This FK703 wireless microphone has two tuned stage for
extra stability and gain. Construction is simplified by the
adoption of etched resonant coils on the PCB. Its small outline
lends itself to use as a lapel microphone or security "bug". The
FK703 may be used with any FM radio receiver in public address
applications.
Technical data
- Power supply : 9VDC.
- Electric current consumption : 10mA. (max.)
- Transmitting frequency : 88 MHz
- IC board dimension : 1.64 in x 1.54 in.
How does it work
The microphone functions as receiver transmitting the sound
signal through C2 into leg B of TR1 which generates radio waves
and mixes them with the generated radio frequency. The mixed
signal will be sent through C5 toward leg B to boost the radio
frequency at leg C and transmit it to the antenna for
broadcasting. For easier assembly, an etched resonant coil is
adopted in lieu of a self made coil at legs C of TR1 and TR2.
Circuit Assembly
The assembly of components is shown in Fig. 2. For good
looking and easy assembly, the shorter components should be
first installed - starting with low resistant components and then
the higher. An important thing is that diodes, electrolyte
capacitors, and transistors shall be carefully assembled before
mounting them onto their right anode/cathode of the IC board
otherwise it might cause damage to the components or the
circuit. Configuration of the anode and the cathode is shown in
Fig 3. Use the soldering iron/gun not exceeding 40 watts and the
solder of tin-lead 60:40 with flux within. Recheck the correctness
of installation after soldering. In case of wrong position, just use
lead absorber or lead extractor wire to avoid probable damage to
the IC.
Testing
Supply 9 VDC to the circuit by connecting the anode at point
+9V and the cathode at point G, and point ANT to the coil by
soldering (extract the plug before soldering). Tune the FM radio
station at 88 MHz and adjust trimmer T1 with a plastic-tipped
screwdriver until the feedback sound is heard. Now test speaking
into the microphone. If no voice from the loudspeaker tune to
100 MHz, and if no voice again, then tune to the farthermost 108
MHz. Try adjusting it one more time and adjust T2 at the same
time in order to attain the longest distance.