This project includes instructions for the oscillator and RF amplifier portions of an FM radio and is designed to be used in tandem with a specially printed circuit board in order to avoid effects from a breadboard such as parasitic capacitance that may interfere with the tuning of the radio. Adapted from the Elenco AM/FM-108ck Superhet Radio kit, copyright 2012.
Solder parts to the provided circuit board according to the key below. Refer to the circuit diagram if needed. A good solder connection should be bright, shiny, smooth, and uniformly flowed over all surfaces. It is helpful to check off each component as you attach it to the circuit board.
C7: .001μF Discap (102)
R4: 33kΩ Resistor (orange-orange-orange-gold)
Q2: 2N3904 Transistor (see Figure R)
R5: 33kΩ Resistor (orange-orange-orange-gold)
R6: 2.2kΩ Resistor (red-red-red-gold)
C8: .005μF Discap (502)
L3: FM Oscillator Coil (5 Turns)
C9: 15pF Discap (15)
C10: 30pF Discap (30)
C11: 220pF Discap (221)
RF Amplifier Components:
R1 - 22kΩ Resistor (red-red-orange-gold)
C3 - .001μF Discap (102)
C2 - .005μF Discap (502)
Q1 - 2N3904 Transistor (see Figure R)
R2 - 6.8kΩ Resistor (blue-gray-red-gold)
C4 - 470pF Discap (471)
L1 - FM RF Amp Coil (6 Turns)
L2 - FM RF Amp Coil (2 Turns)
C5 - 470pF Discap (471)
C6 - 33pF Discap (33)
R3 - 470Ω Resistor (yellow-violet-brown-gold)
Tuning the Oscillator:
This tuning step assumes you have already assembled the rest of the FM radio and are receiving signals.
You will need a short length of brass rod and iron rod (both about 3/16” thick and 1/2" long). Attaching these with a length of heat-shrink tubing constitutes your “magic wand.” About 1/4” of material should be exposed.
Once you have assembled the rest of your radio, tune in to a station around 90MHz. Once a station is found and its broadcast frequency is known, mark the location on the tuner.
Place the brass rod near coil L3.
If the station is heard, this means that L3 needs less inductance. Carefully pull apart L3 until the station is heard. Place the iron end near L3. If the station is heard, this means that L3 needs more inductance. Carefully press together L3 until the station is heard. Pulling apart or pressing together L3 just a small amount will have a great effect on the coils resonant frequency.
Tune the radio to a station around 106MHz. Place the brass end of the “magic wand” near L3. If the station is heard, it means that L3 needs more capacitance. Carefully adjust the FM oscillator trimmer on the back of the tuner until the station is heard. Place the iron end of the “magic wand” near L3. If the station is heard, it means that L3 needs less capacitance. Carefully adjust the FM oscillator trimmer until the station is heard. Adjusting both the oscillator coil L3 and the oscillator trimmer capacitor will affect the oscillator’s frequency, so it is advisable to repeat this procedure until the FM oscillator alignment is optimized. This process sets the FM oscillator range at 98.7MHz to 118.7MHz.
Tuning the Amplifier:
Press together L1 and L2. Spread apart coil L1 so that it resembles the figure shown. The gaps or spaces should be between 1/32” and 1/16” wide.
If no stations are heard, verify that FM signals are present in your location by listening to another FM radio in the same area. If the FM section is still not receiving go back and check each stage for poor soldering.