Shure_U2_Wireless_Mic_Service_Manual.pdf

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Shure Incorporated
222 Hartrey Avenue
Evanston IL 60202-3696 U.S.A.
UHF Wireless System
SERVICE MANUAL CHANGE NOTICE
U2 HAND-HELD TRANSMITTER
Changes and corrections have been made to the Service Manual for the U2 UHF Hand-Held Transmitter.
These changes will make it easier to repair the transmitters. To update your Service Manual, remove the
pages identified in the tables below and replace them with the pages attached to this Change Notice. Note
that there are no changes to pages not specifically identified in the tables below.
U2 SERVICE MANUAL REVISION HISTORY
Release
Original
Revision 1
Revision 2
Revision 3
Part Number
25A1022
25B1022
25C1022
25C1022
Date Code
QE
SA
TD
AG
CHANGES EFFECTIVE JULY 2, 2001
REMOVE
these pages from the
U2 Service Manual
INSERT
these Revised pages into the
U2 Service Manual
22
22
E2000
Shure Incorporated
25–1022–2 (AG)
Printed in U.S.A.
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
1
2
3
GAIN
10
7
8
9
5
4
5
ON
OFF
6
1. Grille
2. Programmable Display
3. Battery Fuel Gauge
4. Battery Cover
5. ON/OFF Switch
Figure 1.
6. Antenna
7. Battery Compartment
8. MODE Button
9. SET Button
10. Audio Gain Control
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 product’s approvals. Contact your country’s communications authorities.
E1999,
Shure Incorporated
25B1022 (AG)
Printed in U.S.A.
Shure U2 Hand-Held UHF Transmitter
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-
Circuit Description
2
25B1022 (AG)
Shure U2 Hand-Held UHF Transmitter
ence from external rf fields. Regulated 5 Vdc power from the dc/dc con-
verter ensures frequency stability even if the battery voltage drops.
The VCO is capable of tuning from 782 to 810 MHz with a 1 to 4 V
tuning voltage range. At the output of VCO U206, the rf signal splits into
two paths. The output of the VCO is coupled by C207 to the frequency
control pin of synthesizer U205.
The synthesizer’s internal circuitry divides the signal as necessary to
the desired reference frequency of 125 kHz. The synthesizer contains a
quartz-controlled reference oscillator circuit operating from a 4.0 MHz
crystal, Y203, that is adjusted by means of trimmer VC201.
The transmitter output frequency is user-selectable in pre-set
increments. The size of the increment and the overall frequency range
depend on the model (KK, JB, MB, MC, MD, etc.). Frequency selection
is made via microprocessor U104, which interfaces with the user through
the mode/select switches.
The output of the synthesizer is a series of pulses which are
integrated by a passive loop filter, R226, C231, R251, C237, R243, and
C257, to produce a control voltage signal. The control voltage signal is
then connected to the VCO through amplifier U210A which is used to
isolate the PLL filter from the audio modulation signals.
The VCO output is also coupled to an rf power resistive pad consist-
ing of R255, R256, R257, R258, and an LC-matching network containing
C270, L202, and C217. The rf power amplifier, a dual gate MESFET,
Q203, is fixed tuned, and configured as a common source device.
Amplifier stability is obtained through resistive loading on input R237.
The output of Q203 contains a low-pass matching network, L207, and
LC-type low pass filter, LP201, providing a high degree of spectral purity.
The output of the low-pass filter feeds a microwave isolator that reduces
the production of reverse third-order intermodulation products.
The transmitter is capable of delivering +10 dBm (10 mW), maxi-
mum to the 50
Ω
helical antenna. During transmitter power up and fre-
quency selection, the rf power is muted by bringing the base of Q209
high. This provides approximately 45 dB rf attenuation until the PLL has
locked.
The transmitter rf is then unmuted by bringing the base of Q209 low.
During transmitter power off conditions, voltage is first removed from the
VCO by bringing the base of Q208 high. In this way, the transmitter
carrier signal is not allowed to drift off frequency during power on or
power off conditions.
U2 Display Board
The Display Board consists of following circuitry blocks:
Microcontroller Section
The microcontroller section consists of microcontroller U104 and the
liquid crystal display (LCD). The microcontroller has an on-board LCD
driver. R104, R105, and R107 supply the microcontroller with the LCD
drive voltage for a 4-plex drive.
The LCD indicates the UHF frequency group and channel, and also
has a battery fuel gauge. A 4.000 MHz oscillator, Y101, provides the
25B1022 (AG)
3
Circuit Description
Shure U2 Hand-Held UHF Transmitter
operating frequency to the microcontroller. The oscillator circuit includes
C102, C103, R106. R108. U105, R113, and C107 form the reset circuit.
U105 is the reset IC that resets U104 microcontroller if the 5 Vdc normal
operating voltage falls below 3.5 Vdc. R112 is the pull-up resistor for the
U104 programming voltage pin.
Memory Section
The memory section consists of U101, a non–volatile EEPRAM chip
that stores current transmitter settings and has the mapping of the com-
patible groups and channels.
Battery Management Section
The battery management section consists of comparator U103. It is
used to measure the effective battery voltage, with reference to +5 Vdc,
by measuring the time taken to charge capacitor C105 to a reference
threshold of 0.1 VDE
±
0.01 Vdc. The battery is checked approximately
every five seconds. Comparator U103 alternately charges C105 with +5
Vdc reference and then the battery voltage (DPLUS signal). The micro-
controller calculates the battery voltage by comparing the time difference
to charge C105, up to a reference threshold of 0.1 Vdc
±
0.01 Vdc, with
the +5 Vdc reference voltage and the battery voltage.
User Interface Section
The user interface section consists of power, MODE, and SET
switches. The LCD provides the user with feedback for all switch
operations.
Circuit Description
4
25B1022 (AG)

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