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CAPACITOR
CARTRIDGE
IMPEDANCE
CONVERTER
CAPACITIVE
ATTENUATOR
CIRCUIT DESCRIPTION
A block diagram of the SM81 is shown in Figure 11. The ca-
pacitor cartridge is followed by a switch-controlled capacitive
attenuator stage which provides for 10 dB attenuation at the
cartridge output. The signal is fed to a field-effect transistor
(FET) impedance conversion stage. The FET output drives an
active low-frequency response (high-pass) filter controlled by a
three-position switch. The filter output from a compound tran-
sistor, Class A, emitter-follower amplifier is transformer-
coupled, providing a balanced output to the RFI protection filter
at the microphone connector. An active, constant-current
power supply circuit regulates the phantom voltage, allowing
the SM81 to operate over a very wide range of voltages. A re-
verse voltage protection diode guards against miswired cables
and equipment. The circuit contains five semiconductors to pro-
vide low noise, low distortion, wide frequency response and
ultra-reliable operation over a very wide range of operating con-
ditions.
SERVICING
Troubleshooting
Due to the high packing density and circuit complexity of
the SM81, only basic servicing is recommended. The following
steps should be taken if problems arise:
1.
Check the power supply output voltage to the microphone.
For the Shure PS1A, this should be 21.5
circuit.
2.
Check the voltage on microphone connector pins 2and 3
(at back of connector; cable connector disassembled from
shell but connected to microphone). The voltage at pins 2
and 3 with reference to pin 1 should be between 10 and 48
Vdc.
3.
If more than one SM81 is available, interchange cartridge
assemblies to determine whether the cartridge or amplifier
is at fault.
Disassembly
The SM81 can be disassembled as follows:
1.
Unscrew the grille and cartridge assembly (counterclock-
wise from top).
2.
Lift the 10 dB attenuator actuator ring up and over the
screw threads.
3.
Grasp the center contact of the 10 dB attenuator switch as-
sembly and lift upward to remove it.
4.
Turn the slotted head screw near the plug counterclock-
wise (inward) as far as it will go. Using long-nose pliers,
grasp one pin of the three-pin connector and withdraw the
RFI filter and plug element from the handle.
LF
FET
RESPONSE
FILTER
1.5 Vdc open
BALANCED
TRANSFORMER
CLASS A
COMPOUND
AMPLIFIER
BLOCK DIAGRAM
FIGURE 11
5.
Remove the low-frequency response switch knob by plac-
ing a small screwdriver or tweezers into the slot under the
ridge of the knob and lifting outward.
6.
Remove the small Phillips screw near the plug. Tap the car-
tridge end of the handle gently on a firm surface; the printed
circuit board and support assembly will slide out of the car-
tridoe end.
ARCHITECTS' SPECIFICATION
The microphone shall be a condenser microphone with a
frequency response of 20 to 20,000 Hz. It shall have a unidirec-
tional pickup characteristic, with cancellation at the sides of 6
dB and a minimum cancellation at the rear of 15 dB at 1 kHz.
The microphone shall have a rated output impedance of 150 Ω
for connection to microphone inputs of 150 ohms or higher. The
open circuit voltage shall be –65 dB (0.56 mV) (0 dB equals 1
volt per microbar).
The microphone shall contain a three-position low
frequency response switch and a lockable 10 dB attenuator
pad.
The overall dimensions shall be 212 mm (8-11/32 in.) in
length by 23.5 mm (15/16 in.) in diameter. The handle diameter
shall be 20.1 mm (25/32 in.). The weight shall be 230 grams (8
oz).
The microphone shall be capable of being powered by a
phantom power supply with an output of 11 to 52 Vdc, or by a
mixer, audio console or tape recorder capable of supplying 11 to
52 Vdc.
The microphone shall be a Shure Model SM81.
FURNISHED ACCESSORIES
Swivel Adapter
10 dB Attenuator Lock
Carrying/Storage Case
Windscreen
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OPTIONAL ACCESSORIES
Pop-Filter Grille
Heavy-Duty Windscreen
Tripod Microphone Stand (4.3 m [14 ftl)
Stereo Microphone Adapter
Cable (7.6m [25ft])
Phantom Power Supply
Omnidirectional Cartridge
REPLACEMENT PARTS
Cartridge and Grille Assembly
For additional service or parts information, please contact
Shure's Service department at 1-800-516-2525. Outside the
United States, please contact your authorized Shure Service
Center.
5
RFI
FILTER
REGULATOR/
REVERSE
VOLTAGE
PROTECT
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SERVICE STATEMENT
A57D
34A830
65A1797
RK311
A81G
A81WS
S15
A27M
C25F
PS1A
R104A
R104
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