Shure lx4 User Manual
Shure Brothers Incorporated
222 Hartrey Avenue
Evanston IL 60202-3696 U.S.A.
LX Wireless System
SERVICE MANUAL CHANGE NOTICE
LX4 DIVERSITY RECEIVER
Changes and corrections have been made to the Service Manual for the LX4 Diversity Receiver. To update
your Service Manual, remove the pages identified in the tables below and replace them with the pages at tached to t his C hange Notice. Note that t here a re n o c hanges t o p ages n ot s pecifically i dentified i n t he t ables
below.
LX4 DIVERSITY RECEIVER SERVICE MANUAL REVISION HISTORY
Release Part Number Date Code Color
Original 25A1008 QH White
Revision 1 25B1008 QL Pink
Revision 2 25C1008 SC Tan
Revision 3 25D1008 SI White
Revision 4 25D1008 TL White
Revision 5 25D1008 BK White
Revision 5 25D1008 DL Red
CHANGES EFFECTIVE DECEMBER 21, 2004
REMOVE
these pages from the
LX4 Service Manual
Pages 26 – 28 Pages 26 – 28
E1999, Shure Incorporated Printed in U.S.A.
25–1008–1 (DL)
these pages into the
INSERT
LX4 Service Manual
LX4 Diversity Receiver
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Characteristics
General
The Shure Model LX4 is a single-conversion superheterodyne
diversity FM receiver operating in the 169 to 240 MHz band. It is
intended for use with compatible Shure VHF wireless transmitters.
Controls and Connectors
Service Manual
25D1008 (DL)
AUDIO
3
OUTPUTS
BAL
9 11
RF A RF B
1
8
2 4
HI Z
MIC LINE
9
10
Figure 1. Identification of Controls and Connectors
1. Diversity signal indicators
2. Rf level indicators
3. Audio level indicators
4. Squelch control
5. Audio level output control
6. Power ON indicator
SQUELCH LEVEL
5
POWER
12.5 – 18.9 VDC
8
MARCAD
DIVERSITY
POWER
6
7
ANTANT
AB
7. Power ON/OFF butto
8. Antenna connectors
9. Output connectors
10. MIC/LINE slide switch
11. Power jack
Service Note: Shure recommends that all service procedures be performed by a Factory-
E1999, Shure, Inc.
Printed in U.S.A.
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.
Characteristics125D1008 (DL)
Shure LX4 Diversity Receiver
Circuit Description
The Shure Model LX4 Diversity Receiver is a single-conversion
superheterodyne diversity FM receiver operating in the 169 – 240 MHz band. It
is intended for use with compatible Shure VHF wireless transmitters, including
the Shure LX, L, EC, SC, and T series hand-held and body-pack transmitters.
The LX4 has balanced LO-Z output (3-pin XLR-type connector), an unbalanced aux level audio output (1/4 inch monophonic phone jack), and audio output
level (volume) control for all outputs.
The LX4 has an externally-accessible squelch control for signal-to-noise muting system. The recessed potentiometer for squelch control is screwdriver-adjustable.
A one quarter wavelength whip type antenna can be mounted directly to the
receiver or to a full rack panel (Shure model WA440), and can then be cabled to
the receiver. Also, a Shure model WA404 antenna splitter can be used to provide
antenna inputs and dc power to the LX4.
Audio output is a 1/4-inch, single circuit (monophonic) phone jack.
Diversity channel selection and muting are provided by a noiseoperated squelch system. Noise signals are obtained from the additional detector outputs at pin 9 of U100 and U200. For diversity channel A,
the noise signal is adjusted by squelch level control R130 before amplification by
U1B. Active high-pass filter U1C removes audio components that could cause
false triggering. The noise is then rectified by D2 and smoothed by C18 to provide a dc voltage that varies in accordance with the amount of noise present on
the detected signal.
An identical scheme is followed for diversity channel B, where noise signals
are adjusted by squelch level control R230 before being amplified by U2B, filtered by U2C, rectified by D1, and smoothed by C17.
Two diversity peak A/B (yellow) LEDS are located on the front of the unit illuminate to indicate which diversity channel (A, B, or both) have been selected by
the receiver. When the quality of the received signal
is below the squelch threshold level, both LEDs are extinguished. Audio peak
LEDs illuminate when the deviation level of the received signal
exceeds the following deviation:
LED Color
1 – Green 1.5
2 – Green 4.0
3 – Green 7.0
4 – Amber 11.0
5 – Red 14.0
Deviation kHz
Characteristics
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25D1008 (DL)
Shure LX4 Diversity Receiver
Diversity Control Signals
The dc noise signals are compared by U3A, U3B, U3C, and U3D to produce
diversity control signals. These signals directly control analog switches U6A and
U6B, and diversity A/B LEDs D9 and D10. These switches control the flow of
audio signals from U1A and U2A.
Squelch Control
Squelch comparators U3A and U3C compare a dc level from the user
squelch control, R66, to the dc noise level signals coming from each diversity
channel. When the noise level in both channels exceeds a preset level, the audio output of the receiver is squelched. Comparator U4A provides additional
signal attenuation in the squelched state by shutting off compander U9B.
However, if the noise level in each channel is comparable and below the
squelch threshold, then U3B and U3D will allow both channels to be active and
both diversity indicators will be illuminated. Because the
signals from each channel are correlated, but the noise is uncorrelated,
a theoretical signal-to-noise improvement of 3 dB is possible with this arrangement.
Balanced LO-Z Output
After passing through analog switches U6A and U6B, audio signals from the
two channels are buffered by U5A before entering three-pole, low-pass filter Q1.
They then enter compander U9B, which provides 2:1 logarithmic expansion. An
additional IC amplifier U5B operates in conjunction with U9B to provide a lower
noise floor. The output of this stage is applied to volume level control R32. In
order to provide a balanced LO-Z output, U5C and U5D operate as a bridging
amplifier. HI-Z audio output connector J3 taps off U5D output.
Audio Indicators
An audio LED indicator display lets the user know when the transmitter (and
audio level) deviation is approaching the limit of 15 kHz. This function is implemented by the U11 display chip. Two rf level LED strings are provided to indicate
relative rf strength of transmitters received by each antenna. They are powered
by display chips U10 and U12, which are controlled by U100 and U200 signal
strength pins.
Characteristics325D1008 (DL)
Shure LX4 Diversity Receiver
Independent Rf Sections
Two complete, independent rf sections are provided for diversity
reception. Signals enter the receiver via detachable, single-element quarterwave antennas. For diversity channel A, they pass through a filter that is doubletuned by L100 and L101 before entering dual gate MESFET amplifier Q100. The
output of this stage is triple tuned by L103, L104, and L105, which filter the signals before they enter gate 1 of
MESFET mixer Q101. The local oscillator (LO) signal, which is applied to gate 2
of Q101, is provided by transistor Q104. A third- or fifthovertone quartz crystal in the 50 to 70 MHz range is used to provide
frequency control. The collector circuit of the oscillator is tuned by
L113 and L114 to the third harmonic of the crystal (150 to 210 MHz)
to provide the correct injection frequency for a 10.7 MHz intermediate frequency
(IF).
The output of mixer Q101 is tuned to 10.7 MHz by L108 before being applied
to ceramic filter FL100. Transistor Q102 provides IF amplification to make up for
filter losses. Signals then pass through a second IF filter, FL101, before entering
the integrated circuit IF amplifier/detector, U100. The detected audio is taken
from pin 8 and is then amplified by operational amplifier U1A.
Rf Channel B
Diversity channel B is identical in design to channel A. Signals first pass
through a double-tuned filter consisting of L200 and L201 before entering amplifier Q200. The output of this stage is triple-tuned by L203, L204, and L205 and
fed to gate 1 of mixer Q201. LO injection is provided by buffer transistor Q103,
which is tuned by L112 and L111 to the third harmonic of the crystal frequency.
The buffer stage helps to isolate the diversity channels from one another by preventing crosstalk through the common LO section.
The output of mixer Q201 is tuned to 10.7 MHz by L208 before being applied
to ceramic filter FL200. Signals are then amplified by transistor Q202 before
passing through the second ceramic filter, FL201, before they enter the integrated circuit IF amplifier/detector U200. The detected audio is again taken from
pin 8 and is amplified in this case by U2A.
Rf Indicators
Rf LEDs indicate approximate rf signal strength for each channel:
LED
1 –95
2 –90
3 –80
4 –65
5 –60
Signal Strength, dBm (±10 dBm)
Characteristics
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25D1008 (DL)
Power
Shure LX4 Diversity Receiver
The Shure LX4 Diversity Receiver operates on 12.5 – 18 Vdc or from an ac
power line with 120 Vac, 60 Hz external power converter;
or 220 to 240 Vac, 50/60 Hz for export models. The dc power connector: is a
male pin coaxial jack with locking connector.
A green power LED, D11, indicates when power is being supplied to the receiver by an external power converter. The circuitry will operate normally with an
input of 12.5 to 18 Vdc. Reverse polarity protection
is provided by D8, and voltage regulation is accomplished by U7. The power input, HI-Z and balanced LO-Z output connectors are filtered to prevent local oscillator radiation from the cables and to protect the
receiver from the effects of external electromagnetic fields.
Characteristics525D1008 (DL)
Shure LX4 Diversity Receiver
Notes:
Notes:
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25D1008 (DL)
Shure LX4 Diversity Receiver
Test Component Locations
Test Component Locations725D1008 (DL)
Shure LX4 Diversity Receiver
Preliminary Tests
Listening Test
Before completely disassembling the receiver, operate it to determine whether it is
functioning normally and try duplicating the reported malfunction. Refer to the User
Guide for operating instructions, troubleshooting suggestions, and specifications.
Review any customer complaint or request, and focus the listening test on any
reported problem. The following, more extensive, functional tests require partial disassembly .
Functional Tests
Refer to the Disassembly section to partially disassemble the receiver for the following functional tests.
Use dc blocks at all rf output to protect test equipment.
Verify operation and reported malfunction, referring to the product User’s
Guide for a description of the unit as well as information on its
operation, troubleshooting, and technical data.
Preliminary Tests
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25D1008 (DL)
Rf Test
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Shure LX4 Diversity Receiver
Receivers that pass this test do not contain any “drop-out” or “range” problems.
MARCAD
DIVERSITY
POWER
RF A RF B
AUDIO
SQUELCH LEVEL
RF SIGNAL GENERATOR
HI Z
OUTPUTS
BAL
MIC LINE
POWER
12.5 – 18.9 VDC
DC BLOCK
NOTE: DC VOLTAGES ARE PRESENT AT MOST
RF TEST POINTS. USE A DC BLOCK ON THE
RF SIGNAL GENERATOR TO PROTECT
TEST EQUIPMENT.
ANTANT
AB
LX4 Receiver Rf Signal Generator
Power: ON Frequency: Receiver frequency
Level (gain): Max Modulation: None
Squelch: Mid
Figure 2. Rf Alignment Functional Test Set-Up
For channel A:
1. Set the receiver’s squelch control to its middle position and the volume
control to its maximum position.
2. Set the rf generator to the receiver’s frequency.
3. Remove the receiver’s antenna. Plug the
BNC end of the 50 Ω test cable
into the signal generator and the other end to the
channel A antenna input.
4. Apply power to the LX4.
5. Verify that the unit unsquelches with rf signals greater than –89 dBm.
6. When the unit unsquelches, verify that the “Rf”
LED glows.
Repeat this test for channel B.
Preliminary Tests925D1008 (DL)
Shure LX4 Diversity Receiver
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Audio Tests
HI Z
RF SIGNAL GENERATOR
AUDIO ANALYZER
OUTPUTS
BAL
MIC LINE
POWER
12.5 – 18.9 VDC
ANTANT
AB
DC BLOCK
NOTE: DC VOLTAGES ARE PRESENT AT MOST
RF TEST POINTS. USE A DC BLOCK ON THE
RF SIGNAL GENERATOR TO PROTECT
TEST EQUIPMENT.
LX4 Receiver Audio Analyzer Rf Signal Generator
Power: ON Output amplitude: Set to turn OFF high and low EXT
Level: – 60 dBm
lights on rf signal generator
Level (gain): Max Filters: Deviation: 15 kHz
Squelch: Mid 400 Hz High-Pass: ON Modulation: EXT 1 kHz
30 kHz Low-Pass: ON Frequency: receiver
frequency
Figure 3. Audio Test Set-Up, steps 1 – 5
Note: The amplitude may have to be adjusted so that neither the HI EXT nor the LO
EXT LEDs on the rf signal generator are on. This amplitude should be between 1.4
and 1.5 V rms.
1. Set the rf signal generator level to –60 dBm, its deviation to
15 kHz, and its external modulation to 1 kHz.
2. Adjust the audio analyzer’s output amplitude so that the high and low
EXT lights on the rf signal generator are off.
3. Using a 3.3 kΩ load, connect the receiver’s unbalanced audio output to
the audio analyzer. The receiver’s volume control should still be in its
maximum position.
4. Engage the 400 Hz high-pass filter and the 30 kHz low-pass filter on the
audio analyzer.
5. Verify the following receiver measurements:
S Audio level is 400 mVrms, "90 mV. Record the
measurement as a reference level for the following steps.
S total harmonic distortion (THD) is <0.75%.
Preliminary Tests
10
25D1008 (DL)
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