Shure T4 User manual

Shure Incorporated
222 Hartrey Avenue
Evanston IL 60202-3696 U.S.A.
T Wireless System
E1999, Shure Incorporated Printed in U.S.A.
25–1020–1 (EA)
SERVICE MANUAL CHANGE NOTICE
T4 DIVERSITY RECEIVER
Changes and corrections have been made to the Service Manual for the T4 Receiver. 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.
T4 RECEIVER SERVICE MANUAL REVISION HISTORY
Release Part Number Date Code Color
Original 25A1020 QG White
Revision 1 25B1020 SB Pink
Revision 2 25C1020 SI White
Revision 3 25C1020 TF White
Revision 4 25C1020 CC White
Revision 5 25C1020 EA Red
CHANGES EFFECTIVE JANUARY 13, 2005
REMOVE
these pages from the
T4 Service Manual
INSERT
these new Revision pages into the
T4 Service Manual
21 & 22 21 & 22

1Characteristics
25C1020 (EA)
®
Service Manual
25C1020 (EA)
T4 Diversity Receiver
Characteristics
General This manual tells how to service and align the Shure T4 Diversity Receiver
(Figure 1). This single-channel, crystal-controlled unit operates within the
169 MHz to 238 MHz VHF-FM band.
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T4G MAX
12-18 VDC
DC INPUT
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56
78
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9
Figure 1. Controls and Connectors
1. “DC Input” jack
2. Squelch control
3. Antennas
4. “Power” LED
5. “Diversity” LEDs
6. “Audio Peak” LED
7. Volume control
8. Audio “Output” (1/4Ȃphone
jack)
9. “Balanced Low Z” audio out-
put (XLR)
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.

Shure T4 Diversity Receiver
2Characteristics 25C1020 (EA)
Circuit Description
The Shure Model T4 is a single-conversion superheterodyne diversity FM
receiver operating in the 169–238 MHz band. It is intended for use with the
matching Shure T Series wireless transmitters.
RF Stages Two complete, independent RF sections provide diversity reception. Signals
enter via the single-element, quarter-wave antennas.
Channel A: The signals pass through a double-tuned filter (L3 and L4) be-
fore entering MOSFET amplifier Q1. The output of this stage is double-tuned by
L5 and L6, which also provide impedance-matching to Gate 2 of GaAs MESFET
(gallium arsenide metal semiconductor field effect transistor) mixer Q3. Gate 1
receives the local oscillator signal from transistor Q5. A third-overtone quartz
crystal in the 50–70 MHz range provides frequency control. The collector circuit
of the oscillator is tuned by L8 to the third harmonic of the crystal (160–230 MHz)
to provide the proper injection frequency for a 10.7 MHz intermediate frequency
(IF).
Channel B: This channel is identical in design to channel A. The signals
from the antenna pass through a double-tuned filter (L12 and L13) before enter-
ing MOSFET amplifier Q6. The output of this stage is double-tuned by L14 and
L15 and fed to Gate 2 of GaAs MESFET mixer Q8. Gate 1 receives the local os-
cillator injection from buffer transistor Q4, which is tuned by L7. The buffer stage
helps isolate the diversity channels from one another by preventing crosstalk
through the common local oscillator section.
IF and Audio-Detection Stages
Channel A: L2 tunes the output of mixer Q3 to 10.7 MHz before the signal
enters ceramic filter FL3. Transistor Q2 provides IF amplification to make up for
the losses in the filters. After passing through a second IF filter, FL2, the signal
enters amplifier/detector U1. The detected audio from pin 6 is amplified by
U105C.
Channel B: L11 tunes the output of mixer Q8 to 10.7 MHz before the signal
enters ceramic filter FL6. Transistor Q7 provides IF amplification before the signal
passes through the second ceramic filter, FL5, and enters amplifier/detector U2.
The detected audio from pin 6 is amplified by U108B.
Noise-Operated Squelch
A noise-operated squelch system provides both diversity-channel selection
and muting. Noise signals are obtained from the additional detector outputs at
pin 7 of U1 and U2.
Noise Signals
Channel A: The squelch level control (R16) adjusts the noise signal from U1
before it is amplified by U105A. Active high-pass filter UI05D removes audio
components that could cause false triggering. The noise is then rectified by D101
and smoothed by C111 to provide a dc voltage that varies with the amount of
noise present on the detected signal.

Shure T4 Diversity Receiver
3Characteristics
25C1020 (EA)
Channel B: This follows an identical scheme: noise signals from U2 are ad-
justed by squelch level control R41 before being amplified by U108C, filtered by
U108A, rectified by D107, and smoothed by C140.
Comparator Circuits
U106C and U106D compare the dc noise signals from the two channels.
When these signals are comparable, U106C and U106D direct analog switches
U103C (Channel A) and U103B (Channel B) to allow both channels to pass
through. The signals from each channel are correlated while the noise is uncor-
related, which makes possible a theoretical signal-to-noise improvement of 3 dB.
When the dc noise signals are not comparable, the channel with less noise is
chosen.
U106A imposes an additional criterion on Channel A, and U106B does the
same for Channel B. Each channel’s dc noise voltage is compared to a threshold
set by user-adjustable R112 (Squelch). A channel whose noise exceeds this
threshold is squelched. If both channels exceed their thresholds, the audio out-
put of the receiver is squelched. Squelching is accomplished by tying together
the outputs of U106A and U106C (Channel A) or U106B and U106D (Channel
B). These outputs also control the yellow “Diversity” status indicators. Compara-
tor U102 provides additional attenuation in the squelched state by shutting off
compandor U3.
Audio Output The audio signals from the two channels pass through analog switches
U103C and U103D, are buffered by U109A, and enter U3, which provides a 2:1
logarithmic expansion. An additional IC amplifier (U109D) operates in conjunction
with U3 to provide a lower noise floor. The output of this stage passes through
the Volume control (R126) to the “Output” connector (J101). A bridging amplifier
formed by U109B and U109C provides the “Balanced Low Z” output.
“Peak” LED This red indicator warns the user when the transmitter deviation is approach-
ing the limit of 15 kHz. This function is implemented by a window comparator
(U102C and U102D) and a pulse stretcher (U102B) that makes short transients
more visually apparent.
Power The green “Power” LED indicates when the external power converter (or a
battery pack) is supplying power to the receiver. The circuitry operates normally
with an input of 12–18 Vdc. D10 provides reverse polarity protection, and U101
supplies voltage regulation. The “Dc Input,” audio “Output,” and “Balanced
Low Z” output connectors are filtered to prevent local oscillator radiation from the
cables.

Shure T4 Diversity Receiver
4Notes 25C1020 (EA)
Notes

Shure T4 Diversity Receiver
5Preliminary Tests
25C1020 (EA)
Preliminary Tests
Listening Tests Before disassembling the unit, operate it to determine whether it is function-
ing normally. First and most important: Review the customer’s complaint (if avail-
able) and focus your tests on the problem. If this proves inconclusive or you want
somewhat more extensive checks, perform the following functional tests.
Functional TestsThe following tests require partial disassembly of the unit:
RF Test The following is the best “fast” test of a receiver’s RF performance. A receiv-
er that passes this test can be removed from the list of suspects for any “drop-
out” or “range” problem.
Initial Set-up
1. Set the receiver’s Squelch control to its middle position and the Volume
control to its maximum position.
2. Set the RF signal generator to the receiver’s frequency.
3. Remove the receiver’s antenna (see “Disassembly,” page 7). Plug the
BNC end of the 50 Ωtest cable into the RF signal generator. Tack-solder
the cable’s center conductor to the receiver’s antenna input (TPA1 or
TPB1), and the shield to a ground plane as close as possible to the an-
tenna input (TPA2 or TPB2).
Test
1. Connect power to the receiver and turn it on.
2. Verify that the unit unsquelches with RF signals greater than –89 dBm.
3. When the unit unsquelches, verify that the “Diversity” LED glows.
Audio Tests
Initial Set-up
1. Set the RF generator as follows:
Level: –60 dBm
Deviation: 15 kHz
Modulation: Ext
2. Set the audio analyzer as follows:
Amplitude: 1.4 Vrms
Frequency: 1 kHz
3. Using a 3.3 kΩload, connect the receiver’s unbalanced audio output to
the input of the audio analyzer, and engage the 400 Hz and 30 kHz fil-

Shure T4 Diversity Receiver
6Preliminary Tests 25C1020 (EA)
ters. The receiver’s Volume control should still be in its maximum posi-
tion.
Tests
1. Verify the following receiver measurements:
SAudio level is 400 mVrms, "90 mV. Record your measurement as a
reference level for the next two steps.
SThd is <0.75%.
2. Change the audio analyzer’s frequency to 100 Hz, and disengage the
audio analyzer’s 400 Hz filter. Verify that the receiver’s unbalanced audio
output is within +2 dB, –1 dB of the reference level recorded in step 1.
3. Set the audio analyzer’s frequency to 10 kHz. Verify that the receiver’s
unbalanced audio output is –7.5 dB to –10.5 dB of the reference level
recorded in step 1.
4. Model T4V only: Place a 150 Ωload across the receiver’s balanced out-
put, then connect this output to the audio analyzer. Verify that the output
is 65 mVrms, "15 mV. Remove the 150 Ωload.
5. If you are finished testing the receiver, remove the test cable and rein-
stall the antenna (see “Reassembly,” page 8).
Units that Pass If the receiver passes these tests, then it is functioning as expected and
shouldn’t require alignment. If you did not use the customer’s microphone trans-
mitter for these tests, check it for proper operation. If it also checks out of it it was
not sent in with the receiver, inform the customer that the product has retested
within specifications.

Shure T4 Diversity Receiver
7Disassembly and Assembly
25C1020 (EA)
Disassembly and Assembly
To access the printed circuit (pc) board, disassemble the receiver.
CAUTION
Observe precautions when handling this static-sensitive device.
Disassembly 1. Disconnect all power to the receiver.
2. Collapse the antennas and rotate them until they lie flat against the case.
Place the receiver upside down. Remove and set aside the four screws
securing the bottom of the case (Figure 2).
Antenna
mounting
screw
Case
mounting
screws
Antenna
mounting
screw
Volume knob
Case
mounting
screws
Figure 2. Bottom View of Case
3. Place the receiver in its normal operating position and rotate the anten-
nas to their vertical positions. Note that an adhesive-backed nameplate
at the at the rear of the unit holds the upper and bottom parts of the case
together. To open the case, carefully separate the upper part from the
bottom part at the front of the unit. Carefully swing the bottom part of the
case away until the two halves are at a 90°angle, to allow removal of the
circuit board and antennas (Figure 3).
4. After the circuit board has been removed, pull off and retain the Volume
knob.

Shure T4 Diversity Receiver
8Disassembly and Assembly 25C1020 (EA)
É
É
É
É
Mounting screw
Case bottom
Case top
Circuit
board
Antenna
Volume knob
Figure 3.
Reassembly 1. Slide the Volume knob back on: note how it is keyed to the flat part of the
shaft.
2. Rotate the antennas to their vertical positions. Place the pc board, com-
ponent-side up, inside the top half of the case: after feeding the anten-
nas through their holes, make sure the notches in the case align with the
Volume knob, the LEDs align with the holes in the upper half of the case,
and that the board is fully seated on the studs.
3. Swing the two halves of the case together until they snap in place.
4. Rotate the antennas until they lie flat against the case. Turn the receiver
upside down and secure the assembly with the four screws removed
earlier (Figure 2, page 7). Check that the Volume knob rotates freely.
Antenna Replacement
The antennas can be replaced without disassembling the case. Rotate the
antenna you are replacing to its vertical position, collapse it, and remove the
mounting screw through its access hole in the bottom of the case (Figure 2). In-
sert the new antenna and start the screw from the bottom, leaving it loose
enough for you to rotate the entire antenna between your fingers. Rotate the an-
tenna in this way as you lightly press it into the case until you feel the base seat
in the slot of the bracket on the pc board. Tighten the screw.

Shure T4 Diversity Receiver
9Service Procedures
25C1020 (EA)
Service Procedures
Reference Material
The Shure Wireless System T Series User’s Guide provides a description of
the unit as well as operating instructions, troubleshooting suggestions, and tech-
nical data.
Special Equipment and Tools
In addition to the standard items described in the Service Equipment manual,
you will need:
Sa wireless microphone with the same frequency (usually a T1, T2, or
T11) to verify that the receiver is working properly
San audio amplifier with a high-impedance input (w10 kΩ) and a
monitor speaker, for listening tests
System Operating Frequencies
Each receiver’s circuit board has a resistor next to the group letter (A–H,
J–L) that identifies the range of frequencies on which the receiver can operate
(see Figure 4). Table 1 shows the Group Letter and its associated frequencies.
Note that this chart applies only to T4 receivers.
Table 1
Pc Board Groups
Group Frequency Range
A169.000–173.975 MHz
B174.000–179.975 MHz
C180.000–185.975 MHz
D186.000–191.975 MHz
E192.000–197.975 MHz
F198.000–203.975 MHz
G204.000–209.975 MHz
H210.000–215.975 MHz
J216.000–222.975 MHz
K223.000–229.975 MHz
L230.000–237.975 MHz
Tables 2 and 3 provide information for identifying the system frequency. The
Crystal Code, together with the appropriate Shure model number, identifies a
specific operating frequency for transmitters and receivers. Note that, although a
Crystal Code always designates a specific frequency, it may be used with differ-
ent Group Letters on other products.

Shure T4 Diversity Receiver
10Service Procedures 25C1020 (EA)
Table 2
FCC-Approved Operating Frequencies
Group Crystal Code Freq. (MHz)
A V 169.445
A W 171.845
B CA 176.200
B CC 177.600
C CE 182.200
C CF 183.600
D CG 186.200
E CL 192.200
F CQ 202.200
G CV 208.200
Table 3
ETSI-Approved Operating Frequencies
Group Crystal Code Freq. (MHz)
A AQ 173.800
BAY 174.100
B AZ 174.500
B ZZ 174.500
B BA 174.800
B NB 175.000
B BB 175.000
B ND 176.600
B NE 177.600
C NH 182.000
C NK 183.600
C NL 184.600
C CS 184.800
C S 184.800
D NP 189.000
D NR 190.600
E NX 197.600
F NY 198.600
F NZ 200.350
F PU 201.650
F PB 203.000
G PD 204.600
J PP 217.000
J PR 218.600
J PS 219.600
L PV 232.825
L PX 233.125
L PY 234.625
L PZ 237.325

Shure T4 Diversity Receiver
11 Service Procedures
25C1020 (EA)
Changing the Frequency
The operating frequency of the T4 Receiver may be changed within a specif-
ic group by changing the crystal on the pc board (for group information, see the
preceding section). Check the receiver for proper operation before attempting to
change its operating frequency. After installing the new crystal, perform the align-
ment procedures. Then run an operational test to ensure the receiver is function-
ing properly. Finally, update the label to show the new frequency and letter identi-
fication code.
Note: To ensure proper operation, obtain the crystal from Shure and verify
that it operates within the frequency range of the pc board. Since crystals are
marked with the nominal oscillating frequency, not with a letter code, you can
use the following equation to determine the frequency at which a receiver will
operate with a given crystal:
Carrier Frequency = (3 nominal crystal freq. in MHz) + 10.71
Alignment The alignment steps must be done together, as a single, continuous proce-
dure. Before beginning, be sure to do the setup described in the following sub-
section, “Test Conditions.”
Test Conditions The following are the standard test conditions for the T4 Receiver:
SWith a #1 Phillips screwdriver, remove the antennas.
SLoad the the unbalanced, audio output (J101) with a 3.3 kΩresistor.
SFor RF adjustments, connect a 33 kΩresistor between ground and
the following points:
Channel A: TPA4 (pin 13 of U1)
Channel B: TPB4 (pin 13 of U2)
SSet the potentiometers as follows:
Volume (R126): Fully CW
Squelch (R112): Midrange
Audio level (R106 and R168): Midrange
Mute level (R16 and R41): Midrange
Test Cable
1. Obtain a 50 Ω coaxial cable for connecting the circuit boards to various
test equipment. (To construct this cable, see “50 Ω Test Cable Assembly”
in the Service Equipment manual.) Refer to Figure 4 for the locations of
the test points, controls, and tuning components.
2. Tack-solder the cable to the pc board as follows:
Channel A: Center conductor to the antenna input (TPA1) and shield to
ground (TPA2)
Channel B: Center conductor to the antenna input (TPB1) and shield to
ground (TPB2)

Shure T4 Diversity Receiver
12Service Procedures 25C1020 (EA)
Presettings
Most field units should already be tuned closer to the desired settings than
these approximations. However, you may need to preset units when you are
changing the frequency or an RF coil, or when an unskilled person has attempted
to retune them.
Minimum inductance occurs when the core is level with the top of the
can. Turning the core further counterclockwise or even removing it will
not appreciably affect the inductance.
Maximum inductance occurs when the core is about two turns less than
its full clockwise rotation—about 3.2 mm (1/8in.) below the top of the
can. Never screw the core against the board.
The manufacturer’s setting of the two smaller, IF coils (L2 and L11) is close
to their ideal setting, so you generally would not preset them. For coils L3–L8
and L12–L15, refer to Table 2 or 3 (page 10) to determine where the desired fre-
quency lies within the board’s group. For frequencies near the bottom of a group,
use the maximum-inductance settings; for frequencies near the top of the group,
use the minimum-inductance settings.
Display Check 1. Connect an external 12–15 Vdc supply to the dc input (J103). The green
“Power” LED should glow.
2. Check for 9 Vdc ("0.35) at TP9 (pin 3 of voltage regulator U101).
Test-Equipment Settings
1. Plug into the RF generator the BNC end of the 50 Ωcable you soldered
to the pcb (antenna and ground). Set the generator’s controls as follows:
S–20 dBm on the receiver’s operating frequency
S"15 kHz FM deviation with 1 kHz modulation
2. Activate the 400 Hz high-pass and 30 kHz low-pass filters on the audio
analyzer.

Shure T4 Diversity Receiver
13 Service Procedures
25C1020 (EA)
L3
L4
L5
L6
L8
L2
Y100
L11
L7
L15
L14
L13
R126
C118
L12
R41
C48
R168
U2
C145
C150
U1
R106
R16 C6
U105
U106
U103
U107
U3
U102
U108
U109
Channel A
Channel B
B
B
B
B
B
B
B
B
A
A
A
A
A
A
A
A
A
B
U101
Group designator
(see Table 1)
XLR audio output
connector (J102)
Note: Used only on
Presenter and Vocal
Artist models
Power input (J103)
Note: Use Shure
PS20 or PS20E
Squelch control
(R112)
Antenna B
Phone jack
!/4-in. (J101)
Edge of
case bottom
Volume control
Crystal (see
Tables 2 and 3)
“Audio Peak”
(red)
“Power” (green)
“Diversity” (RF)
(yellow)
Knob
U104
Antenna A
D108
D109
TPB4
TPB3
TP9
TPA3
TPB7TPA7
TPB5
TPB6
TPA4
TPA5
TP8
TPA6
TPA1
TPA2
TPB1
TPB2
(On earlier
units only.)
L108
L107
Note: L107 and L108 are
not found on earlier units.
Figure 4. Major Components

Shure T4 Diversity Receiver
14Service Procedures 25C1020 (EA)
Coil Adjustments Each channel has to be adjusted separately.
A: Channel A RF and IF
1. Disconnect the power to the receiver. Mute Channel B by grounding
TPB7 (pin 14 of U106). Reconnect the power.
2. Adjust the signal generator’s RF output so that you obtain a reading of
approximately 2 Vdc at TPA4. If this is unattainable even with the full –20
dBm generator output, adjust IF coil L8 until you obtain a reading above
2 V. If you cannot get more than 1.25 V, return L8 to its preset position
and adjust coil L2.
3. Adjust coils L2, L8, L3, L5, L4, and L6 (in that order) for maximum volt-
age at TPA4. Use a hex tuning wrench for all adjustments except L2,
which requires a non-metallic screwdriver (like a Toray driver). Reduce
the generator output as required to keep the signal voltage under 3 Vdc.
Perform the final adjustments with a generator output of approximately
–85 dBm.
Note: For coils other than L2, the “sharpness” of the tuning depends on
the operating frequency within the frequency group. At the upper and
lower edges of a group, a distinct peak may be difficult to observe. In this
case, adjust the core for maximum indication.
B: Channel A Audio
Note: Before making the following adjustments, power up the circuit
board for at least one minute. This allows the FM detector to stabilize.
1. Set the RF signal generator for an output level of –60 dBm, a modulation
of 1 kHz, and a deviation of 15 kHz.
2. Version E and later boards only: With the probe of an ac voltmeter on
TPA5 (pin 8 of U105), adjust L107 for peak output.
3. Adjust R106 for 0 dBu (775 mV), "0.1 dB, at TPA5 (pin 8 of U105).
4. Reset the signal generator to its minimum output level (or “Off”). Check
that both ”Diversity” LEDs turn off.
5. Set the signal generator as follows:
Level: –50 dBm
Ext. Modulation: 50 kHz
Deviation: 15 kHz
Adjust R16 for 1 Vdc ("0.5 Vdc) at TPA6 (the positive end of C111).
6. Turn off the modulation of the signal generator. Set its output to the mini-
mum level, then gradually increase the level until the “Diversity A” LED
turns on. This should occur with an RF input between –102 dBm and
–87 dBm (typically –95 dBm).
7. Disconnect power from the receiver. Remove the jumper from TPB7 and
the RF input from TPA1 and TPA2.
C: Channel B RF and IF
1. Make sure that the RF generator is connected to the antenna input for
Channel B. Mute Channel A by grounding TPA7 (pin 13 of U106).

Shure T4 Diversity Receiver
15 Service Procedures
25C1020 (EA)
2. Connect power to the receiver.
3. Adjust the RF output of the signal generator RF output so that you obtain
a reading of approximately 2 Vdc at TPB4. If this is unattainable even
with the full –20 dBm generator output, adjust IF coil L7 until you obtain a
reading above 2 V. If you cannot get more than 1.25 V, return L7 to its
preset position and adjust coil L11.
4. Adjust coils L11, L7, L12, L14, L13, and L15 (in that order) for maximum
voltage at TPB4. Use a hex tuning wrench for all adjustments except L11,
which requires a non-metallic screwdriver (like a Toray driver). Reduce
the generator output as required to keep the signal voltage under 3 Vdc.
Perform the final adjustments with a generator output of approximately
–85 dBm.
Note: For coils other than L11, the “sharpness” of the tuning depends on
the operating frequency within the frequency group. At the upper and
lower edges of a group, a distinct peak may be difficult to observe. In this
case, adjust the core for maximum indication.
D: Channel B Audio
Note: Before making the following adjustments, power up the circuit
board for at least one minute. This allows the FM detector to stabilize.
1. Set the signal generator for an output level of –60 dBm, a modulation of
1 kHz, and a deviation of 15 kHz.
2. Version E and later boards only: With the probe of an ac voltmeter on
TPB5 (pin 7 of U108), adjust L108 for peak output.
3. Adjust R168 for 0 dBu (775 mV), "0.1 dB, at TPB5.
4. Reset the signal generator to its minimum output level (or “Off”). Check
that both “Diversity” LEDs turn off.
Level: –50 dBm
Ext. Modulation: 50 kHz
Deviation: 15 kHz
5. Set the RF signal generator as follows:
Adjust R41 for 1 Vdc ("0.5 Vdc) at TPB6 (the positive end of C140).
6. Turn off the modulation of the signal generator. Set its output to the mini-
mum position, then gradually increase the level until the “Diversity B”
LED turns on. This should occur with an RF input between –102 dBm
and –87 dBm (typically –95 dBm).
7. Disconnect power from the receiver. Remove the jumper from TPA7 and
the RF input from TPB1 and TPB2.
E: Final Steps
1. If you were unable to align the unit, refer to the “Bench Checks” section,
which follows immediately.
2. When you have finished working on the unit, reassemble it, as described
in the earlier “Reassembly” subsection on page 8.

Shure T4 Diversity Receiver
16Bench Checks 25C1020 (EA)
Bench Checks
Note: In this section, test points for Channel A are given first, and the corre-
sponding test points for Channel B are enclosed in square brackets.
DC Power
nCheck for 9 Vdc (±0.35 Vdc) at TP9 (pin 3 of U101):
SIf the voltage is lower than normal (but not zero): Check for 15 Vdc at the
input of U101 (pin 1), a reversed electrolytic capacitor (C152), or a stage
that is drawing excessive current.
SIf the voltage is zero: Check for solder bridges or shorted foil traces (de-
fective pcb).
SIf the voltage is higher than normal or the other tests prove negative: Re-
place U101.
nCheck for 5 Vdc (±0.25 Vdc) at TP8 (pin 10 of U3).
Tuning
nFirst check that that the RF signal is being applied to the correct diversity
channel and that the other channel is being muted with the jumper.
nIf the voltage reading at TPA4 [TPB4] does not vary when the RF coils are
adjusted, check the operation of the local oscillator. If everything is working
properly, you should obtain a reading of 1–2 Vdc with a –95 dBm RF input
signal, depending upon component parameters and receiver frequency. If
you do not obtain this reading, try the following:
SWith the RF generator turned off, use a spectrum analyzer to measure
the injection level at TPA3 [TPB3] for approximately –15 to –20 dBm.
SIf the preceding level is correct, use a frequency counter to verify that the
local oscillator signal is 10.7 MHz (±.015 MHz) below the operating fre-
quency of the receiver.
Example: To receive 169.445 MHz, the local oscillator frequency
should be 158.745 MHz (±.015 MHz). Crystal Y100 operates at a
third of this frequency, or 52.915 MHz. The nominal crystal frequen-
cy, which is stamped on the part, is 3.333 kHz lower
(52.911667 MHz) in this example. This discrepancy arises because
the load reactance of the oscillator is not precisely zero.
nIf any of the RF coils will not tune properly, check for frequency-dependent
capacitors that are missing or have the wrong value. If the IF coil will not
tune, check C9 [C52].
nIf all the coils tune but the RF signal is low, compare the dc voltages and
components at RF amplifier Q1 [Q6] with those of a properly working unit.
Muting
nWith TPA7 [TPB7] connected to ground, the selected diversity channel should
be squelched and the “A” [“B”] LED should be off, whether or not an RF input

Shure T4 Diversity Receiver
17 Bench Checks
25C1020 (EA)
signal is present. If you obtain different results, compare the dc voltages at
U106, U107 (earlier units only), U103, and U105 [U108] with those of a prop-
erly working unit, or replace the pc board.
nIf the muting circuit works but has insufficient adjustment range, set the RF
generator for a –95 dBm signal and check TPA4 [TPB4] for 1–2 Vdc, as de-
scribed in the preceding, “Tuning,” subsection. If you get a lower reading,
consider the following:
SLow RF levels at the input of U1 [U2] will cause inadequate noise at the
output of U105D [U108A].
SIn order for the noise squelch circuit to function, the detector portion of
U1 [U2] must also be functioning properly (see the following, “Audio,”
subsection).
SIf necessary, check the component values and dc voltages (versus those
of a known working unit) of the following:
Noise amplifier U105A [U108C]
High-pass filter U105D [108A]
Audio
nIf there is no audio at the output of the receiver:
STo verify that the receiver is not squelched, defeat the squelch circuit by
rotating R16 [R41] fully CCW. There should be 9 Vdc at TPA7 [TPB7],
and the yellow “A” [“B”] “Diversity” LED should be on.
SIf the preceding test does not yield the proper results, check for a prob-
lem in the squelch circuitry (see the “Muting” subsection, immediately
above).
nNext, check pin 6 of U1 [U2] for an audio signal of approximately –15 dBV. If
there is no detected audio:
SCheck C1 [C44] and the components connected to pins 7–10 of U1 [U2].
SOn earlier units only: Make sure that FL1 [FL4] is an 86A8920 ceramic
discriminator and not an 86A8910 ceramic filter, which is similar in ap-
pearance.
SCheck the dc voltages at U1 [U2] against those of a known working unit.
If no other problem is found, replace the IC.
SIf audio is present at pin 6 of U1 [U2] but not at the output of the receiver,
trace the signal through signal amplifier U105C (TPA5) [U108B (TPB5)]
and expander U3/U109D. When measuring at audio TPA5 [TPB5], use a
1 kΩresistor in series at the test point to prevent loading the amplifier.
SCheck the Volume control (R126) and the components going to the au-
dio output jack (J101).
SOn vocal models, check for a signal at the Low Z microphone-level out-
put, J102. If no signal is present, check U109B, U109C, and associated
components.
SCheck the dc voltages on any stage that is not working properly; then
look for wrong or missing components. Replace the ICs in the faulty
stage if no other problem is found.

Shure T4 Diversity Receiver
18Bench Checks 25C1020 (EA)
nIf you cannot set the audio level properly with R106 [R168], make sure that
the other diversity channel is squelched: only one channel should be acti-
vated during this adjustment.
nIf you can set the audio levels properly for each channel while the other is
squelched, but these levels change significantly when both channels are ac-
tivated, check U103.
nIf the audio level is correct but the unit exhibits high total harmonic distortion
(thd), vary the RF input frequency 20 kHz above and below the operating
frequency. If the thd drops to an acceptable level, check the local oscillator
frequency. If it is within specifications, repeak the quadrature coil, L107
[L108], for maximum ac output at TP5 [TP8] (see step 1 of “Channel A Au-
dio,” page 14 [step 1 of “Channel B Audio,” page 15]). On earlier units only,
which do not have L107, replace ceramic discriminator FL1 [FL4].
nIf the audio circuitry works properly but the red audio peak LED either fails to
light when the standard test signal is applied or remains on when there is no
modulation, check the dc voltages at U102B, U102C, and U102D against
those in a known working unit. Also check the peak LED itself. Replace any
defective components.

Shure T4 Diversity Receiver
19 Notes:
25C1020 (EA)
Notes: