters lying in my house mainly of 12V and i just made a socket, where in i was able to plug in the 12V directly and from the end of the regulator i would get a steady 5V.
Simple as it sounds. 


And wala my FM transmitter is ready for the long range coverage.

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    96 Discussions

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    LeeR83

    2 months ago on Step 6

    It looks like a Chinese rip-off of a similar looking popular model. I own one and it was delivered with a battery charger, car power plug and it also looks much sturdier than yours. It dislike their copying of products and making the specs so much worse.

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    RockyS21

    2 years ago

    Anyone experimented on a transmitter like this?

    fm.jpg
    1 reply
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    Bob.1RockyS21

    Reply 1 year ago

    Yes. I'm ordering a 50 dB gain amplifier for mine. building a power supply for it and housing unit as well. Costs about 400 bucks. but i will never hear country music again in my life

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    PeterA187

    1 year ago

    Norway anounced its going totaly digital and there will be no more FM radio broadcasts ,FM is to be phased out..

    Hello everyone, here I will reported two links to two videos posted on Youtube showing that transmits digital signals with two low-cost AM and FM transmitters.

    To decode these messages, you can use various software on Linux, Windows and Android.

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    DanielC225

    2 years ago

    this is my transmitter. it has 4 channels. 88.1,88.5,88.9,89.3.

    the pcb i soldered to the battery compartment and i cant find my iron.

    WIN_20161001_13_37_28_Pro.jpgWIN_20161001_13_37_41_Pro.jpg
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    TomT18

    3 years ago

    Hello, mine is similar but still has some differences tough

    Its case says Wireless FM Stereo

    5 replies
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    TomT18TomT18

    Reply 3 years ago

    Here:
    Can someone help me about where to remove the resistor and where to solder the coil

    temp_-1230460138.jpgtemp_-2134084483.jpg
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    ZoltánK15TomT18

    Reply 2 years ago

    Hi! I made this:

    On the picture of PCB i markd the modifyed parts.

    Replace L1 to 3k3 Yellow (i don't know, is the value correct, but it works)

    Replace L3 to 0R (bridge) Purple

    Solder out C3 (blue, red), sold black the blue, red must be open. sold the antenna to the flying side of capa. (lambda/4, ca 0.7...0.85m)

    L2 C2 is a resonant filter. (Orange) This values give ca 98MHz. You can optimize it to your selected FM freq. (I placed a 110nH and 22pF). At the resonant frequency you have much more signal. It's not easy. Dispension of an inductor can be 30%, a capacitor 5%.

    FM1.pngFM2.pngtransmit2.jpg
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    GilbertoM6TomT18

    Reply 2 years ago

    I have a transmitter with pcb identical to his, I am also having trouble connecting an antenna on it, please tell me if he found a way to do!

    I thank you for any information to help me.

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    DavidP246TomT18

    Reply 2 years ago

    Same boat as you. Please let us know if you find out which components to modify.

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    EnglishC1TomT18

    Reply 2 years ago

    Hi

    Would you please tell me if you found the solutions for your fm transmitter? I have the same model and I am looking to add external antenna to increase clarity of sound. If you found the solution I would appreciate if you send it in my email joinwest@gmail.com.

    Thanks

    Jessy

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    bAss Kappa

    2 years ago

    Are you giving instructions or narrating a story?

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    TomT18

    3 years ago

    Pic1

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    rcs-al

    6 years ago on Introduction

    From what I see on your pic, the piece you remove from the antenna circuit is not a resistor, maybe a capacitor or a coil.

    5 replies
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    raikutrcs-al

    Reply 6 years ago on Introduction

    Yes it is a Capacitor. You are right. But i have tested it as if you remove it and add an external antenna along with a small 107pF ceramic capacitor in between the antenna and the ground, the RF signal is much more concentrated between the antenna.

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    Pratyush Natarajan raikut

    Reply 3 years ago

    I'm from Ranchi. Being a teacher wanted to start up a fm transmitter along with my colleague to air educational programmes. Heard that in India to set up radio is a hassle. Licence isn't normally given. I kindly request you to send me the details regarding it at Blackjack241@gmail.com

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    wa7josraikut

    Reply 6 years ago on Introduction

    Increasing the power output of this device would be a violation of FCC rules.
    These devices are limited to 100mw so as not to interfere with licensed operators.
    If your modified device causes interference, you may get a nasty visit from the FCC, who can levy a fine of up to $10,000 PER DAY!

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    chrweiwa7jos

    Reply 6 years ago on Introduction

    Since raikut ins't in the US this may or many not apply directly.

    As always, when dealing with licensed frequencies, be familiar with your local laws.

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    PhilKE3FLchrwei

    Reply 6 years ago on Introduction

    The FCC has jurisdiction over radio frequencies. No local laws apply, as far as I know. Not that they couldn't stop & hold someone & then inform the FCC I guess. Do you know something that I don't?