Shure u2 wireless mic service manual
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Page 1
1999, Shure Incorporated Printed in U.S.A. 25B1022 (AG) Service Manual U2 Hand-Held UHF Transmitter General Characteristics The Shure Model U2 Hand-Held UHF Transmitter is a micro- processor controlled microphone-transmitter operating in the 774 to 862 MHz frequency range. This product is intended for use in high- end installed sound, rental, and concert sound applications. Different frequency variations are available in various countries. Controls and Indicators 9 10 ONOFF 5 1 8 5 4 GAIN 2 6 3 7 1. Grille 2. Programmable Display 3. Battery Fuel Gauge 4. Battery Cover 5. ON/OFF Switch6. Antenna 7. Battery Compartment 8. MODE Button 9. SET Button 10. Audio Gain Control Figure 1.U2 Transmitter Controls and Indicators Service Note: Shure recommends that all service procedures be performed by a Factory-Authorized Service Center or that the product be returned directly to Shure Brothers Inc. Licensing: Operation may require a user license. Frequency or power-output modifications may violate this products approvals. Contact your countrys communications authorities.
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Shure U2 Hand-Held UHF Transmitter Circuit Description25B1022 (AG) 2 Circuit Description Audio Section Audio enters L248, an inductor used as an rf choke. The signal is ac-coupled through C201 into a 26 dB user-adjustable gain stage around U201B. This gain stage is externally accessible to the user. C249, C250, and C209 protect the preamplifier and bias circuits from rf interference. R212 and R241 set up a half-supply bias, and R238 sets the ac input impedance. The amplified audio signal is then passed through a pre-emphasis network before entering the compression stage. R230, R202, and C203 set up two corners for the pre-emphasis network. The pre-emphasis boosts the high frequencies before transmission. This network feeds an NE575 compandor, U203, which utilizes an external amplifier U204B. The compandor performs 2:1 logarithmic com- pression of the audio signal. Additionally, the pre-emphasis network plays a role in setting the hinge point (0 dB gain) of the compandor. Transistors Q211 and Q207, along with crystal Y202, form the tone key oscillator circuit. This circuit provides a stable, continuous 32.768 kHz sine wave. Transistor Q201 buffers the tone key signal before it is added to the audio signal. The tone key signal is used in the receiver to provide audio output only when the tone key signal is present in the transmitted signal; there- fore, if the tone key or the transmitter is turned off, the receiver will be muted. The tone key squelch eliminates receiver noise associated with loss of a carrier. Q206 acts as a switch for toggling the tone key ON/ OFF. It is controlled by the microprocessor. R286 allows for the tone key amplitude level to be set. The tone key circuit is powered via U210B. This is a dc amplifier used to multiply the 3V battery voltage up to 4.5 V (gain = 1.5). As the battery voltage drops, the tone key supply voltage drops, decreasing the amplitude of the oscillator. This is used to detect low battery in the receiver. (This feature was eliminated for JB models.) The tone key signal, along with the processed audio signal, is then fed to a summing amplifier U204A. R231 and R234 set up a half-supply bias. R207, at the output of the summing amplifier, is used to help pre- vent spurious oscillations from the operational amplifier. After passing ac-coupling capacitor C213, the signal is fed to the rf module. Rf Section Processed audio enters an internal potentiometer R227, which is adjusted for 45 kHz deviation (100% modulation) with a -7.2 dBV 1 kHz tone at the output of the front audio stage (pin 1 of U201). (On JB mod- els, R227 is adjusted for 5 kHz deviation with a –67.2 dBV, 1 kHz tone injected into the mic input.) The audio is then fed to the tuning voltage pin of the voltage controlled oscillator (VCO) and modulates the carrier directly. The use of a phase locked loop (PLL) frequency-synthesized system eliminates the need for multiplier stages, resulting in a much higher degree of spectral purity. The VCO is shielded to prevent interfer-