Production of high-quality wireless microphone is the desire of many readers, but amateur
conditions to produce high-performance receiver is not easy. It opens up ways to use scrap cars FM radio cassette players in part, to increase a XR1075 audio processor, redesigned printing plates, produced a stable and reliable, excellent electrical performance dual-band wireless microphone transceiver system. Emitting part BA1404 dedicated FM transmitter IC. This machine is .png">a circuit diagram of the principle shown below, the production process will now be described below. The figure shows that the circuit consists of four parts, namely FM head, IF amplifier, audio processing, and power supply. After JT1 10.7MHz IF filter out the basic principles of the signal induced by the antenna from the FM input pin head, the internal high level, after the local oscillator mixer IF output signal of 10.7MHz from foot from C1 to T1 intermediate frequency input preamplifier after signal to IC1 feet, by the internal limiting, amplification, frequency discriminator demodulation audio signal output from the feet, the C8 is coupled to T2 to zoom level to meet the requirements of IC3. Audio input signal from IC3 feet, after delays in the digital processing portion of the frequency output from the feet. IC1 feet of 2.25V reference voltage source, the tuning indication output pin is used as mute in this circuit. The principle is: When the feet are the 10.7MHz IF signal is input, this pin is low, T3 off, IC3 pin output audio signal without attenuation; conversely pin is high, the power through R11, D2, R13 to provide bias T3, T3 saturated, so that the audio signal T3, c, e pole into the ground, in order to achieve the purpose of squelch. L1, C composition 10.7MHz frequency selective network, it is also in second place IC1 load, the principle C2, C3 intermediate frequency bypass capacitor, W1, W2, respectively treble and bass audio processor gain adjustment potentiometer, see "newsletter" in the relevant articles. Installation and commissioning of two bad looking car cassette players (requires FM tuner includes head), removed from the board and tuner and lower welding 10.7MHz filter JT1, IF filter (shape like in the week, usually magnetic cap light blue), TA7640, FM tuner (see photo top right corner of the shape) and other key components for the backup. According to a cabinet sized printing plates, printing plates should be used epoxy boards and large ground, ready to welding components to be mounted on the T3 and T6 after a good overall debugging. Power (in the way as an example), measuring IC1 pin voltage should be about 6.5V, FM head and feet should be 6V, 0.15V or so feet (M47-type table measurement, digital meter to measure compared to 1.9V), and adjust R1 R10 so that T1, T2 are collector voltage of about 3V. Connected to the amplifier and speakers, at this time if the circuit is normal, the speaker should be larger "rustling" sound, if not rustle, a screwdriver can be used separately touch IC3 feet, T2 base should hum, of taking even IC1 ,, ,, foot, T1 base due louder "giggle" sound, otherwise you should check JT1, L1, IC1 and surrounding components. Everything is normal rotation FM heads to close a local FM radio station, adjust L1 core to make clear sound. Together with the microphone to adjust the transceiver frequency (avoid the attention of local radio frequency), and finally with a microphone in front of the more familiar music sound better regulate W1, W2 clear, natural sound quality receiver so far, if the microphone to pick up the voice of the main, appropriate to increase the treble (adjust W2). Another way to adjust the same way. In this case welded T3 and T6, debugging automatic squelch, turn off the transmitter, the corresponding TA7640 feet should be high; should be open to low, or to retune L1. This circuit has not yet adopted the crystal frequency stabilization, but due to the high stability FM head of the wireless microphone transceiver frequency is very stable, has never deviation from the winter solstice, the author of two microphones are used for voice and zither, zither subjective feeling the string is very crisp and beautiful, have a more significant voice sibilance. As long as elements of quality assurance, the radio microphone with the hundreds of thousands on the finished machine comparable, of course, interested readers can also increase the harmonic generator IC1, IC2 output to improve the clarity of the sound. This machine uses 9.6V rechargeable batteries primary source and the external 16V power supply is designed for battery charging purposes, since R31, limiting the role of regulator D4, so an external power supply for long-term access to the machine.