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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-
