So, you want an FM transmitter, but like me, you are to dang cheap to buy one. All you need is a few common household chemicals and...wait, thats a different story, maybe later. This page will tell you how to a low power (~10mW) mono FM transmitter.
For this project you will need:
1) An old FM receiver, must be completely analog with no IC's in it.
2) An FM receiver, preferably with strength meter and fine tuning (50Khz stepping)
3) Soldering Iron and solder
4) Wire suitable for hookups and the such
5) Various tools: needle nose, screw drivers, wire cutters & strippers
6) Any thing else you think you may need
7) An Audio source (a cheap walkman works good)
8) Some basic electronics skills are assumed
Getting started:
Get out your FM receiver intended to be used as a transmitter, from this point forward I will refer to this radio as the transmitter. The other FM radio will be referred to as the receiver from this point forward. Find an empty spot on the FM band using your receiver, at this point you will be limited to the frequencies between 98-108Mhz so find an empty spot in this range. Now take your transmitter and dial in the frequency approximately 10Mhz below what your receiver is set at, your transmitter will broadcast around 10Mhz higher than what it is set at. For example, if receiver is tuned to 101.7Mhz you will have to tune the transmitter to 91Mhz (the exact offset is near 10.7Mhz I think). You will know when the transmitter is at the right place when you get strong carrier signal on your receiver (the strength meter comes in handy here). This is how the transmitter will work, all we have to do is put an audio signal on the carrier.
Step One: Locating a place to inject audio onto the carrier.
Take apart your transmitter so you have access to the PCB near the tuning area (where the variable air capacitor is), getting to the back side of the PCB works best. You must have the transmitter plugged in and turned on for most of the following instructions. Take an old set of headphones or something and cut the speakers off so all you have is the wires. Hook the ground of your audio source to the ground of the transmitter. Take the other lead and start poking around on the PCB until you hear the audio on your receiver. This is where you will want to solder a plug or wire to, depending on how you plan to feed the audio in. Note that your transmitter may drift from where it is tuned, if it does, just retune one of the radios. We will fix this drift problem later.
Step Three: Location the RF out (antenna jack)
Move your receiver away from the transmitter until it becomes fuzzy. Take a two to three foot long wire and start probing the PCB again. When the signal on the receiver gets stronger, solder a wire or plug here for the antenna. This is the RF out.
Step Four: Stabilizing the signal
Locate the tuner of the transmitter again. In this area there will be two transistor, each with a coil near it. Take a screwdriver or any metal object and put it near one of the coils. If, when you place the metal object near it, the carrier dissapears or radically changes frequency, you have found the coil that should remain in the transmitter (along with the transistor nearest it). Take the other coil and transistor nearest it out. This will greatly improve the stability of your transmitter. If you know how coils work, you can try changing the spacing of the windings of the remaining coil to remove the 10.7Mhz offest mentioned earlier.
Finishing up
Put it all back together and check that it still work. Well there ya go. You are now the somewhat proud owner of a low power FM transmitter. Spruce it up however you like, add jacks and the such if you want. Be creative (think Radio Scrap UHF/VHF amp :)
The skeletal information found here provided by Jams. I just elaborated on his info and tried to make it more clear for the less intelligent people that may stumble on this site.
