18-12-2012, 06:07 PM
Kit 32. THREE STAGE FM TRANSMITTER
THREE STAGE FM TRANSMITTER.pdf (Size: 30.89 KB / Downloads: 15)
ASSEMBLY INSTRUCTIONS
Components may be added to the PCB in any order. It is
usually easier to add the lowest height components first
then proceed to the taller components. The electret
microphone should be inserted with the pin connected to
the metal case connected to the negative rail (that is, to
the ground or zero voltage side of the circuit.) This is
marked with a '-' sign at the MIC on the circuit board
printed overlay.
CIRCUIT DESCRIPTION
The circuit is basically a radio frequency (RF) oscillator
that operates around 100 MHz. Audio picked up and
amplified by the electret microphone is fed into the audio
amplifier stage built around the first transistor. Output
from the collector is fed into the base of the second
transistor where it modulates the resonant frequency of
the tank circuit (L1 coil and the red trimcap) by varying
the junction capacitance of the transistor. Junction
capacitance is a function of the potential difference
applied to the base of the transistor T2.
WHAT TO DO IF IT DOES NOT WORK
Poor soldering is the most likely reason that the circuit
does not work. Check all solder joints carefully under a
good light. Next check that all components are in their
correct position on the PCB. Did you add the connection
from the middle of coil L1 to the pad marked TAP? Check
that the leads of the two coils L2 and L3 are not dry
joints.