FM Transmitter from an earlier time.

I constructed a few of these transmitters using some FM RF boards removed from an old cable head end system. Although I didn't have any schematics of the boards I was able to figure out how they worked. These were used for playing some kind of games on your TV if you had a special box and a subscription to the service. The boards contained an 8038 chip which might have been used for modulating the data and it made an almost perfect SCA generator with a few modifications. The phase locked loop needed to be modified to have a low enough loop gain bandwidth so as to allow the low frequency audio to pass. This was all done and a carrier board was added that has the stereo circuit, 100 KHz reference oscillator, power supply, and.an RF buffer stage that puts out about +17 dbm, Just a little more that the normal part 15 specification. It might actually cause a small amount of harmful interference to a distant station.

This is an unmodified RF board that's been hard wired for 102.9 Mhz. Just click on any of the photos to down load an enlarged view. You can see the MC1648 VCO chip above the third contact pin. Look here for a data sheet. It has a resonator coil and capacitor that controls the oscillating frequency. The phase lock loop cannot move the frequency more than about 1.5 MHz either side of where the coil and capacitor is set. You will need to change the capacitor and adjust the coil if you move it very much.

On the back of this RF board I placed an adjustable capacitor. 
It has voltage on it that if shorted to ground with a metal screwdriver would blow out the VCO chip.

This view shows two resistors and a capacitor all tied together which is the output from the phase lock loop and should be between 0 and 5 volts. When operating normally and the PLL is locked it should stay half way between, at 2.5 volts. If it's 0 or 5 volts then the resonator coil and capacitor are not tuned close enough to the desired frequency.