Shure Audiomaster 1200 User Manual

Model 1200 Powermixer

Price $7.50

SERVICE MANUAL

ISSUE NO. 1

Manufactured by

SHURE BROTHERS INC.

222 Hartrey Avenue

Evanston, Illinois 60202-3696

Copyright 1988, Shure Brothers Inc.
AL954 (HE) 27A8187

Printed in U.S.A.

SPECIFICATIONS

Type

Mono Powermixer

Frequency Response

Flat +1, -3 dB, 40 Hz to 20 kHz (any input to any output)

Inputs

Six input channels: six unbalanced high- and/or bal­anced low-impedance inputs; channels 1 and 2 high­impedance inputs switchable to Aux level; available
expansion modules* each contain 2 high- and 2 low­impedance microphone inputs; two modules (4 chan­nels) can be added to each 1200

Power Output (1 kHz, 120 Vac, 1% THD)

200 watts minimum with 4 ohm speaker
120 watts minimum with 8 ohm speaker

Distortion

THD typically less than 0.15% at 1 kHz, 0.35% at 70 Hz

and 20 kHz (180 watts to 4 ohms, measured from low-

impedance input, Input Attenuation, individual channel

and Master Volume controls at typical user settings)

IM distortion typically less than 0.4% (116 watts to 4
ohms, Input Attenuation, individual channel and Master
Volume controls at typical user settings)

Low- and High-Frequency Equalization (Individual channel

and Monitor)

±10 dB at 100 Hz and 10 kHz

Input OVERLOAD Indicators

Illuminate 3 dB before clipping of input preamplifier or
equalizer stage

Power Amp Limiter

Threshold: 1.5 dB below speaker output clipping level

Input signal range: 15 dB beyond limiting threshold

Output PEAK/LIMIT Indicator

With Limiter Out: Lights 2 dB before clipping at speaker

outputs
With Limiter In: Lights 0.5 dB before onset of Limiter

action

Output NORMAL Indicator

Lights when speaker output level is greater than 1.4
Vrms (0.5 W into 4 ohms)

Input Sensitivity (full power output)

BAL LO Z: -66 dBV (0.5 mV)

HI Z: -44 dBV (6.3 mV)
AUX: -26 dBV (50 mV)

PA INPUT: 0 dBV (1.0 V)

Input Clipping Level

INPUT

CLIPPING LEVEL

ATTENUATION

BAL LO Z -33 to -3 dBV 0 to -30

(22 to 707 mV)

HI Z

-11 to +18 dBV 0 to -30

(281 mV to 8.41 V)

AUX +6 to +36 dBV 0 to -30

PA INPUT -1 dBV

2 to 63 V

--

(891 mV)

Voltage Gain (at 1 kHz)

INPUT OUTPUT

SPEAKERPGM MIXMONITORPHONES

LO Z

HI Z 73 dB

95 dB 65 dB

43 dB 49 dB

AUX 55 dB 25 dB

PA

29 dB - - 16 dB -

71 dB

31 dB

82 dB51 dB
60 dB29 dB

42 dB11 dB

TAPE

Output Clipping Level

OUTPUT
SPEAKER

MONITOR
PROGRAM MIX
PHONES

TAPE

MINIMUM CLIPPING LEVEL

+29 dBV (28 V)
+18 dBV (7.9 V)
+18 dBV (7.9 V)
+16 dBV (6.3 V)
+18 dBV (7.9 V)

Impedance

INPUTS
LO Z MIC

HI Z MIC

AUX

PA

ACTUAL

1 kΩ

130 kΩ

50 kΩ
50 kΩ

FOR USE WITH
75 to 600 Ω

100 kΩ or less
10 kΩ or less
10 kΩ or less

OUTPUTS

MONITOR
PROGRAM MIX

TAPE

PHONES
SPEAKER

2.4 kΩ 2 kΩ or more

2.4 kΩ 2 kΩ or more

2.4 kΩ
430 Ω 4 Ω or more

-

2 kΩ or more
4 Ω or more

Hum and Noise

Less than -126 dBV equivalent input hum and noise

(20 Hz to 20 kHz, LO Z input to SPEAKER output)

Noise

Less than -127 dBV equivalent input noise (300 Hz to

20 kHz, LO Z input to SPEAKER output)

Signal-to-Noise Ratio

Greater than 80 dB (below full output) at typical control

settings

Phantom Power

24 Vdc ± 10% open circuit, 1.8 k series resistance

Power Requirements

120 Vac ± 10%, 50/60 Hz, 420 watts typical with no ac

receptacle load

AC Convenience Receptacle

120 Vac, 100 watts maximum

Certifications

Listed by Underwriters Laboratories Inc. and by

Canadian Standards Association as certified

Environmental Conditions

Operating Temperature: -7 to 43°C (20 to 110°F)
Storage Temperature: -40 to 74°C (-40 to 165°F)

Relative Humidity (Operating and Storage): 0 to 95%

Overall Dimensions

191 mm H x 483 mm W x 343 mm D (7-½ x 19 x 13-½ in.)

Weight

12.3 kg (27 lb)

18.2 kg (40 lb) in A1200C Carrying Case

RECOMMENDED LOUDSPEAKERS

AUDIOMASTER Model 3200, 100 W, 8 Ω, vented two-way
speakers

OPTIONAL ACCESSORIES

Carrying Case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A1200C

Input Expansion Module*

(2 channels, 2 each HI Z and LO Z inputs). . A1200MX

Speaker Cable, 15 m (50 ft) with phone plugs

Line Matching Transformer (LO Z to HI Z)

. . . . . .

. . . . . . . . .

A50SC
A95UF

240-V Conversion Kit* . . . . . . . . . . . . . . . . . . . . . . . . RKC209

Phono Preamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M64A

*For installation by qualified service personnel only.

MODEL 1200 SERVICE MANUAL

TABLE OF CONTENTS

Description
For Years of Dependable Service

Troubleshooting ..................................

Service Instructions

External Parts

Diagnosis .....................................

External Fuse Replacement
Service Access
General Service Procedure
AC Voltage Measurements
DC Voltage Measurements
Resistance Measurements
Chassis-Mounted Boards and Components

A1 Reverb Pan
SHR-11 (and SHR12) Power Amplifier

SHR-12 Temperature Sensor Board
SHR-14 Main Board

Rear-Panel Boards and Components

SHR-10 Input-Output Board
SHR-03 Aux/Mic Board
SHR-02 Hi-Z Input Board

SHR-01 Lo-Z Input Board.......................

SHR-13 Speaker Output Board
B1 Fan
S3 Phantom Power Switch

Front-Panel Boards and Components

SHR-04 Channel Board (6)......................

Front Mounting Panel
SHR-05 Master Control Board
SHR-09 Limiter Board
SHR-06 Headphones Output Board and

SHR-08 Indicators Board

S2 On/Off Switch
A1200MX Input Expansion Module
Checking Active Components
Ferrite Beads
Service Illustrations
Optional Accessories

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Board Assembly

SHR-11 Potentiometer Adjust

VR1101 Output Voltage Offset
VR1102 Driver Balance

VR1103 Idle Current
VR1201 Temperature Sensor Threshold
VR1401 Power Amplifier Limiter Threshold

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SHR-07 Headphones Control Board

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DESCRIPTION

The Shure Model 1200 AUDIOMASTER® Powermixer is a

compact, flexible microphone mixer with six inputs for high- or

low-impedance sources and a 200-watt power amplifier. The

1200 is human-engineered for ease of setup and operation.

Connection of microphones, speakers and other equipment

(tape recorder, equalizer, effects device, remote amp, etc.) is

fast and convenient.

The 1200 is rack-mountable or portable. Its 6 inputs are

expandable to 8 or 10 inputs through the use of A1200MX

Expansion Kits. Inputs accept both high- and low-impedance
microphones (dynamic, ribbon or condenser), as well as ampli-

fied instruments or other high-level sources. Individual channel

input attenuators with overload indicators permit precise gain

control to prevent input overload. The 1200 has built-in rever-

beration with individual channel and overall level adjustments.

Other effects devices or an equalizer can be connected through
a post-master control program loop. An integral power amp
limiter prevents overload distortion over a wide signal input
range. Pre-volume monitor send controls and pre-master vol­ume tape output provide maximum versatility in stage or record­ing applications. The control panel has color-coded knobs for
ease of adjustments, while LED indicators give instant notice of
operating status.

In addition to the A1200MX Expansion Module, the following
accessories are available for the 1200: A1200C Carrying Case,
A95UF Line Matching Transformer, RKC209 240 V Conversion
Kit, and M64A Phono Preamplifier.

The 1200 is Listed by Underwriters Laboratories, Inc., and

listed by Canadian Standards Association as certified.

1

FOR YEARS OF DEPENDABLE OPERATION...

The AUDIOMASTER® System is exceptionally well designed,
with all components of the highest quality, operating well within
their respective ratings to assure long life. The following list of
Do’s and Don’ts describes minimal operating precautions and
maintenance to provide years of dependable service.

DO

use a 16 AWG or larger, heavy-duty, 3-conductor extension

cord when additional line cord length is needed.
DON’T replace the rear-panel fuse with a different size or type.

Use only 5 A, 250 V, Type 3AG.
DON’T operate the Powermixer with its air vents blocked; don’t

place it on a radiator or heat-producing equipment. Avoid oper­ation in direct, hot sunlight.

DO unplug the Powermixer before cleaning. Use only mild
detergent and a damp (never wet) cloth to clean the outer

SYMPTOM

Powermixer is “dead” (no output, POWER LED off)

OVERLOAD LED illuminates, either

A.

Flickering with program material, indicating abnormal
speaker load

B.

Continuously to indicate power amplifier shutdown

No signal at speaker (all other functions appear normal), either

A. NORMAL LED illuminated, or
B. NORMAL LED not illuminated

Fuse blown

One of two inputs on same channel not working properly (both

1/4-inch and 3-pin jacks in use)

surfaces; Don’t use strong solvents or cleaning fluids.
DON't use unbalanced low-impedance microphones with the

PHANTOM 24V switch on; turn off the switch if not required for
powering condenser microphones. If phantom power is in use,
connect unbalanced low-impedance microphones through a
line matching transformer (Shure A95UF) to a HI Z input. Phan­tom power will not affect balanced low-impedance dynamic
microphones.

DON’T risk fire or shock hazard by operating the Model 1200
in the rain.

TROUBLESHOOTlNG

Should any difficulty be encountered in operation of your AUDIO­MASTER System, the trouble can often be traced to some
simple source. The following is offered as a basic guide to
solving this kind of problem.

PROBABLE CAUSE AND CORRECTION

1.

Check that ac power source is “live” and Model 1200 is
plugged in.

2. Check that POWER switch is on.

3.

Check that rear panel fuse (5 A, 250 V) is good.

4.

If the 1200 has turned off due to excessive internal tempera­tures, allow at least an hour for cooling, and reactivate unit.
If difficulty persists, refer to General Service Procedure in
this manual.

1.

Check total speaker load (must be no lower than 4 ohms)
and for possible shorted speaker cable.

1.

Check air vents for blockage.

2.

Check that internal fan is operating (fan located on rear

panel).

3.

Turn MASTER volume control down for several minutes to
allow proper cooling. If difficulty persists, refer to General
Service Procedure.

1.

Check for defective or improperly connected speaker cables.

1. Check settings of channel VOLUME and MASTER VOLUME
controls.

2. Check connections to EXTERNAL DEVICE LOOP

1. Replace with identical fuse (5 A, 250 V, 3AG).

2.

If second fuse blows, refer to General Service Procedure.

1.

Make sure similar microphones are used on both inputs,
and microphone impedances match the inputs used.

2.

Make sure a low-impedance microphone is not used with
aux level equipment on other input.

3.

Make sure MIC/AUX switch is in MIC position (Ch. 1 and 2

only).

4.

Make sure microphone switches are on; check microphone

cables.

2

SYMPTOM

INPUT OVERLOAD LED flashing

NOTE: Occasional flashing is acceptable

Loud clicks when certain microphones or cables are used

No MONITOR output (program output normal)

Sound quality poor (weak or thin)

SERVICE INSTRUCTIONS

PROBABLE CAUSE AND CORRECTION

1.

Adjust INPUT ATTENUATION counterclockwise to reduce
channel input level.

2.

Reduce input signal level at source.

1. Make sure PHANTOM 24V switch is not on (when not needed).

2. Make sure unbalanced low-impedance microphone or cable

is not used when PHANTOM switch is on.

3.

Check for defective microphone cables.

1.

Make sure channel MONITOR and MONITOR MASTER

controls are turned up.

2.

Make sure controls on external equipment connected to
Monitor output jack are turned up.

3.

Check for defective cable from MONITOR output jack.

1.

Check for excessive treble boost or bass cut on equalization
controls.

2.

Check PROGRAM and MONITOR output jacks for signal

quality (if acceptable, problem is in power amplifier section).

3.

Check for defective cables.

4.

Check impedance match (high- or low-impedance mics)

at input.

EXTERNAL PARTS

The knobs and feet can be replaced without disassembling

the 1200. All knobs are of the pull-off type, and are color-coded

by function. The Equalization knobs are dual concentric types;
all other knobs are single.

DIAGNOSIS

A simple method of localizing problems without opening the
1200 is as follows. Turn off the Power switch. Do not connect a
speaker or headphone load. Set all controls of the channel
under test only to full clockwise (other channel volume and
monitor controls full counterclockwise), except: Reverb Return
full counterclockwise, and Eq controls centered. Turn the unit
on and apply a 0.5 mV, 1 kHz test signal across pins 2 and 3 of
one of the Bal Lo Z input connectors. Normal voltmeter readings

taken at each output connector are given in Table 1. Similarly, a

5 mV signal inserted in a HI Z input, 50 mV in an AUX input, or
891 mV in the Power Amp and Monitor Inputs will yield similar
output voltages. This method is useful for saving time in localiz­ing problem areas. Internal servicing should be performed only
by qualified service personnel.

TABLE 1. NOMINAL TEST VOLTAGES

OUTPUTS

INPUT

LO z 0.5 mV 28 V
HI Z
AUX
PA

Input

50 mV

891 mV 25 V

Spkr Prgm Mix

890 mV

5 mV 22 V 708 mV

28 V 890 mV 1.78 V 6.31 V

Mon Phones

1.78 V

1.41 V

--- ---

Tape

6.31 V

5.01 V 141 mV

5.62 V

178 mV

178 mV

---

WARNING

Voltages in this equipment are hazardous to life. Refer

servicing to qualified service personnel.

EXTERNAL FUSE REPLACEMENT

To replace line fuse F1 (with no apparent problems in the
Powermixer), disconnect the line cord from the ac source and
remove the rear panel fuseholder cap by pushing it in and

turning it counterclockwise (either a screwdriver or fingertip

may be used.) Replace the defective fuse only with a 5A 250V
Normal Blow type.

CAUTION

If trouble symptoms (overheating, erratic operation, etc.)
were apparent before the fuse blew, or if the replacement
fuse blows, a qualified service person should trouble-

shoot the Powermixer carefully to find the source of the

problem. Do not continue to replace fuses; have the
trouble corrected.

SERVICE ACCESS

Disconnect the 1200 from its power source. To open the unit
for servicing, remove ten screws and lockwashers from the top
and five from each side of the cover. The cover can then be

lifted off exposing the internal parts (see Figure 1).

3

PRINTED CIRCUIT BOARD AND PARTS LOCATION

FIGURE 1

After replacing high-voltage parts such as the fan, power

WARNING

cord, rear panel convenience receptacle, or printed wir­ing assemblies SHR-11, SHR-12, or SHR-14 (or their
components), make certain that a dielectric voltage
breakdown test is performed before connecting the unit
to a power supply. With the unit disconnected from the ac
source, turn the unit on and measure the resistance
between the ground contact of the power plug and each
of the other two prongs. This resistance should measure
infinite. Then measure the resistance between the plug
ground and exposed metal parts of the chassis, screws,
connectors, fan grille, etc. This resistance should be zero
ohms. If any measurements are recorded outside the
specified limits, a shock hazard exists. The unit must be
repaired and rechecked before it is returned to operation.

IMPORTANT: When removing any printed wiring board, be sure
the wires and connectors detached from the board are identified
for proper reconnection. This may be done by affixing a piece
of masking tape marked with the reference designation
(connectors) or terminal letter or color (wires).
NOTE: Any cable ties that were loosened or removed must be
tightened or replaced after servicing.

Similar wire colors and connectors with the same number

CAUTION

of pins are used in different circuits; make sure proper

reconnections are made. Push-on connectors must be

removed by pulling straight out. The side of a small screw­driver blade inserted between the free and fixed connector
can sometimes be helpful in separating the connector.

Do not apply side force when removing or reconnecting
terminals, or damage may result. Reference to the service

illustrations will be helpful in case of difficulty.

GENERAL SERVICE PROCEDURE

To isolate a problem area, a general procedure is as follows. If
the Powermixer is completely dead, check the ac power source,
line cord, and fuses (external and internal). If the unit turns on
but operation is abnormal, check the chassis power supply
output (-56 Vdc between test point SHR-11-AB and ground,
and +56 Vdc between test point SHR-11-AA and ground; -15

Vdc across C1421, +15 Vdc across C1420, and +24 Vdc

between P1404-A and ground). If the measured dc voltages are
correct, perform additional AC and DC Voltage Measurements
as described below to isolate the problem.

4

AC VOLTAGE MEASUREMENTS

Numbers within rectangular symbols on the circuit diagrams
denote ac voltages at that point under the following test
conditions:

1.

Voltages measured with respect to chassis unless other­wise indicated.

2. Line voltage: 120 V, 60 Hz.

3.

Test signal of 0.5 mV across pins 2 and 3 of connector
J101.

4. AC voltage measurements may vary ±20% from values
shown.

5.

Measurements made with ac voltmeter of 1 megohm or
greater input impedance.

6.

No load on Speaker Outputs.

7.

All controls (of channel under test only) full clockwise

except Equalization controls centered.

8. Reverb Return full counterclockwise.

9. Limiter switch and Phantom switch in Off position.

10.

Monitor Master control clockwise.

DC VOLTAGE MEASUREMENTS

Numbers within elliptical symbols on the circuit diagrams
denote dc voltages at that point under the following test
conditions:

1.

Voltages measured with respect to chassis unless other-

wise indicated.

2. Line voltage: 120 V, 60 Hz.

3.

No input signal applied.

4.

Dc voltage measurements may vary ±20% from values
shown.

5.

Measurements made with dc voltmeter of 11 megohms or
greater input impedance.

6.

Phantom switch in On position.

RESISTANCE MEASUREMENTS

With the ac line cord disconnected from the ac source and
the Power On/Off switch off, the following ohmmeter measure-

ments may be made.

1.

Reverb pan output coil: approximately 360 ohms; input

coils approximately 40 ohms.

2.

Transformers may be checked for continuity of each winding.

3. To test transistors, ICs and diodes, refer to the section on
CHECKING ACTIVE COMPONENTS.

To replace the reverb pan, connect the white lead connector
to the reverb Input and the orange lead connector to the reverb
Output. Use the previously removed screws to secure the reverb
pan to the reverb shield, and to secure the shield to the chassis
bottom and the parallel vertical interior shield. Be sure to replace
any cable ties that were removed or loosened.

SHR-11 AND SHR-12 POWER AMP BOARD ASSEMBLY

The Power Amplifier Board Assembly consists of SHR-11
and SHR-12 mounted on the large aluminum heat sink. To
separate the heat sink and its associated boards from the 1200,
remove the screws holding the sink to the power transformer
shield bracket, and those holding the sink to the metal shield

that divides the fan from the input and output connectors and

boards. Disconnect the connectors from P1101, P1102 and
P1201, and unsolder the red, green, and white leads from AA,

AB and AC, respectively, on SHR-11. (The opposite ends of

leads AA and AB are connected to SHR-14, Main Board; the
opposite end of lead AC is connected to SHR-13, Speaker
Output Board.) The Power Amp Board is now free of the 1200.

To replace parts on SHR-11, it is then necessary to remove
SHR-11 from the large heat sink. Do this by removing the four
Phillips screws holding the power transistors at the corners of

the board, and the two Phillips screws holding the board on the

brass standoffs.

NOTES:

1.

Removal of SHR-11 from the heat sink may damage the
insulators below the power transistors. Therefore, when
servicing SHR-11, be prepared to replace all four 802639FK
insulators. If either predriver transistor Q1103 or Q1104
(mounted on the vertical heat sink fastened to SHR-11)
is replaced, be prepared to replace insulator 802681FK
beneath it.

2.

If transistor Q1105, mounted on the solder side of board
SHR-11, is replaced or if the board has been removed from
the heat sink, make certain that Q1105 physically contacts
the large heat sink after reassembly of the board to the heat
sink is completed. Apply a small amount of Wakefield Type

120 Thermal Joint Compound to the top surface of the
transistor in order to ensure proper heat transfer.

CHASSIS-MOUNTED BOARDS AND COMPONENTS

The following printed wiring boards and components are

chassis-mounted.

SHR-11
SHR-12

SHR-14
A1
T1

POWER BOARD
SENSOR BOARD
MAIN BOARD
REVERB PAN
POWER TRANSFORMER

REVERB PAN

To remove the reverb pan assembly, turn the 1200 on its side

(power transformer down) and remove the three screws holding
the reverb shield to the chassis bottom. Turn the 1200 upright
and remove the screw holding the reverb shield to the standoff
attached to the parallel vertical interior shield. Disconnect the

phono connectors at the bottom of the reverb pan assembly.

Remove the four screws holding the reverb pan to the vertical

metal interior shield, and lift the reverb shield and pan assembly
out of the chassis holding the pan to the shield. (If necessary to
free the reverb shield, remove the cable tie(s) holding the
orange and white leads to the metal shield.)

To replace SHR-11 in the 1200, solder the red, green and
white leads to AA, AB and AC. Connect the cable assembly with
three leads from P1405, SHR-14, to P1101 ; and connect the
cable with a single lead from P1407, SHR-14, to P1102.

SHR-11 POTENTIOMETER ADJUSTMENT

If any components are replaced on SHR-11, trimpots VR1101,
VR1102 and VR1103 may require readjustment. Measure the
SHR-11 voltages described in the following paragraphs. If

measured voltages are not correct, make adjustments as follows.

With power off, no input signal applied, and no load on the

1200’s speaker output, turn VR1103 completely clockwise. Turn
power on and allow unit to warm up or thermally stabilize for ten
minutes prior to making the adjustments. See Figure 2 for
trimpot locations.

5