Shure LX4 User manual

Shure Brothers Incorporated
222 Hartrey Avenue
Evanston IL 60202-3696 U.S.A.
LX Wireless System
E1999, Shure Incorporated Printed in U.S.A.
25–1008–1 (BK)
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 this Change Notice. Note that there are no changes to pages not specifically identified in the tables
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
CHANGES EFFECTIVE NOVEMBER 18, 2002
REMOVE
these pages from the
LX4 Service Manual
INSERT
these pages into the
LX4 Service Manual
Pages 14 & 23 Pages 14 & 23

Characteristics125D1008 (BK)
Service Manual
25D1008 (BK)
LX4 Diversity Receiver
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
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
7. Power ON/OFF button
8. Antenna connectors
9. Output connectors
10. MIC/LINE slide switch
11. Power jack
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12.5 –18.9 VDC
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1
RF A RF B AUDIO
SQUELCH LEVEL
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ANTANT
OUTPUTS
HI Z BAL
MIC LINE
POWER
AB
89911 8
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, Inc.
Printed in U.S.A.

Shure LX4 Diversity Receiver
2
Characteristics 25D1008 (BK)
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 trans-
mitters, 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 direct-
ly 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 noise-
operated 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 amplifi-
cation 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 Deviation kHz
1 –Green 1.5
2 –Green 4.0
3 –Green 7.0
4 –Amber 11.0
5 –Red 14.0

Shure LX4 Diversity Receiver
Characteristics325D1008 (BK)
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 pre-
set 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 con-
junction 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 transmit-
ter (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.

Shure LX4 Diversity Receiver
4
Characteristics 25D1008 (BK)
Independent Rf Sections
Two complete, independent rf sections are provided for diversity
reception. Signals enter the receiver via detachable, single-element
quarter-wave antennas. For diversity channel A, they pass through a
filter that is double-tuned 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 fifth-
overtone 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 opera-
tional 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 pro-
vided 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 Signal Strength, dBm (±10 dBm)
1 –95
2 –90
3 –80
4–65
5–60

Shure LX4 Diversity Receiver
Characteristics525D1008 (BK)
Power 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.

Shure LX4 Diversity Receiver
6
Test Component Locations 25D1008 (BK)
Test Component Locations

Shure LX4 Diversity Receiver
Preliminary Tests725D1008 (BK)
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 sugges-
tions, 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 TestsRefer 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.

Shure LX4 Diversity Receiver
8
Preliminary Tests 25D1008 (BK)
Rf Test Receivers that pass this test do not contain any “drop-out”or “range”
problems.
RF SIGNAL GENERATOR
NOTE: DC VOLTAGES ARE PRESENT AT MOST
RF TEST POINTS. USE A DC BLOCK ON THE
RF SIGNAL GENERATOR TO PROTECT
TEST EQUIPMENT.
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ANTANT OUTPUTS
HI Z BAL
MIC LINE
POWER
AB
MARCAD
DIVERSITY
DC BLOCK
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.

Shure LX4 Diversity Receiver
Preliminary Tests925D1008 (BK)
Audio Tests
RF SIGNAL GENERATOR
NOTE: DC VOLTAGES ARE PRESENT AT MOST
RF TEST POINTS. USE A DC BLOCK ON THE
RF SIGNAL GENERATOR TO PROTECT
TEST EQUIPMENT.
AUDIO ANALYZER
12.5 –18.9 VDC
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ANTANT OUTPUTS
HI Z BAL
MIC LINE
POWER
AB
DC BLOCK
LX4 Receiver Audio Analyzer Rf Signal Generator
Power: ON Output amplitude: Set to turn OFF high and low EXT
lights on rf signal generator Level: –60 dBm
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 kΩload, 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:
SAudio level is 400 mVrms, "90 mV. Record the
measurement as a reference level for the following steps.
Stotal harmonic distortion (THD) is <0.75%.

Shure LX4 Diversity Receiver
10
Preliminary Tests 25D1008 (BK)
RF SIGNAL GENERATOR
NOTE: DC VOLTAGES ARE PRESENT AT MOST
RF TEST POINTS. USE A DC BLOCK ON THE
RF SIGNAL GENERATOR TO PROTECT
TEST EQUIPMENT.
AUDIO ANALYZER
12.5 –18.9 VDC
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ANTANT OUTPUTS
HI Z BAL
MIC LINE
POWER
AB
DC BLOCK
LX4 Receiver Audio Analyzer Rf Signal Generator
Power: ON Output amplitude: Set to turn OFF high and low EXT
lights on rf signal generator Level: –60 dBm
Gain: Max Filters: Deviation: 15 kHz
Squelch: Mid 400 Hz High-Pass: OFF Modulation: EXT 1 kHz
30 kHz Low-Pass: ON Frequency: Receiver
frequency
Figure 4. Audio Functional Test Set-Up, steps 6 –14
6. Use external modulation only on the rf signal generator. Input a
100 Hz signal from the audio analyzer to the external modulation
input of the rf signal generator.
7. Disengage the 400 Hz high–pass filter on the audio analyzer.
8. Verify that the receiver’s unbalanced audio output is within
+2 dB, –1 dB of the reference level recorded in step 5.
9. Change the audio analyzer’s frequency to 10 kHz.
10. Verify that the receiver’s unbalanced audio output is –7.5 dB to
–10.5 dB from the reference level recorded in step 5.
11. Place a 150 Ωload across the receiver’s balanced output. Verify
that it is in the MIC position.
12. Connect this output to the audio analyzer and select the float
position. Verify that the output is 65 mVrms, "15 mV.
13. Remove the 150 Ωload.
14. Take the audio analyzer input out of the float position.
Repeat this test for channel B.
After all testing has been completed, remove the test cable and reinstall
the antenna. If the receiver passes these tests, the unit is functioning
correctly and does not require alignment.

Shure LX4 Diversity Receiver
Disassembly and Assembly1125D1008 (BK)
Disassembly and Assembly
! IMPORTANT SAFETY INSTRUCTIONS !
1. READ these instructions.
2. KEEP these instructions.
3. HEED all warnings.
4. FOLLOW all instructions.
5. DO NOT use this apparatus near water.
6. CLEAN ONLY with a damp cloth.
7. DO NOT block any of the ventilation openings.
Install in accordance with the manufacturer’s
instructions.
8. DO NOT defeat the safety purpose of the ground-
ing-type plug. The third prong is provided for
your safety. When the provided plug does not fit
into your outlet, consult an electrician for
replacement of the obsolete outlet.
9. PROTECT the power cord from being walked on
or pinched, particularly at plugs, convenience
receptacles, and the point of exit from the
apparatus.
10. USE only attachments/accessories specified by
the manufacturer.
11. USE only with a cart, stand, tripod, bracket, or
table specified by the manufacturer or sold with
the apparatus. When a cart is used, use caution
when moving the cart-apparatus combination to
avoid injury from tip-over.
12. UNPLUG this apparatus during lightning storms
or when unused for long periods of time.
13. REFER all servicing to qualified service person-
nel. Servicing is required when the apparatus
has been damaged in any way, such as when the
power-supply cord or plug has been damaged,
liquid has been spilled or objects have fallen into
the apparatus, the apparatus has been exposed
to rain or moisture, does not operate normally, or
has been dropped.
! CAUTION !
Observe precautions when handling this static-sensitive device.
! WARNING !
Voltages in this equipment are hazardous to life. No user-serviceable parts are inside.
Refer all servicing to qualified service personnel. The safety certifications of the LX4 Diversity Receiver
do not apply when the operating voltage is changed from the factory setting.
Disassembly Remove the case:
1. Remove the six screws from the case top, which includes two
screws on the top and two screws on either side.
2. Slide the case off the frame from front to back.
The front panel must be removed to access the printed circuit board:
3. Remove the clip from the center front panel frame screw tabs.
4. Unlock the four snap-in tabs to detach the front panel. Begin
with the bottom two tabs, placing your fingers on the bottom of
the chassis just behind each tab and squeezing down with your
thumbs on the top of the front panel.
5. Next, release the two top tabs.

Shure LX4 Diversity Receiver
12
Disassembly and Assembly 25D1008 (BK)
Remove the printed circuit board (pcb):
1. Remove the nuts and washers from the antenna input
connectors and the HI-Z audio output connector.
2. Remove the two screws on either side of the balanced
output XLR connector.
3. Unplug the dc power input from the pcb.
4. Remove the six screws anchoring the pcb to the bottom
of the chassis.
5. Lift the pcb from the front and pull forward. Note the lock
washers on the inside of the antenna connectors and HI-Z
audio output connector.
Reassembly Replace the printed circuit board:
1. Carefully replace the six screws that anchor the pcb to the
bottom of the chassis and replug the dc power input to the pcb.
Note the lock washers on the inside of the antenna connectors
and HI-Z audio output connector.
2. Replace the two screws on either side of the balanced output
XLR connector.
3. Replace the nuts and washers from the antenna input
connectors and the HI-Z audio output connector.
Replace the front panel:
1. Slide the case on the frame from front to back.
2. Lock the four snap-in tabs to attach the front panel.
3. Replace the six screws from the case top (two screws on the top
and two screws on each side).
4. Replace the clip from the center front panel frame screw tabs.

Shure LX4 Diversity Receiver
Service Procedures1325D1008 (BK)
Service Procedures
Test Equipment Most test equipment needed is described in the Shure Wireless
Service Equipment Manual. The following test equipment (or approved
equivalent) is also needed.
Table 1
Test Equipment
Equipment Type Model
Audio analyzer *Hewlett-Packard 8903B
Digital multimeter Fluke 87
Rf signal generator Hewlett-Packard 8656B
Frequency counter Hewlett-Packard 53181A
Spectrum analyzer Hewlett-Packard 8590L
*Audio levels in dBu are marked as dBm on the HP8903B.
Changing the Frequency
System Operating Frequencies
The LX4 Diversity Receiver printed circuit board is marked with
a group letter that identifies the range of frequencies the receiver can
operate within.
The operating frequency may be changed to a different frequency
within the same group. Before changing the operating frequency, refer
to the Functional Test section and check the receiver for proper opera-
tion.
Change the frequency by changing the crystal on the rf board.
For proper operation, the crystal must be obtained from Shure and must
operate within the frequency range of the pc board. Use the following
equation to determine the operating frequency:
Carrier frequency = 3 x (nominal crystal frequency in MHz) + 10.71 MHz
After the new crystal is installed, operational tests and alignment pro-
cedures must be performed to ensure that the receiver is operating cor-
rectly. The pcb label should also be updated to show the new frequency
and letter identification code.

Shure LX4 Diversity Receiver
14
Service Procedures 25D1008 (BK)
Table 2 Wireless Frequency Selections
Code Frequency Printed Circuit Board Assembly
169.445
169.505
170.000
170.245
A170.305 A171.045
171.105
171.845
171.905
173.800
174.100
174.400
174.500
175.000
B176.200 B176.600
177.000
177.600
180.400
182.000
182.200
C183.200
C183.600
184.600
186.200
186.350
D186.600 D189.000
190.600
191.900 T90__8632
192.200 T90__8632
192.600
E195.000 E196.600
197.600
198.600
200.300
200.350
F201.650 F202.200
203.000
204.200
204.600
206.000
G208.200 G208.300
209.000
H216.100
216.300
217.000
218.200
J218.600
J219.600
221.350
232.825
L233.125 L234.625
237.325

Shure LX4 Diversity Receiver
Service Procedures1525D1008 (BK)
Alignment The following steps are for channel A; channel B, is in parentheses.
For example, TPA7 (TPB7) means to use TPA7 for channel A and
TPB7 for channel B.
RF SIGNAL GENERATOR
NOTE: DC VOLTAGES ARE PRESENT AT MOST RF TEST POINTS. USE A DC
BLOCK ON THE RF SIGNAL GENERATOR TO PROTECT TEST EQUIPMENT.
TP9
TPA4
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ANTANT OUTPUTS
HI Z BAL
MIC LINE
POWER
AB TPB4
J2
DC BLOCK
LX4 Receiver Rf Signal Generator
Power: ON Frequency: carrier
Gain: Max Modulation: OFF
Squelch: Mid Amplitude: –60 dBm
Figure 3. Alignment Test Setup
1. Solder a 33 kΩ resistor between TPA4 (TPB4) and ground for rf
adjustments. Do the same for channel B with TPB4.
2. Set the rf signal generator frequency to the same as the LX4,
with no modulation, and its amplitude at –60 dBm.
3. Connect the power supply (PS40) to 120 Vac power and to the
receiver’s dc input connector (J2). The green power on LED of
the LX4 should now be illuminated. If not, there is a circuit mal-
function.
4. Connect the rf signal generator to the 50 Ωantenna input for
channel A or channel B, as appropriate.
5. Verify that 10.0 ±0.35 Vdc is present at TP9 (U7, pin 3), using a
digital multimeter (DMM).
Preset the adjustable circuit components as follows:
6. Set volume (R32) control fully clockwise.
7. Set squelch (R66) control to midrange.
8. Turn the LX4 power OFF.

Shure LX4 Diversity Receiver
16
Service Procedures 25D1008 (BK)
Received Signal Strength Indicator (RSSI) Adjustment
RF SIGNAL GENERATOR
NOTE: DC VOLTAGES ARE PRESENT AT MOST
RF TEST POINTS. USE A DC BLOCK ON THE
RF SIGNAL GENERATOR TO PROTECT
TEST EQUIPMENT.
TPA4
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RF A RF B AUDIO SQUELCH LEVEL
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ANTANT OUTPUTS
HI Z BAL
MIC LINE
POWER
AB
MARCAD
DIVERSITY
TPB4
TPA7TPB7
L100
L101
L103 L104
L105
L114
L108 L113
L112
L111
L208
L205 L204L203
L200
L201
DC BLOCK
LX4 Receiver Rf Signal Generator
Power: OFF Frequency: carrier
Gain: Max Modulation: off
Squelch: Mid Mod source: none
Deviation: none
Amplitude: off
Figure 4. RSSI Test Setup
1. Verify that the LX4 power is OFF.
2. Place a jumper between TPB7 and ground to mute channel B for
channel A alignment. (Place a jumper between TPA7 and
ground to mute channel A for channel B alignment.)
3. Turn the LX4 power ON.
4. Measure dc voltage at TPA4 (TPB4).
5. Adjust rf signal generator rf output so that approximately
2.0 Vdc is measured at TPA4 (TPB4).
6. If this is unattainable even with 0 dBm generator output, adjust
L108 (L208) until the voltage is above 2.0 Vdc. If adjusting L108
(L208) does not produce more that 2.0 Vdc at TPA4, return it to
the preset position and adjust L113 and L114 (L111, L112).
7. Adjust coils L100, L101, L103, L104, L105, L108, L113, and L114
(L200, L201, L203, L204, L205, L208, L111, and L112) for maxi-
mum voltage at TPA4 (TPB4). Use a hex-tuning wrench for all
adjustments except L108 (L208), which requires a non-metallic
screwdriver.
8. Reduce the rf signal generator output level as required to keep
signal voltage under 3.0 Vdc.
9. Final adjustments should be performed with an rf signal
generator output level of approximately –85 dBm.
Repeat this test for channel B.
Note: Inductance increases as the core is turned clockwise into
the coil. The maximum inductance position is approximately two

Shure LX4 Diversity Receiver
Service Procedures1725D1008 (BK)
turns before full clockwise rotation, or 13 mm (1/4-inch) below
the top of the can. Minimum inductance occurs when the core
is level with the top of the can.
Use care when turning the core to avoid locking it against the
pcb and causing it to break. Should this happen, the core can be
repaired either by removing the coil assembly and replacing it or
by drilling out the core and inserting a new one, after removing
any broken pieces that remain. Sometimes a broken core may
be removed using a metallic Allen wrench of the correct size.
For coils other than L108 and L208, “sharpness”of tuning
depends on the operating frequency within the frequency group.
At the upper and lower edges of the group, a distinct peak may
be difficult to observe. In this case, the core should be adjusted
for maximum inductance and remain there. At frequencies near
the high end of the group (where coils are adjusted to minimum
inductance), do not back cores beyond the point where they are
level with the top of the can. Only minimum change in induc-
tance occurs beyond this point.

Shure LX4 Diversity Receiver
18
Service Procedures 25D1008 (BK)
Audio Level Adjustments
RF SIGNAL GENERATOR
NOTE: DC VOLTAGES ARE PRESENT AT MOST
RF TEST POINTS. USE A DC BLOCK ON THE
RF SIGNAL GENERATOR TO PROTECT
TEST EQUIPMENT.
AUDIO ANALYZER
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HI Z BAL
MIC LINE
POWER
AB
L110 L210
R58
TPB5
TPA5
R59
DC BLOCK
LX4 Receiver Audio Analyzer Rf Signal Generator
Power: ON Output amplitude: 1.46 V rms Frequency: carrier
Gain: Max Frequency: 1 kHz Modulation: FM
Squelch: Mid Filters: Mod source: EXT
400 Hz High-Pass: ON Amplitude: –60 dBm
30 kHz Low-Pass: ON Deviation: 15 kHz
Figure 5. Audio Level Adjustments Test Setup
Note: Remove the 33KΩresistor from test point TPA4 and TPB4.
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.
Note: Place a jumper between TPB7 and ground to mute channel B for channel A alignment.
(Place a jumper between TPA7 and ground to mute channel A for channel B alignment.)
1. Set the rf signal generator frequency to the same as the LX4, its
modulation to FM, its modulation source to EXT, its FM deviation
to 15 kHz, and its amplitude to –60 dBm.
2. Set the audio analyzer’s audio frequency to 1 kHz and its
amplitude to 1.46 V rms.
3. Connect the output of the audio analyzer to the input of the
rf signal generator.
4. Place a probe on TPA5, U1, pin 1 (TPB5) and adjust
L110 (L210) for maximum ac voltage.
5. With the probe still on TPA5 (TPB5), adjust R59 (R58) for
0.775 V ±50 mV rms (1 kHz tone).
Repeat this test for channel B.

Shure LX4 Diversity Receiver
Service Procedures1925D1008 (BK)
Noise Squelch Level Adjustment
RF SIGNAL GENERATOR
NOTE: DC VOLTAGES ARE PRESENT AT MOST
RF TEST POINTS. USE A DC BLOCK ON THE
RF SIGNAL GENERATOR TO PROTECT
TEST EQUIPMENT.
AUDIO ANALYZER
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ANTANT OUTPUTS
HI Z BAL
MIC LINE
POWER
AB
TPA6 R130
TPB6
R230
R60
DC BLOCK
LX4 Receiver Audio Analyzer Rf Signal Generator
Power: ON Output amplitude: 1.46 V rms Frequency: carrier
Gain: Max Frequency: 50 kHz Modulation: FM
Squelch: Mid Filters: Mod source: EXT
400 Hz High-Pass: ON Amplitude: –60 dBm
30 kHz Low-Pass: OFF Deviation: 15 kHz
Figure 6. Noise Squelch Level Adjustment Test Setup
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 frequency to the same as the LX4, its
modulation to FM, its modulation source to EXT, its FM deviation
to 15 kHz, and its amplitude to –60 dBm.
2. Set the audio analyzer’s audio frequency to 50 kHz and its
amplitude to 1.46 V rms. Disengage 30 kHz lowpass filter.
3. Connect the output of the audio analyzer to the input of the
rf signal generator’s modulation input.
4. For channel A, adjust R130 for 2.0 ±0.05 Vdc at TPA6 (positive
end of C18). For channel B, adjust R230 to 2.0 ±.05 Vdc at
TPB6 (positive end of C17).
5. Set the rf signal generator for 1 kHz modulation, amplitude to
–50 dBm, with 15 kHz deviation. Engage 30 kHz lowpass filter.
6. Measure total harmonic distortion (THD). Adjust R60 for
minimum distortion at the unbalanced output. Distortion should
be < .75%.
7. Remove the external modulation from the rf signal generator.
Repeat this test for channel B.