Shure lx2 Service Manual
Shure Incorporated
222 Hartrey Avenue
Evanston IL 60202-3696 U.S.A.
LX Wireless System
SERVICE MANUAL CHANGE NOTICE
LX2 WIRELESS HAND-HELD TRANSMITTER
Changes and c orrections h ave b een made t o t he S ervice M anual f or t he L X2 H and-Held Transmitter. To u pdate your Service M anual, r emove t he p ages i dentified i n t he t ables b elow a nd r eplace t hem w ith t he p ages
attached to this Change Notice. Not e that there are no changes to pages not specifically identified in the
tables below.
LX2 HAND-HELD TRANSMITTER SERVICE MANUAL REVISION HISTORY
Release Part Number Date Code
Original 25A1006 QH
Revision 1 25B1006 QL
Revision 2 25C1006 SC
Revision 3 25D1006 SI
Revision 4 25D1006 TF
Revision 5 25D1006 BB
Revision 6 25D1006 BK
Revision 7 25D1006 CG
CHANGES EFFECTIVE JULY 10, 2003
REMOVE
these pages from the
LX2 Service Manual
these new Revision pages into the
INSERT
LX2 Service Manual
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E1999, Shure Incorporated Printed in U.S.A.
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LX2 Hand-Held Transmitter
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Characteristics
General
The Shure LX2 Hand-Held Transmitter is part of a wireless system
designed for public address, educational radio and television broadcasting, school and community theater productions, and similar applications.
The LX2 is intended for use with the matching LX3 and LX4 receivers.
Controls and Connectors
Service Manual
25D1006 (CG)
1
LX2/BETA 58
6
5
2
3
BAT
4
MUTE
Figure 1. Identification of Controls and Connectors
1. Grille
2. OFF/PWR power switch
3. Battery LEDs
4. ON/MUTE mic switch
5. Audio gain control
6. Battery cup
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.
E1999, Shure, Inc.
Printed in U.S.A.
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Licensing: Operation may require a user license. Frequency or
power-output modifications may violate this product’s approvals.
Contact your country’s communications authorities.
Characteristics1
Shure LX2 Hand-Held Transmitter
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 externallyaccessible 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).
Characteristics
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Shure LX2 Hand-Held Transmitter
The base-emitter circuit of Q201 operates as a crystal-controlled
Colpitts oscillator in the 20 MHz region. Fundamental-mode crystal Y201
is tuned 10 kHz below series resonance by the series combination of
frequency-netting coil L201, diode D201, capacitor C203, and capacitor
divider C206 and C207.
Frequency Stability
To ensure frequency stability with changes in battery voltage,
regulated 5 Vdc bias is applied to the varactor diode and to the base
of Q201. Temperature compensation is provided by C206 and C207.
The collector circuit of Q201 is tuned to the third harmonic of the
oscillator frequency (approximately 60 MHz) by means of L202 and
C208. C210 couples rf energy to a second tuned circuit consisting
of L203, C214, and C215, which also forms a capacitively-tapped
voltage divider for matching into the base of Q202, operating as a
buffer amplifier.
Regulated dc bias is again employed to minimize changes in loading
on the oscillator stage and stabilize drive levels. The collector circuit of
Q202 is again tuned to the third harmonic of the oscillator frequency by
L204 and C216. C219 couples rf energy to another tuned circuit consisting of L205, C222, and C224, which also forms a capacitively-tapped
voltage divider for matching into the base of Q203.
Frequency Tripler
The third stage (Q203) is operated as a frequency tripler with the
collector circuit tuned to the output frequency (for example, 180 MHz).
In this case L206, C226, C229, L207, C230, and C232 perform tuning
and impedance matching functions. As in the preceding stage, regulated
dc bias is applied to the base circuit to stabilize the drive level, and the
output is double-tuned to provide spectral purity.
The final output stage (Q204) is operated as a tuned amplifier.
Resistive loading on the input enhances stability. The output circuit
consists of a tuned tank circuit (L209, C237, and C239) which also
provides impedance matching to the output low-pass filter (C239,
L210, and C241). L211 and C242 match the antenna to the filter.
Stable Operation
To promote stable operation, the LX2 is designed to minimize the
production and radiation of spurious emissions and harmonic energy.
In addition to the features described previously, the collector of each rf
stage is separately decoupled from the 9 Vdc supply using ferrite chokes,
resistors, and bypass capacitors of the appropriate value. The base circuits are similarly decoupled except that they use resistor–capacitor
(R–C) networks that are more appropriate, due to the higher impedance
level. The use of surface mount components results in better performance because of a reduction in stray inductances and unwanted
radiation from component leads. Double-sided printed circuit board
construction improves ground–plane performance.
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Characteristics3
Shure LX2 Hand-Held Transmitter
Rf: Euro (ETSI)
Audio Input
Processed audio enters R201, an internal potentiometer that is
adjusted for 15 kHz deviation (100% modulation) when the audio
section provides a –2.2 dBV, 1 kHz tone.
Oscillation
The audio then goes to varactor diode D201, which is part of the
modulated oscillator-tripler stage (Q201). The latter’s base-emitter circuit
operates as a crystal-controlled Colpitts oscillator in the 20 MHz region.
Fundamental-mode crystal Y201 is tuned 10 kHz below series resonance
by the series combination of frequency-netting coil L201, diode D201,
capacitor C203, and capacitor divider C206 and C207.
Frequency Stability
To ensure frequency stability despite changes in the battery voltage,
regulated 5 Vdc bias is applied to the varactor diode and to the base of
Q201. C203, C206, and C207 provide temperature compensation.
Tuned Circuits
Stage 1: The collector circuit of Q201 is tuned to the third harmonic
of the oscillator frequency (approximately 60 MHz) by L202, C208, C244,
C210, L203, C214, C213, and C215. The latter components also form a
capacitively-tapped voltage divider for matching into the base of Q202.
The output is double-tuned to provide high spectral purity. Regulated dc
bias is again employed to minimize changes in loading on the oscillator
stage and to stabilize the drive levels.
Stage 2: Q202 operates as a buffer with the collector circuit tuned to
the output frequency (for example, 60 MHz). In this case, L204, C216,
C245, C218, L205, C222, C221, and C224 perform tuning and impedance-matching functions. As in the preceding stage, regulated dc bias
is applied to the base circuit to stabilize the drive level, and the output
is double-tuned to provide spectral purity.
Stage 3: Q203 operates as a frequency tripler, with the collector
circuit tuned to the output frequency (for example, 180 MHz). In this
case, L206, C226, C227, C229, L207, C230, and C232 perform tuning
and impedance-matching.
Stage 4: Q204 operates as a tuned amplifier. Resistive loading on
the input provides stability. The output circuit consists of a resonant tank
circuit (L203 and C213) capacitively coupled to a low-pass filter (C219,
L206, and C218). C213 and C219 provide a capacitively-tapped voltage
divider for matching into the low-pass filter.
Characteristics
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Shure LX2 Hand-Held Transmitter
Final Output
Transmitter: The transmitter can deliver up to +11.5 dBm (14 mW)
to the antenna. No user adjustments permit this value to be exceeded.
Testpoint TP6 provides a termination point for the 50 Ω output. C243 and
L211 act as a series-resonating network for the battery, which acts as the
antenna. The unit should be powered exclusively by a 9 Vdc dry battery.
An alkaline type such as the Duracell MN1604 is recommended.
Voltage Measurements
With 9 Vdc applied to the unit, the following voltages should appear
at the terminals of the output transistor:
S Vc = 8.83 Vdc
S Vb = .097 Vdc
S Ve = .32 Vdc
S Base current = 0.27 µAdc
S Emitter current = 9.67 mAdc
S Collector current = 9.67 mAdc
S Power input = 87 mW
The output power is +9.5 dBm (8.9 mW) into a 50 Ω load, at a
frequency of 169.445 MHz. At the lowest acceptable battery voltage
of 6 Vdc, the final collector current drops to 8.84 mAdc and the output
power to +7.3 dBm (5.4 mW).
Power Section
Spurious Emissions
To promote stable operation and to minimize the production and
radiation of spurious emissions and harmonic energy, the collector of
each rf stage is separately decoupled from the 9 Vdc supply by ferrite
chokes, resistors, and bypass capacitors. The base circuits are similarly
decoupled, except for using resistor-capacitor (R-C) networks whose
higher-impedance levels are more appropriate.
The transmitter delivers a maximum of +3 dBm (2 mW) ERP, to
comply with ETSI Class 1 or 2 limits. There are no user adjustments
which would permit this value to be exceeded. The low effective radiated
power is due to the low radiation resistance of the battery antenna, which
is only a small fraction of a wavelength at this frequency.
The transmitter is intended to be powered exclusively by a 9 Vdc dry
battery (alkaline type recommended) such as a Duracell MN1604, which
also serves as the antenna. The ground plane of the circuit board serves
as an untuned counterpoise.
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Characteristics5
Shure LX2 Hand-Held Transmitter
Notes:
Notes:
6
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Preliminary Tests
Listening Test
Before completely disassembling the transmitter, operate it to deter-
mine whether it is functioning normally and try to duplicate the reported
malfunction. Refer to the product’s User Guide for operating instructions,
troubleshooting, 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
transmitter for the following functional tests.
Use dc blocks at all rf outputs to protect test equipment.
Shure LX2 Hand-Held Transmitter
25D1006 (CG)
Preliminary Tests7
Shure LX2 Hand-Held Transmitter
Test Component Locations
Audio Board (top)
Power
(GREEN) D103
Power ON/OFF
S101
Low battery
(RED) D101
R130
Mic ON/OFF
Head Board (front view)
Audio OUT
(to rf board)
+5 V
Audio IN
(from audio board)
+9 V
Gnd Gnd
U101
S102
+9 V
(to cartridge)
R125
U102
Gnd
+9 V
Audio IN
(to audio board)
+9 V
J102
J101
–
+
Gold-plated contacts
Audio IN
(from cartridge)
Rf Board (top) (FCC/IC)
L209
R217
Y201
C217
C216
Rf Board (top) (ETSI)
L201
R201
Y201
C214
C216
C222
Figure 2. Test Component Locations
I210
I2GN
C226
C240
J101
+
–
J102
J101
+
–
J102
Preliminary Tests
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Rf Tests
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Shure LX2 Hand-Held Transmitter
SPECTRUM ANALYZER
BAT
MUTE
LX2 Transmitter Spectrum Analyzer
Power switch: PWR Center frequency: transmitter frequency
Mute switch: MUTE Span: 30 MHz
Reference level: + 20 dBm
Scale: 10 dB/div
Figure 3. Rf Functional Test Set-Up
1. Install a new 9 Vdc battery and turn on the LX2.
2. Measure the current drain: it should not exceed 35 mA.
3. Maximize the signal received on the spectrum analyzer by
attaching a telescoping whip antenna. Then measure the nearfield output power. It should measure 3 dBm .
4. Verify that the carrier frequency of the transmitter varies from its
nominal value by no more than "6 kHz.
5. Check for an intermittent problem by shaking the transmitter and
tapping on it, keeping the transmitter a constant distance from
the spectrum analyzer. Verify that the output power on the spectrum analyzer shows no large and sudden drops in power level.
It will, however, vary a few dB according to hand position.
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6. Turn the LX2 OFF.
Preliminary Tests9
Shure LX2 Hand-Held Transmitter
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Audio Tests
Check the Matching Receiver
AUDIO ANALYZER
LX4 RECEIVER
HI Z
OUTPUTS
BAL
MIC LINE
POWER
12.5 – 18.9 VDC
ANTANT
AB
RF SIGNAL GENERATOR
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 switch: ON Frequency: 1 kHz Amplitude: –20 dBm
Gain: Max Filters: Modulation:: 1 kHz
Squelch: Mid Low-Pass (30 kHz): ON Deviation: 15 kHz
High-Pass (400 Hz): ON Frequency: LX2 operating frequency
Figure 4. Matching Receiver Functional Test
1. Connect the matching receiver’s antenna input to the rf
signal generator. Set the rf signal generator’s amplitude to
–20 dBm, modulation to 1 kHz, deviation to 15 kHz, and set
the frequency to the LX2 unit’s operating frequency.
2. Connect the audio from the unbalanced output to the audio
analyzer with a 3.3 kΩ load. Turn the volume control to
maximum.
Preliminary Tests
3. Engage the audio analyzer’s 400 Hz high-pass filter and 30 kHz
low-pass filter..
4. For the LX4, verify the following:
S audio level is 400 mVrms ("90 mV)
S total harmonic distortion (THD) is < 0.75%
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Audio Test Head
PT1840
Check the Transmitter
TRANSMITTER SHOWN WITH
BATTERY CUP REMOVED, AND
AUDIO TEST HEAD ATTACHED.
LX2 Transmitter LX4 Receiver Audio Analyzer
Power switch: PWR Power switch: ON Frequency: 1 kHz
Gain: Min Gain: Max Amplitude: .775 V
Mute switch: MUTE Squelch: Mid Filters:
AUDIO ANALYZER
LX4 RECEIVER
Shure LX2 Hand-Held Transmitter
HI Z
OUTPUTS
BAL
MIC LINE
POWER
12.5 – 18.9 VDC
Low-Pass (30 kHz): ON
High-Pass (400 Hz): ON
ANTANT
AB
Figure 5. Transmitter Functional Test
1. Disconnect the rf signal generator from the receiver and turn the
rf power off. Monitor the receiver’s unbalanced audio output with
a 3.3 kΩ load and the audio analyzer. Turn the receiver’s volume
control to its maximum setting.
2. Use an audio test head to input audio into the hand-held transmitter.
3. Turn the LX2 ON.
4. Inject a 0.775 V, 1 kHz signal from the audio analyzer into
the audio test head. Transmitter gain should be set to minimum.
Verify the following:
S the amplitude from the receiver’s unbalanced output equals
400 mVrms ("90 mV)
S total harmonic distortion (THD) is < 0.75%
5. Change the frequency of the audio generator to 100 Hz and
disengage the 400 Hz high-pass filter from the audio analyzer.
Verify that the audio level is –.5 dB ("0.5 dB) relative to the level
measured above.
6. Change the frequency of the audio generator to 10 kHz and
reengage the 400 Hz high-pass filter. Verify that the audio level,
relative to that measured above, is 0 dB, "2 dB.
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Preliminary Tests11
Shure LX2 Hand-Held Transmitter
Tested Units
If the system components pass the RF and Audio tests, then the sys-
tem is functioning correctly and does not require tuning and alignment.
Disassembly and Assembly
! CAUTION !
Observe precautions when handling this static-sensitive device.
Disassembly
To access the printed circuit boards of the unit, open the case:
1. Make sure the PWR/OFF switch is in the OFF position.
Handle
2. Unscrew the battery cup to access and remove the battery.
3. Unscrew the microphone cartridge and grille assembly.
4. If removal of the microphone-transmitter subassembly is necessary, use snap ring pliers to remove the retaining ring from inside
the microphone handle.
5. To remove the bezel, carefully insert a small screwdriver into the
microphone handle. Press against the plastic bezel tab and lift
upward. Remove the bezel, associated nameplate, and switch
actuators.
6. Remove the transmitter subassembly (audio, rf, and head circuit
board) from the handle while noting the internal guides they were
positioned on.
Bezel
Right side view of microphone
Tab
Preliminary Tests
Tab
Figure 6. Bezel Removal
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Circuit board subassembly
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Retaining ring
A
Ball screen
Head
board
Switches
Shure LX2 Hand-Held Transmitter
Microphone
cartridge
Audio circuit
board
Gain control
A
Nameplate
Bezel
Reassembly
Rf circuit
board
Circuit
board
guides
Handle
Gain control
access
Battery cup
Switch
actuators
9 V alkaline
battery
Figure 7. LX2 Hand-Held Transmitter, exploded view.
1. Make sure the PWR/OFF switch is in the OFF position.
2. Replace the transmitter subassembly (audio, rf, and head circuit
board) into the handle. Position it using the internal guides.
3. Carefully replace the nameplate, switch actuators, and bezel.
25D1006 (CG)
4. Replace the retaining ring.
5. Replace and screw in the microphone cartridge and
grille assembly.
6. Replace the battery.
7. Replace and screw in the battery cup.
Preliminary Tests13
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