Shure lx2 wireless mic service manual

Page 2

Shure LX2 Hand-Held Transmitter 2 Characteristics 25D1006 (CG) Circuit Description The Shure LX2 Hand-Held Transmitter contains three circuit boards; an audio section, an rf section, and a microphone head interface board. It is intended for use with the matching LX3 and LX4 receivers. Audio Section Audio signals from the microphone head enter the transmitter via a printed circuit board (pcb) with spring contacts, which also interconnects the audio and rf printed circuits. The audio signal next enters a preamplifier stage consisting of one section of operational amplifier (U102C). The voltage gain of this stage may be adjusted over a 40 dB range by means of an externally- accessible potentiometer (R125) to enable the user to compensate for variations in sound level at the microphone. The preamplified audio signal is then passed through a passive pre-emphasis network consisting of R142, C110, C111, R112, and R115, that has a pole at 63 microseconds and a zero at 12 microseconds. Audio Signal Compression The signal then enters the NE571D integrated circuit compander (U101A). This provides 2:1 logarithmic compression of the audio signal. A lower noise floor is achieved by U102A. An internal potentiometer (R130) is provided for nulling system audio distortion. Operational amplifier U102B operates as a two-pole active low-pass filter to restrict the bandwidth of the system to audio frequencies. Reverse Battery Protection and Low Battery Warning The NE571D also contains an identical second channel (U101B), which in this case is used to supply regulated, low-noise 5 Vdc bias to various audio and rf circuit points. Transistor Q105 provides reverse battery protection to the circuit. Q110 drives a green LED (D103) that serves as a power ON indicator. Q107 drives an amber LED (D102) that provides a low battery warning signal to the user. Q106 and Q108 drive a red LED (D101) that serves as a final low battery indicator. Rf: Domestic (FCC/IC) Processed audio enters R201, an internal potentiometer that is adjusted for 15 kHz deviation (100% modulation) with a –2.2 dBV, 1 kHz tone at the output of the audio section. The audio is then fed to varactor diode D201, which is part of the modulated oscillator-tripler stage (Q201).