Mackie FR Series M.1400 User manual

M•1400 / M•1400i / M•1200
SERVICE MANUAL
1998MACKIEDESIGNS, INC.
820-179-00
Version1.2

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®
TABLE OF CONTENTS
INTRODUCTION............................................................. 2
SYSTEM OVERVIEW.......................................................3
SUMMARY OF FEATURES .....................................................................4
SPECIFICATIONS...................................................................................6
CIRCUIT THEORY........................................................... 8
INPUT CIRCUITRY..................................................................................8
POWER AMPLIFIER CIRCUITRY...........................................................8
PROTECTION CIRCUITS.......................................................................9
THERMAL MANAGEMENT..................................................................10
SERVICE PROCEDURES...............................................11
REQUIRED EQUIPMENT LIST..............................................................11
TROUBLESHOOTING TIPS - OUTPUT FAILURES................................12
TEST AND BIASING PROCEDURE......................................................13
PARTS............................................................................15
QUICK PARTS......................................................................................15
CHASSIS ASSEMBLY............................................................................16
HEATSINK ASSEMBLY..........................................................................18
COMPLETE PARTS LIST........................................................................20
SCHEMATICS / PCB LAYOUTS....................................23
INPUT PCB SCHEMATIC.....................................................................23
INPUT PCB LAYOUT............................................................................24
DISPLAY PCB SCHEMATIC................................................................25
DISPLAY PCB LAYOUT........................................................................26
MAIN PCB SCHEMATICS...................................................................27
MAIN PCB LAYOUT ............................................................................30
SOFT START PCB SCHEMATIC...........................................................31
SOFT START PCB LAYOUT...................................................................32

M•1200/M•1400 SERVICE MANUAL
INTRODUCTION
2
INTRODUCTION
This manual contains complete service information for the M•1200, M•1400 and M•1400i
audio power amplifiers. Operating instructions will be touched on briefly. For complete
operating instructions refer to the owner’s manual.
The FR SeriesM•1200 and M•1400 are high power audio amplifiers designed for
professional applications. They feature audiophile quality sound, excellent stability,
extensive protection circuitry, and legendary Mackie reliability. Patented Fast Recovery
circuitry allows the M•1200/M•1400 to recover from clipping without generating
undesirable artifacts.
To service the M•1200/M•1400, technicians should be familiar with op-amps, discrete
analog circuitry, and troubleshooting high power solid state amplifiers. Presentation of
this manual does not constitute endorsement of qualifications by Mackie Designs.
TECHNICAL SUPPORT
Mackie Designs Technical Support Department is available 8AM - 5PM PST, Monday -
Friday at 1-800-258-6883 (outside US call 011-425-487-4333) Please feel free to call with
any questions (Better safe than sorry!).
DISCLAIMER
The information contained in this manual is proprietary to Mackie Designs, Inc. The
entire manual is protected under copyright and may not be reproduced by any means
without express written permission from Mackie Designs, Inc.
SERVICE ON THE M•1200/M•1400 IS
TO BE PERFORMED BY EXPERIENCED
TECHNICIANS ONLY!
! SMD !
The M•1200/M•1400 makes extensive
use of surface mount components.
Servicing technicians should have the
tools and experience to perform
surface mount rework.

VERSION 1.1
SYSTEM OVERVIEW 3
SYSTEM OVERVIEW
This section provides a quick summary of the M•1200/M•1400’s major features. It is not
intended to take the place of the owner’s manual. It is included here to help servicing
technicians familiarize themselves with the M•1200/M•1400.
DIFFERENCES BETWEEN THE M•1400, M•1400i AND M•1200
The M•1200 and M•1400 are electronically identical products with the following
exceptions:
1. The M•1400 has 12,000uF main filter caps, where the M•1200 and M•1400i have
10,000uF.
2. The M•1400 and M•1400i use a more powerful fan than the M•1200.
3. The M•1400 has Speakonconnectors where the M•1200 and M•1400i have ¼”
speaker outputs (both in addition to binding posts).
The M•1200 was discontinued in 1998 and replaced by the M•1400i.
Export M•1400’s also have a “Soft Start” circuit that is not installed on American units.
FAST RECOVERY DESIGN
Solid state power amplifiers have a reputation for sounding harsh when driven into
clipping. The usual explanation is odd harmonics generated by hard clipping, but In
fact much of the unpleasant sound is caused by latching and parasitic oscillations that
occur as the amplifier recovers from clipping.
TYPICAL AMPLIFIER MACKIE FR SERIES
ARTIFACTSCAUSEDBY
LATCHINGANDPARASITIC
OSCILLATION
Mackie’s Fast Recovery design recovers from clipping quickly without generating any
undesirable artifacts. This performance is achieved by two unique design elements; the
Full Symmetry Dual Differential front end provides excellent linearity while minimizing the
need for negative feedback, and the Baker Clamp prevents output/driver device
saturation. How these circuits operate is covered in depth in the circuit theory section.

M•1200/M•1400 SERVICE MANUAL
SUMMARY OF FEATURES
4
SUMMARY OF FEATURES
FRONT PANEL
1. GAIN
Detented gain controls adjust the level sent to each output section. The gain structure
is designed so that a +4dBu (1.23V rms) input signal drives the amplifier to full rated
power into 4Ω.
2. METERS
The M•1200/M•1400’s meters indicate the relative output level of the amplifier
referenced to full power. The SIG (signal present) senses the input prior to the gain
control.
3. PROTECT
The Protect LEDs light indicating the channel has been muted for one of the following
reasons:
•4 second turn on delay
•Output devices beyond Safe Operating Area (SOA), also lights SHORT LED(s).
•Heatsink temperature beyond 80 degrees Celsius.
4. SHORT
The Short LEDs light to show that the amplifier has engaged protection due to a short or
low impedance on the output.
5. TEMP STATUS
The HOT LED lights to indicate that the amplifier has engaged protection due to
heatsink temperature or power transformer core temperature.
6. POWER SWITCH
This remarkable device requires years of study to full comprehend.
OL
-3
-6
-2
0
-3
-6
-9
-2
0
SIG
O
L
SIG
CH
1
30
28
26
20 2218
2
4
1
6
1
4
8
1.23v(+4dBu)
SENSITIVITY
GAIN/dBCH
2
PROTECT
COL
DHOT
SHORT
TEMPSTATUS
INTERNALSTATUS
CH
1
CH
2
CH
1&2
ON
OFF
POWER
3v
2v
1v3
0
2
8
2
6
2
02
2
1
82416
14
8
1.23v(+4dBu)
SENSITIVIT
Y
GAIN/dB
3
v
2
v
1v
PROFESSIONALPOWERAMPLIFIERFULL SYMMETRYDUALDIFFERENTIALHIGHCURRENTDESIGN
-9
1 12 3 4 5 6

VERSION 1.1
SYSTEM OVERVIEW 5
REAR PANEL
1. SPEAKER OUTPUTS
The M•1400 has Speakonconnectors where the M•1200 has ¼” speaker outputs. Both
models have binding posts. For bridged mono operation, the + output comes from
channel one’s positive terminal and - from channel two’s positive.
2. INPUT
The M•1200/M•1400 has both XLR and ¼” TRS inputs. They are electrically identical
(wired in parallel). In bridged mono or parallel mono mode channel 2 input is disabled.
3. THROUGH
Yet another jack wired in parallel with the Inputs, the through jack provides a
convenient way to chain amplifiers or pass the signal to any other device.
4. LOW CUT FILTER
Sweeps the built in high pass filter from 10Hz to 170Hz.
5. CONSTANT DIRECTIVITY HORN EQ/AIR EQ
Built in shelving EQ provides high frequency compensation when using constant
directivity horns.
6. AMP MODE
Three position slide switch selects STEREO, PARALLEL MONO or BRIDGE MONO operation.
Channel One’s input signal is used for PARALLEL MONO and BRIDGE MONO operation.
AMP MODE and OUTPUT APPLICATION switches should not be changed while the unit is
on.
7. OUTPUT APPLICATION
Placing the three position slide switch in its center position defeats the built in LIMITER for
both channels. Selecting SUB WOOFER engages the built in low pass filter. The
FREQUENCY switch selects between 125Hz and 63Hz cutoff for the low pass filter. In SUB
WOOFER mode, inputs 1 and 2 are summed regardless of AMP MODE setting.
(Remember, human hearing is unable to discern direction at frequencies where the
wavelenth is greater than the distance between your ears, ~250Hz unless you have a
grossly large head!)
CHANNEL
21
CHANNEL
CH
1
CH
2
SPEAKEROUTPUTS
+
--
+
63Hz
125Hz
CONSTANTDIRECTIVITY
HORNEQ/AIREQCONSTANTDIRECTIVITY HORNEQ/AIREQ
INPUT
THRUTHRU
INPUTAMPMODE
FREQUENCY
4.5 k HzON
6kHz2kHz
MONOBRIDGE
LIMITER
( CH1&CH2)
OFF
FULL
RANGESUB
WOOFER
LOWCUT
FILTERLOWCUT
FILTER
OUTPUTAPPLICATION
CAUTION
BALANCED
O
R
UNBALANCED
BALANCED
O
RUNBALANCED
AIREQ
MONO
BRIDGE
ON
/BRIDGE/MONO
170HzOFF
4.5kHz
6kHz2kHzAIREQ 120VAC50/60Hz
1500WATTS
+
-
170Hz
100Hz
OFF
STAGE
MONITOR
100Hz
STAGE
MONITOR
OFF
TYPICAL
ON
OFF
TYPICAL
TYPICAL
35HzTYPICAL
35Hz
(MONOBRIDGE)
1200W A TTS
4OHMLOADMIN.
600 WATTSCH
2OHMSLOADMIN.
STEREO
TYPICAL
STEREO
TYPICAL
AIREQAIREQ
1 5
4 62
3 7
2
53
4

M•1200/M•1400 SERVICE MANUAL
SPECIFICATIONS
6
SPECIFICATIONS
CONTINUOUS AVERAGE OUTPUT
POWER, BOTH CHANNELS DRIVEN:
M•1200
225 watts per channel into 8 ohms from 20Hz to
20kHz, with no more than 0.025% THD
400 watts per channel into 4 ohms from 20Hz to
20kHz, with no more than 0.050% THD
600 watts per channel into 2 ohms from 20Hz to
20kHz, with no more than 0.095% THD
Bridged mono operation:
800 watts into 8 ohms from 20Hz to 20kHz, with
no more than 0.050% THD
1200 watts into 4 ohms from 20Hz to 20kHz, with
no more than 0.095% THD
Maximum Power at 1% THD:
250 watts per channel into 8 ohms
425 watts per channel into 4 ohms
640 watts per channel into 2 ohms
850 watts into 8 ohms bridged
1280 watts into 4 ohms bridged
M•1400 / M•1400i
250 watts per channel into 8 ohms from 20Hz to
20kHz, with no more than 0.012% THD
425 watts per channel into 4 ohms from 20Hz to
20kHz, with no more than 0.025% THD
630 watts per channel into 2 ohms from 20Hz to
20kHz, with no more than 0.050% THD
Bridged mono operation:
850 watts into 8 ohms from 20Hz to 20kHz, with
no more than 0.025% THD
1260 watts into 4 ohms from 20Hz to 20kHz, with
no more than 0.050% THD
Maximum Power at 1% THD:
300 watts per channel into 8 ohms
500 watts per channel into 4 ohms
700 watts per channel into 2 ohms
1000 watts into 8 ohms bridged
1400 watts into 4 ohms bridged
Note: Power ratings are specified at
120VAC (U.S. and Canada) and 240VAC
(Export) line voltages.
The M•1200/M•1400 power amplifiers draw
large amounts of current from the AC line with
continuous sine wave testing. Accurate
measurement of power requires a steady and
stable AC supply. This means the line
impedance must be very low to insure that the
peak AC line voltage does not sag to less than
97% of its value.
If driving highly reactive loads, we recommend
that the limiter circuit be engaged.
POWER BANDWIDTH:
20Hz to 70kHz (+0, –3 dB)
FREQUENCY RESPONSE:
20Hz to 40kHz (+0, –1 dB)
10Hz to 70kHz (+0, –3 dB)
DISTORTION:
THD, SMPTE IMD, TIM < 0.025% @ 8Ω
< 0.050% @ 4Ω
< 0.150% @ 2Ω
SIGNAL-TO-NOISE RATIO:
> 107 dB below rated power into 4 ohms
CHANNEL SEPARATION:
> 80 dB @ 1kHz
DAMPING FACTOR:
350 minimum
INPUT IMPEDANCE:
20k  balanced bridging
INPUT SENSITIVITY:
1.23 volts (+4 dBu) for rated power into 4 ohms
GAIN:
30.25 dB (32.5V/V)
MAXIMUM INPUT LEVEL:
9.75 volts (+22 dBu)

VERSION 1.1
SPECIFICATIONS 7
RISE TIME:
< 4.4µs
SLEW RATE:
Voltage Slew Rate > 50V/µs
> 100V/µs bridged
Current Slew Rate > 32A/µs at 2Ω
CMRR:
> 40 dB, 20Hz to 20kHz
LOAD ANGLE:
8(±jx) time independent at 8Ω
4(±jx) time dependent, T > 6 min. at 4Ω
2(1±jx) time dependent, T > 2 min. at 2Ω
TRANSIENT RECOVERY:
< 1µs for 20 dB overdrive @ 1kHz
HIGH FREQUENCY OVERLOAD AND
LATCHING:
No latch up at any frequency or level.
HIGH FREQUENCY STABILITY:
Unconditionally stable driving any reactive or
capacitive load.
TURN ON DELAY:
3 seconds
VARIABLE LOW-CUT FILTER:
10Hz (Off) to 170Hz, 2nd Order Bessel
SUBWOOFER LOW-PASS FILTER:
Switched: 63Hz/125Hz, 3rd Order Bessel
CONSTANT DIRECTIVITY HIGH
FREQUENCY BOOST:
2kHz to 6kHz (+3 dB points)
6 dB/octave high-frequency shelving filter,
(shelving occurs at approximately 30kHz)
LIMITER SECTION:
Complementary Positive and Negative Peak
Detecting
INDICATORS:
6 meter LEDs per channel
SIG (Signal Present), –20, –9, –6, –3, OL
(Overload)
CH 1 & 2
PROTECT LEDs
SHORT LEDs
TEMP STATUS
COLD/HOT LEDs
POWER CONSUMPTION:
65 watts at idle
900 watts with musical program fully loaded
(2 ohms per side, or 4 ohms bridged)
550 watts with musical program fully loaded
(4 ohms per side, or 8 ohms bridged)
850 watts at full power into 8 ohms
(continuous sine wave)
1500 watts at full power into 4 ohms
(continuous sine wave)
2500 watts at full power into 2 ohms
(continuous sine wave)
AC LINE POWER:
US 120VAC, 60Hz
Europe 240VAC, 50/60Hz
Japan 100VAC, 50/60Hz
Korea 220VAC, 60Hz
AC DROP-OUT VOLTAGE:
At approximately 50% of rated line voltage
PHYSICAL:
Height 3.5 inches (89mm)
Width 19.0 inches (483mm)
Depth 15.25 inches (387mm)
Overall Depth 16.25 inches (413mm)
Handle Depth 1.25 inches (32mm)
Weight 36 pounds (16.3kg)
Since we are always striving to make our stuff
better at Mackie Designs by incorporating new
and improved materials, components, and
manufacturing methods, we reserve the right to
change these specifications at any time
without notice.

M•1200/M•1400 SERVICE MANUAL
CIRCUIT THEORY
8
CIRCUIT THEORY
Much of the circuitry in the M•1200/M•1400 is self explanatory from the schematic. This
section will explain the unique circuits and architecture. Samples in this section will refer
to Channel 1 (Left) for circuitry that is identical on both channels.
INPUT CIRCUITRY
Refer to schematic on page 23.
The signal path begins with the INPUT BOARD. Following Channel 1’s input, signal is fed
to a unity gain differential op-amp, U1A. The signal is next sent to U1B which serves as
both HPF and CD horn EQ. If not in SUBWOOFER mode, the summing amp (U2B) and
LPF’s (U3A, U3B) are bypassed. The signal is sent via J11 to the gain control on the
DISPLAY BOARD. After the gain control, the signal is buffered by U7A, returned to the
INPUT BOARD and routed to the MAIN BOARD via J16.
Channel 2’s input signal path is electrically identical to Channel 1’s in STEREO mode. In
PARALLEL MONO mode Channel 2’s input signal is ignored and Channel 1’s input is sent
to Channel 2’s gain control via SW5A. For BRIDGE MONO operation, Channel 1’s input
signal is inverted by U2A and routed to the MAIN BOARD via SW5C.
POWER AMPLIFIER CIRCUITRY
Refer to schematic on page 27.
The M•1200/M•1400 use a class AB triple darlington output with complementary output
devices. The main power supply is +/- 80V, there is also a +/- 90V supply for the front
end circuitry. Channels are muted for 4 seconds on power up as C53 charges via R203.
Muting is achieved by turning off Q73, removing current sources for the differential pairs.
(Wait a minute! PAIRS? Read on…)
An immediately obvious departure from standard designs is the Dual Differentials and
Symmetrical Voltage Amps. The reasoning behind this front end architecture is
actually quite simple. Transistors transfer characteristics are not entirely linear, so even
the best conventional front end design will introduce some distortion. Most amplifiers
use negative feedback to reduce this problem (creating a few more in the process).
Mackie FR Series amplifiers take a different approach. By using two complementary
“mirror image” front end circuits any distortion caused by non-linear transistor curves is
effectively canceled out in the bias string, without feedback!
Another design feature unique to Mackie FR Series Amplifiers is the Baker Clamp. The
Baker Clamp has two functions; 1) Prevent output/driver transistors saturating, 2) Drive
the LIMITER LDR. Referring to the left main amp schematic, Q27 is a common base
amplifier, it will turn on if Q31’s collector rises more than two diode drops above the
+80V supply, preventing it from rising further. There are three diode drops between
Q31’s collector and the base of driver transistor Q28 (D3, D2, Q29Vbe). Q29 will never
see a voltage greater than one diode drop below the +80V supply, even if the +80V
supply fluctuates. Q27 also drives the limiter LDR LED U2A. The negative side has the
same circuit mirrored.

VERSION 1.1
CIRCUIT THEORY 9
PROTECTION CIRCUITS
Refer to schematics on pages 27-29.
The M•1200/M•1400 has two output protection circuits in addition to the rail fuses. The
first mutes the amp if the output devices are loaded beyond their safe operating area
(SOA). The second is a crowbar circuit that ensures the rail fuses blow if there is DC on
the output.
SOA PROTECTION
The SOA protection circuit senses the voltage drop across the emitter resistors much like
standard VI current limiting transistors. Q76 and Q77 will turn on if the emitter drop
exceeds ~1V. If they are on for long enough to charge C38 (through R159), the SOA
protection circuit is activated by transistor switches Q79 and Q78.
If SOA detection is tripped, the channel is muted by discharging C53 through R200 and
D65, fan speed is increased, and the “SHORT” LED lights.
DC CROWBARS
The DC Crowbar circuit protects speakers in the event of an output failure by blowing
all rail fuses. SCRs Q21-24 are used to short the +/- 80V supply supplies to ground.
Output signal from both sides is fed through 100K resistors R90, R89 to C21, essentially
forming a LPF. DC on either output will trigger comparator U4A (positive) or U4B
(negative). The SCRs are then fired via driver transistors Q87, Q88.
NOTE:
The LM339 comparator IC is used extensively in the
M•1200/M•1400. It differs from a standard op-amp
used as a comparator in that it has an open
collector output. If the voltage at the inverting input
exceeds the non-inverting, the LM339’s output
switches to it’s negative supply. Otherwise, the
output is open.

M•1200/M•1400 SERVICE MANUAL
CIRCUIT THEORY
10
THERMAL MANAGEMENT
Refer to schematic on page 29.
The M•1200/M•1400 T-Design Heatsink/Fan cools output devices evenly and does not
collect dust on the circuitry. The fan operates at two speeds, controlled by the
amplifier. An LM35DZ mounted in the center of the heatsink provides temperature
information to the fan control and overtemp circuits. There is also a thermal breaker in
the power transformer.
FAN CONTROL CIRCUIT
The fan voltage comes from a feedback regulator circuit formed by Q91 and Q92. The
output voltage is programmed by the ratio of R209 to R211// R207 (//=parallel). When
the unit is powered up C83 charges through R241 providing a 40 second delay before
the fan starts. As C83 charges, U6D’s output switches to -16V, starting the fan. Then
U6A’s output switches to open, allowing U6B to increase the fan speed, if triggered. R240
and C82 give the fan extra voltage when starting.
The fan speed is increased by U6B adding R211 in parallel with R207. (Q91 wants it’s
base a diode drop above ground, so Q92 doubles its Emitter voltage). U6B is triggered
by U4D to increase fan speed if;
•Channel 1 SOA protection is activated (U5C), OR
•Channel 2 SOA protection is activated (U5D), OR
•The heatsink is above 60 deg C (U5B) AND program is detected (U4C), OR
•The heatsink temperature is above 80 deg C (U5A).
U6C and Q89 are used to reset the circuit on power down. Under normal operation
C50 is charged via D57, Holding Q89 off. When power is removed C50 discharges and
Q89 switches on as the rails collapse. When the voltage on Q89’s collector falls below
ground U6 switches to the negative rail discharging C82 and C83.
THERMAL SHUTDOWN
Extreme heatsink or transformer core temperature will activate OVERTEMP. U5A
activates overtemp if the LM35DZ detects heatsink temperature above 80 deg C.
Feedback resistor R137 rescales U5A’s voltage reference so that the heatsink must cool
to below 50 deg C before overtemp is deactivated. Overtemp can also be activated
by the power transformer thermal breaker (via Q89). In either case, both channel are
muted via D69 and D66.

VERSION 1.1
SERVICE PROCEDURES 11
SERVICE PROCEDURES
MECHANICAL PROCEDURES
The M•1200/M•1400 design allows easy removal of all components and assemblies.
Disassembly and reassembly should be self evident. This manual has exploded
diagrams in the parts section.
Several important points concerning the assembly:
•4 of the screws that secure the top cover also hold the front extrusion (curved
piece) in place. When removing the top cover, be careful that the front extrusion is
not dropped.
•If the MAIN PCB is removed, be sure the smaller head machine screws (750-050-04)
are replaced next to the filter caps. The larger head machine screws can arc to the
capacitor body.
•Be sure all tywraps are replaced and that no wires are in danger of being pinched
in-between the cover and heatsink.
REQUIRED EQUIPMENT LIST
•30 MHz dual trace oscilloscope
•DMM (Must be capable of measuring 8mV DC with a minimum of 2 digits of
accuracy, and down to .1Ω)
•Sine wave oscillator / Function generator (20Hz - 20KHz)
•4 load resistors (8Ωwith a minimum 500W power rating)
•10A / 120V Variac With AC voltmeter and a defeatable ammeter (1 Amp full range)
or wattmeter (300 Watts full range)
•0.1uF 400V film capacitor installed across dual banana plug
•16 AWG shorting jumper installed across dual banana plug
RECOMMENDED EQUIPMENT LIST (PERFORMANCE VERIFICATION)
•Distortion analyzer and low distortion oscillator.
(Capable of resolving distortion as low as 0.002% from 20Hz to 20KHz)
•2500W AC power source.

M•1200/M•1400 SERVICE MANUAL
SERVICE PROCEDURES
12
TROUBLESHOOTING TIPS - OUTPUT FAILURES
After a catastrophic failure, it is likely that the four +/-80V supply fuses will be blown.
Replace fuses and very slowly bring up the Variac while monitoring line consumption. It
is likely that substantial line current will be pulled due to shorted output parts. At the
same time, look for DC offset at the output. Offset of greater than +/- 2.8V will trigger
the protection circuits and re-blow the four fuses. Keep the output offset below +/- 2V,
or temporarily defeat the DC-offset detector (Lift one side of D56).
Remove the main board from the unit and check for shorted output transistors. If one
bad output is found, replace all eight in the channel. When an output device shorts it
can place high current stresses on the other output parts. These output parts can fail
over time. Since long term reliability is paramount, please replace all the outputs.
All eight of the 3W 0.22Ωemitter resistors must be verified for proper value. Any off-
tolerance, or open parts, need to be replaced. An off tolerance (higher resistance)
emitter resistor will prevent it’s related output transistor from “doing it’s share” and will
place more stress on it’s mates in the output section. Also verify the 8 base drive resistors
(2.2Ω, 1/4W, fusible) are all OK. Verify that the drivers and pre-drivers are not shorted. If
one driver is shorted replace it’s mate. Do the same with the pre-drivers. Also check all
the resistors surrounding the drivers and pre-drivers.
Check the VI limiters and detectors. It is not uncommon to damage these parts when
the amplifier fails in a spectacular way! Look for shorts on Q76, Q77, Q94, Q95, Q83,
Q84, Q96, and Q97. Verify proper value on the resistors that go to the bases of these
devices. It is critical that these sections are working correctly. Shorted transistors can
cause some odd asymmetrical clipping problems. Open parts will not allow the current
limiting to operate effectively. If problems exist in these sections the amplifier might fail
into a short, or might clip prematurely when loaded to 2Ω.
All of the above trouble shooting (Not including part replacement) takes perhaps 15 to
30 minutes to do. If you take the time, and do all that is indicated above, it will allow
99% of the amplifiers to come up the first time! Trying to hurry, and skipping what is
suggested, can lead to a frustrating and time consuming repair.
Slowly bring up the supply and verify that line consumption isn’t excessive and that the
output is centered (no DC offset). It might be desirable to defeat the amplifier muting
(Connect U5-12, 13, & 14 together) temporarily so the amplifiers are active even when
the supplies are still very low. If the amp stays centered, verify that it will pass a nice
clean sine wave. Remove any test jumpers (I.E. DC-fault defeat & muting defeat) and
proceed to the “Reliability Verification” section.

VERSION 1.1
SERVICE PROCEDURES 13
BIAS AND TEST PROCEDURE
After the unit has been repaired, the following should be done to assure long term
reliable operation. If a distortion analyzer is present, distortion specifications should be
verified.
1. Adjusts bias in both channels (R1 & R51) for 8mV +/- 0.3mV at bias test points (J53 &
J54) after unit has idled for a few minutes. With Full AC line voltage applied to unit,
it will pull around 65W from the line (900mA at 120V). Measure for DC offset on both
output connectors. DC offset should be less than +/- 50mV.
2. Apply a 1KHz sine wave to the inputs and verify that the unloaded outputs have a
waveform that is symmetrical and undistorted. Drive the outputs into clipping and
verify symmetrical “flat-topping” on the waveform.
3. Reduce output levels, install the 0.1uF capacitor jumper from the output to ground
connections, and verify that clipping behavior is proper. Verify that no high
frequency oscillation occurs near and at clipping (parasitic oscillation).
4. Remove capacitive loading and minimize sine output. Verify and re-adjust bias if
required. Note that the bias will not drift appreciably in a unit that is functioning
properly.
5. Connect the amplifier directly to the AC line and connect an 8Ωdummy load to
both channels. Bring sine wave level up on both channels and verify symmetrical
clipping. The output will clip somewhere between 120V - 140V pk/pk depending on
how stiff the line is. Clipping should be as described above. Add the 0.1uF
capacitive loading and verify clipping is still well behaved.
6. Individually load Channel 1 and Channel 2 with 2Ω. Clipping should be
symmetrical, well behaved , and occur somewhere around 80V - 100V pk/pk. Verify
that clipping is well behaved after adding the 0.1uF capacitive loading.
7. Reduce output level to 20V pk/pk and short first channel one and then channel
two. The front panel “short” and “protect” LEDs should flash on the respective
channels.
8. Place amplifier in bridge mode and connect 4Ωloading to bridge outputs (across
both “+” output binding posts). Slip some card stock between the heatsink-outlets
and chassis sides. Monitor one of the outputs and adjust for a 20V pk/pk signal.
Short across both outputs and verify that all four ‘Short” and “Protect” LEDs light and
the amplifier mutes.
9. Remove short, monitor one of the outputs, and adjust for a 60V pk/pk sine output
(450W of output power bridged). After a few minutes the fan will begin running fast
(heatsink at 60°C) and a short time later the amplifier will mute (heatsink at 80°C).
The “Hot” and “protect” LEDs will come on. Remove card stock and after a few
minutes the amplifier will come out of mute mode and the “cold” LED will return.
10.Disconnect loading and remove drive. Reconnect amplifier to Variac and confirm
that the idle consumption is roughly 65W or 900mA, as before. Connect the
amplifier to speakers and verify that it sounds OK with music.

M•1200/M•1400 SERVICE MANUAL
SERVICE PROCEDURES
14
(THIS PAGE UNINTENTIONALLY LEFT BLANK), THOSE RESPONSIBLE HAVE BEEN PUT TO THE SWORD

VERSION 1.1
PARTS 15
PARTS
QUICK PARTS
FRONT PANEL
OL
-3
-6
-2
0
-3
-6
-9
-2
0
SIG
O
L
SIG
CH
1
30
28
26
20 2218
2
4
1
6
1
4
8
1.23v(+4dBu)
SENSITIVITY
GAIN/dBCH
2
PROTECT
COL
DHOT
SHORT
TEMPSTATUS
INTERNALSTATUS
CH
1
CH
2
CH
1&2
ON
OFF
POWER
3v
2v
1v3
0
2
8
2
6
2
02
2
1
82416
14
8
1.23v(+4dBu)
SENSITIVIT
Y
GAIN/dB
3
v
2
v
1v
PROFESSIONALPOWERAMPLIFIERFULL SYMMETRYDUALDIFFERENTIALHIGHCURRENTDESIGN
-9
REAR PANEL
CHANNEL
21
CHANNEL
CH
1
CH
2
SPEAKEROUTPUTS
+
--
+
63Hz
125Hz
CONSTANTDIRECTIVITY
HORNEQ/AIREQCONSTANTDIRECTIVITY HORNEQ/AIREQ
INPUT
THRUTHRU
INPUTAMPMODE
FREQUENCY
4.5 k HzON
6kHz2kHz
MONOBRIDGE
LIMITER
( CH1&CH2)
OFF
FULL
RANGESUB
WOOFER
LOWCUT
FILTERLOWCUT
FILTER
OUTPUTAPPLICATION
CAUTION
BALANCED
O
R
UNBALANCED
BALANCED
O
RUNBALANCED
AIREQ
MONO
BRIDGE
ON
/BRIDGE/MONO
170HzOFF
4.5kHz
6kHz2kHzAIREQ 120VAC50/60Hz
1500WATTS
+
-
170Hz
100Hz
OFF
STAGE
MONITOR
100Hz
STAGE
MONITOR
OFF
TYPICAL
ON
OFF
TYPICAL
TYPICAL
35HzTYPICAL
35Hz
(MONOBRIDGE)
1200W A TTS
4OHMLOADMIN.
600 WATTSCH
2OHMSLOADMIN.
STEREO
TYPICAL
STEREO
TYPICAL
AIREQAIREQ
GAIN CONTROLS
5KL POT, 130-036-00
KNOB, 760-061-00
NUT, 705-013-00
STAR WASHER, 710-023-00
POWER SWITCH
500-022-00
ALL LEDS
RED T-1, 304-001-00
GREEN T-1, 304-004-00
HANDLE ASSEMBLYS
RIGHT, 080-023-10
LEFT, 080-023-20
SCREW(TO CHASSIS),700-028-04
AC LINE CORD
US, 640-006-00
EUROPE, 640-007-01
JAPAN, 640-007-02
ENGLAND, 640-007-03
ITALY, 640-007-04
AUSTRALIA, 640-007-05
SWITZERLAND, 640-007-06
STRAIN RELIEF
740-004-00
INPUT SECTION
ALL KNOBS, 760-048-04
LOW CUT, 50KZx2 POT, 130-017-02
CD HORN EQ, 10KC POT, 130-037-02
¼” INPUT JACK, 400-214-00
¼” JACK WASHER, 710-002-00
¼” JACK NUT, 705-003-00
MALE XLR JACK, 400-142-00
FEMALE XLR JACK, 400-141-00
XLR JACK SCREWS,700-055-00
DP3T SWITCHES, 500-023-00
DPDT SWITCHES, 500-024-00
SPEAKER OUTPUTS
BINDING POSTS, 400-119-00
BINDING POST SCREW, 700-011-00
M•1200/M•1400i
¼” JACK, 400-118-00
WASHER, 710-019-00
SLOTTED NUT, 705-015-00
M•1400
SPEAKON, 400-138-00
SCREW, 700-055-00
INTERNAL FUSES
RAIL FUSES;
10A 5X20MM, 510-021-00
MAIN FUSE;
20A, 510-017-00
8A, 510-023-00

M•1200/M•1400 SERVICE MANUAL
PARTS
16
CHASSIS ASSEMBLY
4
5
8
12
3
6
1 7
1
11
9
10
12
13
15
16 17 18
14

VERSION 1.1
PARTS 17
CHASSIS ASSEMBLY PARTS LIST
ITEM # PART # DESCRIPTION NOTES
1700-028-02 SEMS 6-32X3/8 PHP BLKZC ALL SIDE PANEL SCREWS
2550-221-00 TOP COVER - M1200
3700-041-04 MCH 6-32X3/8 FL 100DG BLK TOP COVER TO CHASSIS
4551-029-00 EXTR SCRN DSPLY BZL-M1200
4551-029-10 EXTR SCRN DSPLY BZL-M1400
5700-041-04 MCH 6-32X3/8 FL 100DG BLK MANIFOLD TO FAN
6550-277-00 SCREEN MANIFOLD - M1400
7550-223-20 SIDE RAIL LEFT - M1200
8550-223-10 SIDE RAIL RIGHT - M1200
9550-222-00 BTM COVER - M1200
9550-244-00 BOTTOM COVER - M1400
10 080-023-20 PNT HANDLE ASSY LEFT
11 080-023-10 PNT HANDLE ASSY RIGHT
700-028-04 SEMS 6-32X3/4 PHP BLKZC HANDLE ASSY TO CHASSIS
700-041-04 MCH 6-32X3/8 FL 100DG BLK HEATSINK TO CHASSIS BOTTTOM
12 TRANSFORMER / MOUNTING
600-016-00 XFMR M1200 120V
600-016-01 XFMR M1200 230V
600-016-02 XFMR M1200 100V
705-019-00 NUT HEX 5/16-18
710-017-00 WASH SPLTLCK 5/16 HEAVY
710-024-00 WASH FLAT 5/16 HARD (USS)
550-249-00 PLATE XFMR MTG - M1200
780-111-00 RUBBER PAD
700-059-00 HEX 5/16-18X3IN GD-5
13 550-247-00 SHIELD INPUT - M1200
14 780-042-00 INSUL MYLAR INPUT SHIELD
15 055-079-00 PCB ASSY INPUT 1200/1400
16 700-028-02 SEMS 6-32X3/8 PHP BLKZC INPUT PCB TO SHIELD
17 760-048-04 KNOB 9MM DARK GRAY
18 550-275-00 SPACER PLATE - AMPLIFIER
710-002-00 WASHER FOR ¼” JACKS
705-003-00 NUT FOR ¼” JACKS
700-055-00 MCH 4-24X3/8 PHP BLK HILO XLR JACK SCREWS
055-080-00 PCB ASSY DISPLY 1200/1400
055-081-00 PCB ASSY MAIN - 1200
080-072-01 PCB ASSY MAIN - M1400
080-072-02 PCB ASSY MAIN - M1400i
700-028-02 SEMS 6-32X3/8 PHP BLKZC MAIN PCB TO CHASSIS EXCEPT FOR:
700-050-04 MCH 6-32X3/8 FIL PHL CLR ! USE THESE NEXT TO CAPS !
700-011-00 MCH 4-40X1/4 BTNSKT BLKOX OUPTUT TERMINALS TO CHASSIS
710-019-00 WASH FIBRE BLK (W/JACK)
705-015-00 NUT SLOT NCKL
700-055-00 MCH 4-24X3/8 PHP BLK HILO

M•1200/M•1400 SERVICE MANUAL
PARTS
18
HEATSINK ASSEMBLY