Introduction
AM radio circuits and kits abound. Some work quite well. But, look around and you will find virtually no FM radio kits. Certainly, there are no simple FM radio kits. The simple FM radio circuit got lost during the transition from vacuum tubes to transistors.
In the late 1950s and early 1960s there were several construction articles on building a simple superregenerative FM radio. After exhaustive research into the early articles and some key assistance from a modern day guru in regenerative circuit design, I have developed this simple radio kit. It is a remarkable circuit. It is sensitive, selective, and has enough audio drive for an earphone. Read more about theory behind this radio on the low-tech FM page.
Adjustment
If the radio is wired correctly, there are three possible things you can hear when you turn it on: 1) a radio station, 2) a rushing noise, 3) a squeal, and 4) nothing. If you got a radio station, you are in good shape. Use another FM radio to see where you are on the FM band. You can change the tuning range of C3 by squeezing L1 or change C1. If you hear a rushing noise, you will probably be able to tune in a station. Try the tuning control and see what you get. If you hear a squeal or hear nothing, then the circuit is oscillating too little or too much. Try spreading or compressing L1. Double check your connections. If you don't make any progress, then you need to change R4. Replace R4 with a 20K or larger potentiometer (up to 50K). A trimmer potentiometer is best. Adjust R4 until you can reliably tune in stations. Once the circuit is working, you can remove the potentiometer, measure its value, and replace it with a fixed resistor. Some people might want to build the set from the start with a trimmer potentiometer in place (e.g., Mouser 569-72PM-25K).
Substituting other components
Many of the parts are fairly common and might already be in your junk box. Only certain component values are critical. The RF choke should be in the range of 20 to 30 uh, although values from15 to 40 uh might work. The tuning capacitor value is not critical, but if you use values below 50 pf you should reduce or remove C1. The circuit is designed for the high impedance type earphone. Normal earphones can be used, but the battery drain is much greater and the circuit must be changed. To use normal earphones, change R3 to 180 ohms. Q1 can be replace with any high-frequency N-channel JFET transistor, but only the 2N4416, 2N4416A, and J310 have been tested. A MPF102 probably will work. C2 is not too critical; any value from 18 to 27 pf will work. C7 is fairly critical. You can use a .005 or .0047 uf, but don't change it much more than that.