QSC RMX Series Operating manual

Technical Service Manual
RMX™ Series
2RU models
▲▲
▲▲
▲RMX 850
▲▲
▲▲
▲RMX 1450
▲▲
▲▲
▲RMX 1850HD
▲▲
▲▲
▲RMX 2450
*TD-000098-00*
TD-000098-00
Rev. C
QSC Audio Products, LLC
Costa Mesa, CA 92626 USA
www.qscaudio.com

RMX Series Technical Service Manual (2RU models) 1
TD-000098-00 rev. C
QSC Audio Products, LLC
Technical Services Group
Phone: 1-800 QSC AUDIO (1-800-772-2834) USA only
[phone]
Fax: [phone]
Postal: 1665 MacArthur Blvd.
Costa Mesa, California 92626 USA
E-mail: [email protected]
Web: http://www.qscaudio.com (product information and support)
http://www.qscstore.com (parts and accessory sales)
RMX 850
RMX 1450
RMX 1850HD
RMX 2450
RMX Series
Technical Service Manual
Copyright 2001, 2002, 2005, 2008 QSC Audio Products, LLC All rights reserved.
Document # TD-000098-00, Rev. C. Released June 2008.

2QSC Audio Products, LLC
RMX 850 RMX 1450 RMX 1850HD RMX 2450
OUTPUT POWER in watts
FTC: 20 Hz–20 kHz @ 0.1% THD, both channels driven
8Ωper channel 185 260 350 450
4Ωper channel 280 400 550 650
EIA: 1 kHz @ 0.1% THD, both channels driven
8Ωper channel 200 280 360 500
4Ωper channel 300 450 600 750
1 kHz @ 1% THD, typical, both channels driven
2Ωper channel 430 700 900 1200
Bridge Mono:
8Ω, 20 Hz–20 kHz, 0.1% THD 530 800 1300
8Ω, 1 kHz, 0.1% THD 600 900 1200 1500
4Ω, 1 kHz, 1% THD, typical 830 1400 1800 2400
DYNAMIC HEADROOM 2 dB @ 4Ω
DISTORTION
SMPTE-IM < 0.01% < 0.01% < 0.02% < 0.02%
FREQUENCY RESPONSE 20 Hz–20 kHz, +0/-1 dB
(at 10 dB below rated output power) -3 dB points: 5 Hz and 50 kHz
DAMPING FACTOR > 300 @ 8Ω
NOISE (unweighted 20 Hz to 20 kHz, below rated output) 100 dB 100 dB 100 dB 100 dB
VOLTAGE GAIN 31.6× (30 dB) 40× (32 dB) 46× (33 dB) 50× (34 dB)
INPUT SENSITIVITY, V RMS
full rated power @ 8Ω1.15v (+3.4 dBu) 1.15v (+3.4 dBu) 1.16v (+3.5 dBu) 1.23v (+4.0 dBu)
INPUT IMPEDANCE 10 kΩunbalanced
20 kΩbalanced
CONTROLS Front: AC switch, Ch. 1 and Ch. 2 gain
Rear: 10-position DIP switch
INDICATORS POWER: Green LED CLIP: Red LED, 1 per channel
SIGNAL: Yellow LED, 1 per channel
CONNECTORS Input: Active balanced; XLR and ¼" (6.3 mm) TRS, tip and pin 2 positive, and barrier strip
Output: “Touch-Proof” binding posts and Neutrik Speakon™
COOLING Continuously variable speed fan, back-to-front air flow
AMPLIFIER PROTECTION Full short circuit, open circuit, thermal, ultrasonic, and RF protection
Stable into reactive or mismatched loads
LOAD PROTECTION Turn-on/turn-off muting (all models)
AC coupling AC coupling Triac crowbar (each channel) Triac crowbar(each channel)
OUTPUT CIRCUIT TYPE AB AB H H
AB: Class AB+B complementary linear output
H: Class AB+B complementary linear output with Class H 2-step high efficiency circuit
DIMENSIONS 19.0" (48.3 cm) wide, 3.5" (8.9 cm) tall (2 rack spaces)
15.9" (40 cm) deep (rack mounting to rear support ears)
WEIGHT Shipping: 41 lb. (18.6 kg) 46 lb. (20.9 kg) 50.5 lb. (23 kg) 50.5 lb. (23 kg)
Net: 35 lb. (15.9 kg) 40 lb. (18.2 kg) 44.5 lb. (20.2 kg) 44.5 lb. (20.2 kg)
POWER REQUIREMENTS Available for 120 or 220–240 VAC, 50/60 Hz
POWER CONSUMPTION @ 120 VAC
(both channels driven)
Multiply currents by 0.5 for 230V units
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1
lluF
2
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3
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A5.0
8 ΩΩ
Ω
ΩΩ
A3A1.4A4.7
4 ΩΩ
Ω
ΩΩ
A5.4A6.6A5.11
2 ΩΩ
Ω
ΩΩ
A5.6*A5.9*A71
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8 ΩΩ
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A7.3A4.5A01
4 ΩΩ
Ω
ΩΩ
A6A6.9A61
2 ΩΩ
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ΩΩ
A3.9*A7.41*A52
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4 ΩΩ
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A3.6A6.51A72
2 ΩΩ
Ω
ΩΩ
A2.9*A32*A14
RMX Series Performance Specifications
Typical
1
Full
2
Max
3
Idle
0.8 A
8 ΩΩ
Ω
ΩΩ
3.9 A 6.9 A 12.6 A
4 ΩΩ
Ω
ΩΩ
6.1 A 11.2 A 20.7 A
2 ΩΩ
Ω
ΩΩ
8.7 A 16.6 A* 32 A*
1. Typical—Measured with pink noise signal on all channels at 1/8 of full average power. Producing peaks that occasionally reach the amp’s clipping point, this level approximates the maximum operating
level before clipping affects sound quality. These data would be the amp’s typical worst-case current.
2. Full—Measured with pink noise signal on all channels at 1/3 of full average power. Music at this level would typically produce massive clipping. In a class B amplifier, this would be the approximate
point of maximum heat production.
3. Max—Measured with continuous sine wave signal on all channels at a point of slight clipping (1% THD). This represents a “test bench” scenario and does not resemble any real-world usage.
US patents pending SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE

RMX Series Technical Service Manual (2RU models) 3
TD-000098-00 rev. C
Table of Contents
RMX Series Performance Specifications ...................................................................................................................................................... 2
1. Introduction ..................................................................................................................................................................................................... 5
1.1 Restriction of Hazardous Substances Directive (RoHS)................................................................................................................................ 5
1.2 QSC serial numbers ....................................................................................................................................................................................... 5
1.3 Service bulletins ............................................................................................................................................................................................. 5
1.4 The well-equipped service bench .................................................................................................................................................................. 6
1.5 Working with surface-mount components ................................................................................................................................................... 6
1.6 Series description .......................................................................................................................................................................................... 8
1.7 Technical descriptions and theory of operation ........................................................................................................................................... 8
2. Component identification and pinout ..................................................................................................................................................... 11
3. Troubleshooting: Symptoms, causes, & remedies ................................................................................................................................ 13
3.1 Excessive current draw ................................................................................................................................................................................ 13
3.2 Protection, muting, and turn-on/turn-off delay problems ......................................................................................................................... 13
3.3 Faults with signal present ........................................................................................................................................................................... 14
3.4 Instability ...................................................................................................................................................................................................... 14
3.5 Power supply and rail balancing problems ................................................................................................................................................. 15
4. RMX calibration procedures ...................................................................................................................................................................... 16
4.1 Setting bias .................................................................................................................................................................................................. 16
4.2 Setting positive and negative current limits .............................................................................................................................................. 16
5. Servicing RMX amplifiers ........................................................................................................................................................................... 18
5.1 Mechanical disassembly and re-assembly .................................................................................................................................................. 18
6. Replacement parts ....................................................................................................................................................................................... 19
6.1 RMX850 Replacement Parts ....................................................................................................................................................................... 19
6.2 RMX1450 Replacement Parts ..................................................................................................................................................................... 22
6.3 RMX1850HD Replacement Parts ................................................................................................................................................................. 25
6.4 RMX2450 Replacement Parts ..................................................................................................................................................................... 29
7. Schematics and diagrams .......................................................................................................................................................................... 33
7.1 RMX Assembly/Disassembly Diagram 1 of 2
(All models) ................................................................................................................................................................................................. 33
7.2 RMX Assembly/Disassembly Diagram 2 of 2
(All models) ................................................................................................................................................................................................. 34
7.3 RMX 850 Schematic Diagram 1 of 3
Channel 1 ................................................................................................................................................................................................... 35
7.4 RMX 850 Schematic Diagram 2 of 3
Channel 2 ................................................................................................................................................................................................... 36
7.5 RMX 850 Schematic Diagram 3 of 3
Power Supply ............................................................................................................................................................................................. 37
7.6 RMX 1450 Schematic Diagram 1 of 3
Channel 1 ................................................................................................................................................................................................... 38
7.7 RMX 1450 Schematic Diagram 2 of 3
Channel 2 ................................................................................................................................................................................................... 39
7.8 RMX 1450 Schematic Diagram 3 of 3
Power Supply ............................................................................................................................................................................................. 40
7.9 RMX 1850HD Schematic Diagram 1 of 3
Channel 1 ................................................................................................................................................................................................... 41
7.10 RMX 1850HD Schematic Diagram 2 of 3
Channel 2 ................................................................................................................................................................................................... 42
7.11 RMX 1850HD Schematic Diagram 3 of 3
Power Supply ............................................................................................................................................................................................. 43
7.12 RMX 2450 Schematic Diagram 1 of 4
Channel 1 ................................................................................................................................................................................................... 44

4QSC Audio Products, LLC
7.13 RMX 2450 Schematic Diagram 2 of 4
Channel 2 ................................................................................................................................................................................................... 45
7.14 RMX 2450 Schematic Diagram 3 of 4
Power Supply ............................................................................................................................................................................................. 46
7.15 RMX 2450 Schematic Diagram 4 of 4
Power Supply Revision .............................................................................................................................................................................. 47
7.16 RMX Power Transformer Configurations
March 2007 and later ................................................................................................................................................................................ 48
7.17 RMX 850 Chassis Wiring Diagram ............................................................................................................................................................ 49
7.18 RMX 1450 Chassis Wiring Diagram .......................................................................................................................................................... 50
7.19 RMX 1850HD and RMX 2450 Chassis Wiring Diagram ........................................................................................................................... 51
Table of Contents (continued)

RMX Series Technical Service Manual (2RU models) 5
TD-000098-00 rev. C
1. Introduction
1.2 QSC serial numbers
Serial numbers on QSC RMX series amplifiers are nine digits long. The first
four digits are a manufacturing date code in mmyy format; the last five digits
are the sequential identifying number for each individual amplifier. For
example, a serial number of 0807xxxxx would indicate that the amplifier was
built in 08/2007, or August 2007. The date code is important for determining
whether an amplifier is subject to a service bulletin, covered by a warranty (in
the absence of original purchase records), etc.
The amplifier’s serial number can be found on a label on the rear panel. The
label also indicates the model number, power rating, 4Ωcurrent draw, and the
AC mains voltage the amplifier was made for.
Certain design changes that the RMX amplifiers have undergone commenced
with a particular date code; for example, the RMX amplifiers with a date code
of 0406 or later conform to the European Union’s RoHS Directive and must be
serviced only with lead-free solder and soldering irons that are not contami-
nated with lead or other heavy metals covered by the directive.
1.1 Restriction of Hazardous Substances Directive (RoHS)
Since April 2006, all RMX Series amplifiers are manufactured to conform to the European Union’s RoHS Directive, which reduces the
amount of hazardous substances allowed in products for sale within its member nations. In electronic equipment such as audio power
amplifiers, this applies primarily to certain toxic heavy metals, such as lead, which may be present in electronic components, solder, and
other parts.
RoHS-compliant materials
When servicing RoHS-compliant amplifiers, it is important for the service technician to use only RoHS-compliant components and solder
(lead-free). All replacement parts provided by QSC for RoHS-compliant products are certified as RoHS compliant.
RoHS-compliant tools
Soldering irons and desoldering apparatus used on RoHS-compliant products must also not be contaminated by hazardous substances,
such as lead. Therefore, you cannot use the same soldering and desoldering tools for RoHS-compliant products and solder as you do for
non-compliant products and solder. You must either use separate soldering irons, desoldering tools and braid, etc., or at the very least
designate separate tips and braids and use only the appropriate ones. If you contaminate a tip or braid, even inadvertently, by using it on
a non-compliant product or solder, you should no longer use it with RoHS-compliant products or solder.
1.3 Service bulletins
Contact QSC Technical Services to make sure you have the most up-to-date service bulletins for RMX Series amplifiers. Service bulletins
may be distributed in hard copy, via fax, and electronically (Adobe Acrobat PDF) via CD-ROMs, FTP from the QSC web site
(www.qscaudio.com), and e-mail.
These service bulletins had been issued at the time this manual was printed:
•RMX0001, “Q205 Lead Stress” (RMX2450 only)
•RMX0002, “RMX Turn-on Delay” (all RMX models)
•RMX0003, “RMX2450 Turn-off Mute” (RMX2450)
•RMX0004, “RMX2450 AC Wire Routing” (RMX 2450)
•RMX0005, “RMX2450 AC Wire Replacement” (RMX2450)
•RMX0006, “RMX2450 IRFZ44N Field Effect Transistors (RMX2450)
Figure 1.1. The label displays two vital pieces of
information: the amplifier’s serial number and
operating voltage

6QSC Audio Products, LLC
Solder
1.4 The well-equipped service bench
To properly service RMX amplifiers, a technician needs the right tools. The
technician’s service bench should have the following equipment:
• Digital multimeter with RMS AC voltage and current
• Digital clamp-on ammeter
• Dual-trace oscilloscope
• Audio distortion analyzer
• Non-inductive load resistors, configurable as 8 ohms (min. 500 watts capacity),
as 4 ohms (min. 750 watts capacity), and 2 ohms (min. 1200 watts capacity);
see Figure 1.2 and Table 1.1.
• Variable AC voltage source, such as a Variac or Powerstat variable trans-
former, with a rated current capacity of up to 25A (for 120V models) or 12A (for
230V models)
• Low-distortion audio sine wave generator
• Philips and flat screwdrivers
• Soldering iron with a fine tip, 25–60W recommended (if you
service both RoHS-compliant and non-compliant amplifiers, you
will need separate soldering and desoldering equipment for
each)
• Appropriate rosin-core solder (use only RoHS compliant lead-
free solder for RoHS-compliant amplifers; for non-RoHS
amplifiers, use a 60/40 or 63/37 Pb/Sn alloy)
• Long-nose pliers
• Diagonal cutters
• Wire strippers
Automated test equipment, such as an Audio Precision workstation, is very useful for servicing RMX amplifiers. Contact QSC Technical
Services to obtain applicable AP test files.
Solder braid
1.5 Working with surface-mount components
RMX amplifiers, like many modern electronic products, use surface-mount technology (SMT) components
where appropriate in order to make high-density circuitry that is reliable and economical to manufacture.
SMT components in the RMX amps are used in the small-signal and control circuits, so they do not handle
significant amounts of power; therefore, they are subject to very little stress and should seldom fail.
Sometimes they do fail, or they require replacement for a performance upgrade or modification. Thus, it is
important to know how to work with SMT components.
Specialized tools and equipment exist for soldering, unsoldering, and removing SMT components quickly
and efficiently, but they are often expensive. Most SMT repairs, though, can be handled reasonably well with
common tools and equipment, such as tweezers, solder braid, and fine-tip soldering irons. The original
factory components are tacked to the board with a spot of glue, so you might have to apply some force to
break the adhesive.
Two-terminal components (resistors, capacitors, diodes, etc.)
Removal
1Use two soldering irons, preferably about 25 to 40 watts, with fine tips.
2With a soldering iron in each hand, hold one tip on the solder at one end of the component and the
other tip on the other end (Figure 1.3).
3Once the solder melts on both ends, grip the component between the two tips and lift it from the
circuit board.
4Use solder braid and a soldering iron to remove the solder from the two pads (Figure 1.4).
Figure 1.2. Load resistor bank
Table 1.1. Load resistor bank switch truth table
SW1 SW2 SW3
SW4 8Ω8Ω8Ω8Ω
R1: 8 , 500W non-inductiveΩ
R2–R4: 8 , 300W min. non-inductiveΩ
R1 R2 R3 R4
SW1 SW2 SW3 SW4
∞Ω (no load) OFF•••
8ΩON OFF OFF OFF
4ΩON ON OFF OFF
2ΩON ON ON OFF
0Ω(short circuit) ON • • ON
Figure 1.5.
Figure 1.3.
Figure 1.4.

RMX Series Technical Service Manual (2RU models) 7
TD-000098-00 rev. C
Solder
Tweezers
Solder
Insertion
1With a soldering iron and appropriate solder, melt just enough solder onto one pad to create a small
mound (Figure 1.5).
2Grasp the component in the middle with tweezers. Melt the small mound of solder with the iron and
place the component across the two pads (in the correct orientation, if the component is sensitive to
direction) and press it flat against the circuit board, with one end of the component immersed in the
melted solder (Figure 1.6).
3Hold the component in place and take the soldering iron away. Let the solder harden to tack the
component in place.
4Fully solder the other end of the component to its pad. Let the solder harden (Figure 1.7).
5Fully solder the tacked end of the component to its pad (Figure 1.8).
3When the solder on both ends melts, grip the component between the two tips and lift it from the circuit board. You might need to
quickly touch the pad on the middle terminal with a soldering iron to melt any remaining solder that might be holding the component
down.
4Use solder braid and a soldering iron to remove the solder from the three pads.
Insertion
1With a soldering iron and appropriate solder, melt just enough solder onto one pad to create a small mound of solder.
2Grasp the component with tweezers. Melt the small mound of solder with the iron and place the component in the correct orientation
across the three pads and press it flat against the circuit board, with one terminal of the component pressed into the melted solder.
3Hold the component in place and take the soldering iron away. Let the solder harden to tack the component in place.
4Fully solder the other terminals of the component to their pads. Let the solder harden.
5Fully solder the tacked terminal of the component to its pad.
Multi-pin components (ICs, etc.)
Removal
Removing a multi-pin SMT component is a delicate procedure. Ideally, you should use a soldering iron with an attachment that allows you
to heat all the pins simultaneously.
If such a soldering device is not available, use this procedure:
1Use a soldering iron and solder braid to remove as much solder as possible from the pins of the component.
2With fine tweezers, carefully try to lift each pin to see if it’s free. If it’s not, touch it with the tip of the soldering iron and if necessary,
use the solder braid to remove the remaining solder.
3Repeat the process until all the pins are free and you can remove the component.
Insertion
1With a soldering iron and appropriate solder, melt just enough solder onto one pad to create a small mound of solder. It is usually
easiest to use a pad that corresponds to one of the end or corner pins of the component.
2Grasp the component with tweezers. Melt the small mound of solder with the iron and place the component in the correct orientation
upon its pads and gently press it flat against the circuit board, with the appropriate terminal of the component pressed into the
melted solder.
3Hold the component in place and take the soldering iron away. Let the solder harden to tack the component in place.
4Fully solder the other terminals of the component to their pads. Let the solder harden.
5Fully solder the tacked terminal of the component to its pad.
Three-terminal components (transistors, etc.)
Removal
1With a soldering iron and solder braid, remove as much solder as possible from the middle terminal of
the component.
2With a soldering iron in each hand, hold one tip on the solder at the terminal at one end of the
component and the other tip on the terminal at the other end.
Figure 1.6.
Figure 1.7.
Figure 1.8.

8QSC Audio Products, LLC
+Vcc
-Vcc
+Vcc
-Vcc
Channel 1
Channel 2
+110V
-110V
+55V
-55V
Channel 1
Ch. 1 Center Tap
To Channel 2 Center Tap
12 5W
×2
Ω
0.047 µF
Note: Some of these descriptions concern circuitry that is
duplicated in the amplifier’s two channels. For the sake of
simplicity, the descriptions are of Channel 1 only. Components in
Channel 1 have a 3-digit designation with “1” as the first digit;
their equivalents in Channel 2 have a “2” as the first digit,
followed by the same two numerals. For example, R122 and R222
have identical functions in their respective channels.
Power supplies
Unlike other recent QSC amplifiers, the RMX line uses strictly
conventional power supplies, with large transformers that operate
at the 50 or 60 Hz frequency of the AC line. The electrical current
in the secondary circuitry is converted to DC through a full-wave
bridge rectifier. The resulting 100 or 120 Hz ripple is filtered out by
large capacitors that also serve as current reservoirs for short-
term, transient demands.
The supply provides a bipolar set
of supply rails for each channel,
with equal quiescent positive and
negative voltages, as shown in
Figure 1.9. Note that unlike many
bipolar supplies for complemen-
tary transistor arrangements, the
secondary windings are not
connected to ground at the center.
This is because the output transistors are directly mounted to the
heat sink, metal-to-metal, to
maximize heat transfer; this
grounds the collectors,
requiring somewhat
different output and power
supply arrangements. The
grounded-collector concept
is described later in this
chapter.
In the RMX1850HD and
RMX2450, the secondaries
are tapped to provide an
intermediate set of bipolar rails for the Class H output circuitry.
Figure 1.10 shows one channel. Class H operation is described
later in this chapter.
The 24-volt cooling fan is driven by a separate DC supply that is
powered by a 20-volt tap on the transformer primary. To minimize
fan noise, the fan speed is controlled by varying its actual DC
voltage in response to the amplifier’s heat sink temperatures. An
optocoupler isolates the fan control circuitry from the thermal
sensors.
Audio circuitry
The audio inputs are balanced to offer a reasonably high amount
of common-mode noise rejection. The input balancing is done
using a single op amp (one half of an NE5532 dual op amp)
arranged as a differential amplifier. The degree of common-mode
rejection is dependent on a close match between the input
resistors (R100 and R101 in Figure 1.11) and between the feedback
resistor and the shunt resistor (R105 and R106). The circuitry uses
1% precision resistors to ensure at least 40 dB of common-mode
rejection.
The feedback and shunt capacitors, C101 and C103, add a first-
order high-frequency roll-off, down 3 dB at 88.4 kHz (over two
octaves above the high end of the audio spectrum). This makes the
amplifier less susceptible to RF interference, high-frequency
oscillations, etc.
Also in this stage, the feedback loop contains one half of an NE5517
(equivalent to the more familiar LM13600) dual operational
transconductance amplifier (Figure 1.12). The OTA is part of the clip
limiter circuitry; when the clip limiter is activated, a control voltage
increases the transconductance of the OTA, which essentially
decreases the impedance of the feedback loop and reduces the
gain of the stage in order to reduce the amount of clipping.
The gain control uses a linear potentiometer, but the impedances
loading the wiper to ground make the pot approximate an audio
taper over most of its rotation. After the wiper, RC networks roll
off the low end, if the LF filter is set for that channel, at either 30
or 50 Hz, depending on the DIP switch setting on the rear panel.
1.7 Technical descriptions and theory of operation
QSC’s RMX Series amplifiers are entry-level professional audio
products, designed for good, basic performance and reliability at
low price. The series comprises six two-channel models; this
manual covers the four two-rack-space models: the RMX850,
RMX1450, RMX1850HD and RMX2450. See page 2 for complete
specifications. The three-rack-space models—the RMX4050HD
and the RMX5050—are covered by another service manual.
The RMX850 and RMX1450 have single-sided printed circuit
boards. The RMX1850HD and RMX2450 use double-sided boards.
1.6 Series description
Figure 1.9
Figure 1.10

RMX Series Technical Service Manual (2RU models) 9
TD-000098-00 rev. C
the arrangement of the output transistors might look somewhat
backwards: PNP transistors connected to the positive voltage rail,
and NPN on the negative rail. Not only that, but the transistors
attached to the positive rail are for pulling the output voltage
negative, and the ones on the negative rail swing the output
positive. The collectors all connect to ground, which allows them
to be mounted directly to the heat sink—metal-to-metal, without
insulators in between—for the best possible transfer of heat away
from the transistors. The emitters of the PNP and NPN transistors
are coupled through resistors to the positive and negative supply
rails, respectively, forming banks of common-emitter circuits
driving the supply rails. Consequently, the devices drive the rails
with the audio signal, which rides atop the DC. The output to the
speaker load is taken from the point between the positive and
negative reservoir capacitors; this is also where the negative
feedback is taken from. The nature of this arrangement, with audio
signal riding on the supply rails, is why the power supply has no
ground reference.
Another unusual characteristic of the grounded-collector output
section is that the signal at the output to the speaker is actually
opposite in polarity to the signal at the op amp output. This is why
the negative feedback resistor, R122, connects to the op amp’s
non-inverting input instead of the inverting input.
The output point of the circuit couples to the output connector
through an RLC network (R160, R161, R162, L100, and C124) that
serves as a high-frequency snubber and also helps keep the amp
circuitry stable when driving capacitive loads.
Clip detection
The output of the op amp also drives a group of four diodes (D102,
D103, D105, and D106) arranged as a full-wave rectifier. Normally,
the op amp’s output signal level is about 1 volt or less, which is all
it takes to drive the driver transistors.
But because this point is within the overall feedback loop, when
clipping occurs, the op amp approaches full open-loop gain and
puts out a much higher signal voltage to try to make the output
signal track the input. The four diodes rectify the voltage to drive
the clip indicator LED, LD100. The current exiting the full-wave
rectifier passes to ground through R127 and also drives the base of
transistor Q100 through R115. If the clip limiter is switched on,
Q100’s emitter is grounded, and when the voltage across R127
goes sufficiently negative to forward-bias Q100, which sends
current through R111 and R103 into the amplifier bias input of the
operational transconductance amplifier (OTA), U10:1. The OTA is in
the negative feedback loop of U101:1, and increasing its
transconductance essentially reduces the impedance of the
feedback loop, which reduces the gain of the op amp stage. This
reduces the signal level until the amount of clipping is minimal.
When the clipping stops, Q100 is no longer forward-biased, and
the gain returns to normal.
The next active device is another 5532 op amp, U101:2. Its output
drives the driver transistors, which in turn drive the output transis-
tors. The output section has a Class AB+B configuration; the
drivers (a complementary pair, Q105 and Q106, comprising an NPN
MJE15032 and a PNP MJE 15033) are class AB. A series network
of two diodes and a 100-ohm trimpot provide the small amount of
forward bias on the transistor pair to keep crossover distortion
minimal. In parallel with the trimpot is a 50-ohm thermistor with a
negative temperature coefficient; as the circuitry warms, its
resistance decreases. This reduces VBE on both Q105 and Q106,
decreasing the bias current to reduce the threat of thermal runaway.
The base of each driver transistor is tied to ground through a diode
and a 2.2 kΩtrimpot in series; these set the current limiting
threshold for their respective signal polarities.
The collector of each driver transistor directly drives the bases of
its output transistors, which are the main power-handling signal
devices. If you’re not familiar with the grounded-collector scheme,
R111
100
^R_0805
R109
100
^R_0805
10.0K
^R_0805
R103
3
+
8
75
1
4
-
2
U10:1
LM13600M
R108
7.50K
^R_0805
+14V
A1
R104
150K
^R_1206
-14V
R110
270
^R_0805
C105
100-25V R107
^R_0805
39K
Q100
3906
R115
820
^R_1206
To input op amp U101:1
11
CLIP LIMIT
SWITCH
(Open to defeat
clip limiter)
From clip
detection
2.5K LINEAR
RIGHT ANGLE POT
R106
10.0K
^R_0805
R101
10.0K
^R_0805
R100
10.0K
^R_0805
R105
10.0K
^R_0805
CW
CCW
W
R112
2.5K
A1
R102
1K
^R_0805
+IN_A
-IN_A
C101
180p-5%
^C_0805
C103
180p-5%
C106
47µF NP
3+
2-
1
NE5532
U101:1
A1
R113
274
^R_0805
To LM13600 operational
transconductance amp
Figure 1.12
Figure 1.11

10 QSC Audio Products, LLC
DC protection
The RMX1850HD and RMX2450 have a crowbar circuit, based on a
triac and two silicon controlled rectifiers, on the output to protect
against DC faults. If an amp channel puts out a DC voltage, which
could be the result of a component or circuit failure, it will first
trigger either D119 or D120, depending on the polarity of the
voltage. The triggered SCR will in turn trigger triac Q113, shorting
the output to ground through fuse F100. The fuse will blow,
safeguarding the speaker load from the DC fault.
The output sections of the RMX850 and RMX1450 are AC coupled.
Class H
The RMX1850HD and RMX2450 utilize a two-step
Class H output section. It is essentially a Class AB+B
circuit but with two sets of bipolar supply rails. On
both the positive and the negative sets of rails, a
comparator circuit, called a “step driver,” compares
the audio signal to the lower rail voltage. When
necessary to fully reproduce the signal’s voltage
swing—just before the instantaneous signal voltage
reaches the lower rail voltage—the step driver turns
on a TMOS power FET to pull the output transistors’
supply rail up from the lower voltage to the higher
one, and then back down again when the signal
allows. By keeping the transistors’ supply rails low
whenever possible, the devices dissipate less unused
power and generate less waste heat, making the
amplifier more efficient than a straight class AB
amplifier with the same power points (Figure 1.13).
The comparators are 311-type ICs: U170 on the
positive step and U171 on the negative. Each one
drives a high-gain complementary transistor pair
(2N3904 + 2N3906), which drive the gate of their
respective MOSFET.
Bridged mono operation and protection
When the amplifier is operated in bridged mono, its two channels
work in tandem to produce up to twice the voltage swing that a
single channel is capable of. To do this, Channel 2 produces a
signal identical to Channel 1’s, but opposite in polarity—in other
words, a mirror image.
Channel 2’s signal feed (bus BR_MONO_FEED) is an attenuated
version of the signal on Channel 1’s speaker bus. Closing DIP
switch #6 (set to “BRIDGE MONO ON”), connects the
BR_MONO_FEED bus on Channel 1 to the BR_RET bus on Channel
2. The BR_RET bus drives the non-inverting input of op amp
U201:2 directly.
With two channels operating as one, but each having its own
feedback and protection circuitry, it is vital to keep both running as
mirror images. A protection circuit monitors the balance between
Channel 1’s and Channel 2’s signals. Resistors R22 and R23 (R22A,
R22B, R23A, and R23B on the RMX2450) are equal in value and
form a voltage divider between the two channel outputs. If the
output signals are mirror images, the voltage at the junction of the
resistors (bus BR_BAL) will be zero. If the signals are not mirror
images—for example, one channel is defunct, distorting, or
reduced in gain—a voltage will appear on BR_BAL. Through DIP
switch 7, the BR_BAL bus becomes bus BR_CUT and feeds the
bases of transistors Q8 and Q6, which are part of a 4-transistor
circuit across the +15V and -15V rails that supply the op amps and
the input circuitry. If the voltage on BR_CUT goes positive enough
to forward-bias Q8, the transistor’s collector will collapse the +15V
rail. At the same time, the emitter current from Q8 will flow
through R25 and into the emitter of Q7, forward-biasing it, too.
The collector of Q7 will then collapse the -15V rail.
Similarly, if BR_BAL goes sufficiently negative, it will forward-bias
Q6, in turn forward-biasing Q9, and these will collapse the ±15V
rails.
With the rails collapsed, the op amp and the input circuitry will not
function, which will mute the audio.
-V
HI
0
+V
HI
+V
LO
-V
LO
Power saved
Power saved
Figure 1.13. A two-tier class H amplifier circuit operates on the lower voltage
rails until the signal amplitude requires the higher voltage.

RMX Series Technical Service Manual 11
2. Component identification and pinout
1
2
3
4
8
7
6
5
AB
OUTPUT A
INVERTING
INPUT A
NON-INVERTING
INPUT A
V-
OUTPUT B
INVERTING
INPUT B
NON-INVERTING
INPUT B
V+
1
2
3
4
8
7
6
5
GROUND 1
NON-INVERTING
INPUT
INVERTING
INPUT
V-
OUTPUT
BALANCE/
STROBE
BALANCE
V+
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
AB
NON-INVERTING
INPUT A
V-
AMP BIAS INPUT B
NON-INVERTING
INPUT B
INVERTING INPUT B
OUTPUT B
BUFFER INPUT B
BUFFER OUTPUT B
V+
DIODE BIAS B
AMP BIAS INPUT A
DIODE BIAS A
INVERTING INPUT A
OUTPUT A
BUFFER INPUT A
BUFFER OUTPUT A
NE5532AN Dual operational amplifier
(part number IC-000211-GP; was 3130-2430-0)
LM311 Voltage comparator
(part number IC-000214-GP; was 3130-9240-0)
NE5517D Dual operational transconductance
amplifier (may be substituted by LM13600 or LM13700)
(part number IC-000212-GP; was 3131-9730-0)
4N29 Opto-isolator
(part number IC-000213-GP; was 3130-9270-0)
1
2
3
6
5
4
2N5064 Sensitive gate thyristor
(part number QD-000350-10; was 4860-5250-5)
KAG
12
3
BE
C
GDS
2N3904 (NPN) Small-signal transistor
(part number QD-000204-GP; was 4860-0640-3)
2N3906 (PNP) Small-signal transistor
(part number QD-000205-GP; was 4853-9060-3)
IRFZ44 TMOS power field effect transistor
(part number QD-000347-GP; was 490F-Z440-5)

12 QSC Audio Products, LLC
B C E
TOSHIBA
BCE
MJE15032 (NPN) Driver transistor
(part number QD-000207-GP; was 4860-5020-5)
MJE15033 (PNP) Driver transistor
(part number QD-000210-GP; was 4860-5030-5)
2SC5200 (PNP) Power transistor
(part number QD-000209-GP; was 4860-5050-5)
2SA1943 (NPN) Power transistor
(part number QD-000208-GP; was 4860-5060-5)
MAC224 Triac (discontinued; see BTA41 triac
below)
(part number QD-000221-GP; was 8910-0482-0)
MT1
MT2
G
BTA41 Triac
(part number QD-000261-GP; replaces discontinued
MAC224 Triac)
A1 A2 G
Note: To install the BTA41 Triac on a channel module circuit board
that originally held a MAC224 Triac, first mount it to an adapter
(QSC part number SR-000117-00) and then solder the adapter to
the circuit board. See Service Bulletin RMX0008 for more
information. The Triac and adapter are available together as a kit,
QSC part number SG-000522-00.
10 kΩΩ
ΩΩ
ΩPotentiometer (gain control)
(part number PT-310006-GP; was 4750-6200-0; used
April 2002 and earlier with 10 µF C106)
2.5 kΩΩ
ΩΩ
ΩPotentiometer (gain control)
(part number PT-225000-GP; was 4751-0420-0; used May
2002 and later with 47 µF C106)
potentiometer value
printed on back

RMX Series Technical Service Manual 13
When first checking the operation of an amplifier on the bench, always turn your variable transformer down to zero before plugging the
amplifier in. After you turn the amplifier on, gradually turn up the AC voltage as you observe the amplifier’s behavior and its current
draw; this will help you determine what, if anything, is wrong with it. If you see or smell smoke, flames, or any other signs of short
circuits or excessive current draw, quickly turn the AC back down to zero. If no such problems occur, it is usually safe to turn the AC up to
the amplifier’s full operating voltage for further testing.
Slow increase in current draw (current doesn’t become
excessive until about half of the amplifier’s full AC
operating voltage; amplifier may pass signal)
• The bias is severely misadjusted, or bias diodes D108, D109,
D208, or D209 is defective.
• An oscillation is causing excessive current demand.
Runaway current draw (current increases sharply at about
25 to 33% of the amplifier’s full AC operating voltage)
One or more reservoir capacitors is reversed. CAUTION: the gas
buildup in a reversed electrolytic capacitor can cause it
to vent explosively. Immediately turn off power and let the
capacitor cool down before replacing it.
3. Troubleshooting: Symptoms, causes, & remedies
3.2 Protection, muting, and turn-
on/turn-off delay problems
The customer complains of amplifier locking up, or not turning on
and off correctly.
Symptoms covered:
• Both channels do not come out of protect
• Amplifier will not thermally shut down when it should
• Power LED doesn’t light
• Too little or too much muting delay
• No clip limiting
• Fan doesn’t run, or runs always at high speed
Possible situations:
Both channels stay in protect after turn-on
• Q4 or Q5 is shorted base-to-emitter.
• Voltage across D8 should be 14 to 15 volts DC. If it is low, check
D8 and R10; also, C7 and C8 is leaky.
• D9, R4, and/or R7 is open.
3.1 Excessive current draw
The customer complains of blowing circuit breakers or fuses, or
burning smell or smoke.
Symptoms covered:
• Fuses blow immediately
• The amplifier quickly gets very hot
• Line circuit breakers trip at turn-on
• The amplifier hums loudly and the chassis vibrates
• The amplifier emits smoke
• The amplifier gives off a burning smell
If the symptoms indicate a possible problem in the channel circuits
or output sections, you can isolate either channel module from the
power supply by pulling its fuses from the AC board.
Possible situations:
Excessive current with no signal present
If the amplifier seems to run hot and draws higher-than-normal
current when idling at full AC voltage, the cause could be bias
misadjustments in the output circuitry of one or both channels. See
the calibration procedures in the next section. In the RMX2450, the
cause might also be blown step FETs; see service bulletin RMX0006.
Fast increase in current draw (current increases rapidly at
only a few volts AC)
• The main bridge rectifiers BR100 and/or BR200 (all models) and
BR101 and/or BR201 (RMX2450) is reversed or shorted.
• Supply clamp diode pairs D117 and D118 and/or D217 and D218
is reversed or shorted.
• The drivers and/or power transistors is shorted on both polarities
(NPN and PNP) on one or both channels.
Moderate increase in current draw (current increases
slowly, doesn’t become excessive until about ¼ of the
amplifier’s full AC operating voltage)
• One polarity’s drivers and/or power transistors (NPN or PNP) is
shorted, on one or both channels.
• Individual supply clamp diodes D117, D118, D217, or D218 is
reversed or shorted.
• Bias diodes D108, D109, D208, or D209 or bias trimpots R131 or
R231 is open.

14 QSC Audio Products, LLC
The amplifier will not thermally shut down when it over-
heats
Check for shorted D9, R4, or R7; check also for open LD1 (“POWER”
LED), R2, or R5.
Too much or too little muting delay
• Excessive delay at turn-on: check for open LD1, R2, or R5; check
for incorrect R10 or R15; check for shorted D9; also see service
bulletin RMX0002.
• Too short at turn-on (amplifier unmutes before the circuits stabilize,
causing a thump): check for incorrect or bad C7, C8, or R10.
• Amplifier doesn’t stay muted at turn-off: see service bulletin
RMX0003
Power LED doesn’t light
Check for open or shorted LD1; check R2.
No clip limiting
• U10 is defective.
• DIP switches SW1:1 or SW1:10 is defective.
Fan doesn’t run
• The fan is defective.
• Check the fan connection to the AC board.
• Check R1 and BR1 on the AC board.
Fan runs always on high speed
• Q3 and/or U2 are shorted.
• Check PTC thermistors R4 and R7.
The output signal collapses when driving a normal speaker
load
• R139, R140, R239, and/or R240 is misadjusted.
• Check R118, R119, R218, and R219.
The supply rails are OK with no signal but collapse with a signal
• C112, C113, C212, and/or C213 is leaky.
• Check C114, C124, C214, and C224.
The amplifier gets too hot with no load
• Bias trimpots R131 or R231 are misadjusted, burned, or open.
• Bias diodes D108, D109, D208 and/or D209 are incorrect (should
be 1N4934).
• If the amplifier is producing high-frequency oscillations, check
C114, C124, C214, and C224.
• Check resistors R136, R137, R236, and R237.
• Op amp U101 or U201 is unstable.
One channel clips prematurely
• R146 or R147 (Channel 1) or R246 or R247 (Channel 2) are open.
• R139 or R140 (Channel 1) or R239 or R240 (Channel 2) are misadjusted.
• Check R157, R158, R198 (RMX1850HD and RMX2450 only),
D115, and D116 (Channel 1) or R157, R258, R298 (RMX1850HD
and RMX2450 only), D215, and D216 (Channel 2).
• (RMX850 and RMX1450) Check R118 and R119 (Channel 1) or
R218 and R219 (Channel 2).
• R130 or R132 (Channel 1) or R230 or R232 (Channel 2) is open.
Excessive hum in loudspeaker when no signal is present
(RMX850 and RMX1450)
Check R118, R119, R218, and R219.
Excessive current draw with signal present (RMX1850HD
and RMX2450)
• Check triacs Q113 and Q213.
• Check D119, D120, D219 and D220.
3.3 Faults with signal present
The customer compains that the amplifier passes a signal but
doesn’t run correctly.
Symptoms covered:
• The output signal breaks up or is distorted
• “Ringing” sound in loudspeaker when no audio signal is present
• The output signal collapses when driving a normal speaker load
• Supply rails OK with no signal, but collapse when a signal passes
• The amplifier gets too hot
• One channel clips prematurely
• Excessive hum in loudspeaker when no audio signal is present
Possible situations:
The output signal breaks up or is distorted
• (RMX850 and RMX1450) Check the hum-null resistors R118,
R119, R218, and R219.
• Check the ground traces for continuity among speaker ground,
input ground, and AC ground.
“Ringing” sound in loudspeaker when no audio signal is
present
• Check C114, C124, C214 and C224.
• Check or replace dual op amp U101 or U201.
3.4 Instability
The customer complains of gain problems, spurious noises, or oscilla-
tions.
Symptoms covered:
• General output distortion
• “Ringing” sound in loudspeaker trailing an audio signal
• Excessive crossover distortion
• The output waveform appears fuzzy on an oscilloscope
Distinguish among the different symptoms of fuzziness (instability),
ringing (momentary instability after a transition), crossover distor-
tion (often causing ringing), or general distortion.
Possible situations:
General distortion in the output signal
• Severe distortion, at any load, often with abnormally high
current draw: check the slew rate capacitors C114, C115, and

RMX Series Technical Service Manual 15
C116 (Channel 1) or C214, C215, and C216 (Channel 2).
• Moderate distortion, especially with light loading: stability
capacitors C124 and C126 (Channel 1) or C224 and C226 is too
high in capacitance; also check the slew rate capacitors C114,
C115, and C116 (Channel 1) or C214, C215, and C216 (Channel
2), and the output filter resistors R161, R162, R154, and R155
(Channel 1) or R261, R262, R254, or R255 (Channel 2).
• Distortion with low gain: check the feedback shunt components
R120, R138, and C125 (Channel 1) or R220, R238, or C225
(Channel 2); also check for broken circuit traces around the
components; U101 (Channel 1) or U201 (Channel 2) is defective
or its socket is contaminated.
Ringing sound trailing the audio signal
• This usually indicates marginal instability and is usually
triggered by the signal passing through zero volts (the crossover
point). Check the stability components and output filters.
• With a sine wave test signal, use an oscilloscope to check for
excessive crossover notch at the output signal’s zero crossings.
Excessive crossover distortion (unbalanced, asymmetrical,
or excessively large crossover notch)
• Severe crossover discontinuity: bias diodes D108 or D109
(Channel 1) or D208 or D209 (Channel 2) are shorted.
• Moderate discontinuity: bias diodes D108 or D109 (Channel 1)
or D208 or D209 (Channel 2) are out of spec.
• R131 (Channel 1) or R231 (Channel 2) is defective.
• Base resistors R136 or R137 (Channel 1) or R236 or R237
(Channel 2) are open. Also check the NTC thermistors R134
(Channel 1) or R234 (Channel 2).
3.5 Power supply and rail
balancing problems
Symptoms covered:
• Insufficient or excessive current limiting into a shorted load
• Op amp rails too high with a shorted load
• Uneven voltages on supply rails
A channel’s output current should remain unaffected when driving
resistive loads as low as 2 ohms per channel. When driving a short
circuit, the current limiting circuit should collapse the output to a
lower current. This is done by dropping the op amp’s supply rails
from a normal ±14–15 volts down to about ±5–6 volts. Normally,
the output signal helps replenish the op amp supply rails through
D115 and D116 (Channel 1) and D215 and D216 (Channel 2), but a
short circuit or excessively low load impedance prevents the
replenishment, and the op amp rails collapse because they can’t
supply enough current to let the op amp drive both the clip LED and
the driver transistors. The current limit trimpots R139, R140, R239,
and R240 permit adjustment of the current limit thresholds. See
the RMX calibration section of this manual for adjustment
procedures.
Possible situations:
Excessive current into short (insufficient limiting)
• If the op amp rails are dropping to ±5 to 6 volts as they should:
the 5.6V zener diodes D107 and/or D110 (Channel 1) or D207
and/or D210 (Channel 2) are reversed or shorted.
• If high crossover distortion is present: bias diodes D108 or D109
(Channel 1) or D208 or D209 (Channel 2) are shorted.
Excessive current into short (op amp rails are not drop-
ping)
• The op amp U101 (Channel 1) or U201 (Channel 2) is defective,
with insufficient output current.
• Clip LED LD100 (Channel 1) or LD200 (Channel 2) and/or its
rectifying diodes (Channel 1: D102, D103, D105, D106; Channel 2:
D202, D203, D205, D206) are open.
• When driving a short circuit, the output section’s positive and
negative supply rail voltages should be equal, within 3 volts. If
they aren’t, check D107, D110, R146, and R147 (Channel 1) or
D207, D210, R246, and R247 (Channel 2).
Weak current into 2ΩΩ
ΩΩ
Ωor short (excessive or premature
limiting)
• Bias resistors R130 and R132 (Channel 1) or R230 and R232
(Channel 2) are too high.
• Driver transistors (Channel 1: Q105, Q106; Channel 2: Q205,
Q206) have very low gain.
• One or more emitter resistors in the output section are open.
Current OK at 2ΩΩ
ΩΩ
Ω, weak into short
• LD100, D102, D103, D105, or D106 (Channel 1), or LD200, D202,
D203, D205, or D206 (Channel 2), are shorted.
• Zener voltage of diodes D107 or D110 (Channel 1), or D207 or
D210 (Channel 2), is too high.
Current limits properly into short, but current is weak at 2ΩΩ
ΩΩ
Ω
• If the op amp rails are low (< 14–15 volts) when driving a 2-
ohm load
without clipping, check the resistors R157 and R158
(Channel 1, all models), R198 (Channel 1, RMX1850HD and
RMX2450 only) and diodes D115 and D116 (Channel 1, all
models), or R257 and R258 (Channel 2, all models), R298
(Channel 2, RMX1850HD and RMX2450 only) and diodes D215
and D216 (Channel 2, all models).
• If the op amp rails are normal (14–15 volts) when driving a 2-ohm
load without clipping, usually the output section gain is too low:
weak driver transistors, open output transistors, or open emitter
resistors. Check the value of the driver transistors’ emitter
resistors, too: R146 and R147 (Channel 1) or R246 and R247
(Channel 2).
Rail voltages unequal
The balance between the positive and negative rail voltages is set
by a voltage divider comprising resistors R118 and R119 (Channel
1) and R218 and R219 (Channel 2). If the amplifier channel passes
a signal but clips unevenly due to unequal rail voltages, this
voltage divider is the likely culprit.

16 QSC Audio Products, LLC
4.1 Setting bias
Always set the bias
• after replacing any output or driver transistor.
• after replacing any diode or resistor in the driver/output circuitry.
• if the amplifier seems to run too hot at idle.
• if the amplifier exhibits crossover distortion.
The bias network sets the quiescent base current in the NPN and
PNP driver transistors, which in turn sets the quiescent current in
the output transistors. The driver transistors should both be slightly
“on” at idle so that the transitions of the signal voltage between
positive and negative are smooth and free of gaps or glitches. Too
much bias current will cause the amplifier to run hotter than it
should, especially at idle, while too little will cause noticeable
crossover distortion, especially at low signal levels.
The amplifier circuitry must be cool, or at least within a couple
degrees of ambient air temperature, and the top cover must be
removed. If the driver and output transistors are significantly
warmer than the ambient air, leave the amplifier off and let it cool
before proceeding.
Before turning the amplifier on to set bias on one or both channels,
familiarize yourself with the locations of the trimpots (R131 and
R231) and the voltage measuring points so you can work quickly but
thoroughly. If the amplifier warms up before you finish setting the
bias, you will need to shut the amplifier off and let it cool down
before you resume.
Tools and resources you will need:
• Small flat screwdriver (non-conductive) for adjusting trimpots
• DC voltmeter
• AC power
Procedure
1. Turn the amplifier’s gain controls all the way down. No test
signal is needed.
2. Plug the amplifier into an appropriate AC source. Turn the
amplifier on.
3. Channel 1: While measuring the DC voltage across resistor R146,
adjust trimpot R131 to obtain the voltage listed in Table 1.
4. Channel 2: While measuring the DC voltage across resistor R246,
adjust trimpot R231 to obtain the voltage listed in Table 1.
After setting the bias, calibrate the positive and negative current
limiting; instructions for the procedure follow below.
4. RMX calibration procedures
4.2 Setting positive and negative
current limits
Tools and resources you will need
• Oscilloscope
• 2-ohm resistive load (rated for at least 1200 watts)
• Shorting connector for amplifier output
• Variable AC transformer (e.g., Variac, Powerstat, etc.) rated for
25A (120V) or 12A (230V). Make sure the AC supply is appropri-
ate for the amplifier.
• 1 kHz audio sine wave generator
• Digital multimeter
• Clamp-on digital current meter (e.g., Fluke 30 Clamp Meter)
• Small flat screwdriver (non-conductive) for adjusting trimpots
Procedure
1. Set the audio sine generator to 1 kHz at 1 volt RMS and
connect it to Channel 1's input. Connect a 2-ohm load and the
oscilloscope probe across Channel 1's output.
2. Turn up Channel 1's gain control partway. On the oscilloscope
you should see the amplitude of the sine wave increase
accordingly.
3. Turn the gain control back down and apply a short circuit
across the output terminals of Channel 1. Clamp a current
probe either onto one of the brown wires running to the AC
switch or onto the gray output wire from channel 1's module.
4. Turn the gain control all the way up. Adjust trimpots R139 and
R140 equally until the current measured falls within the range
shown in Table 1.
5. Turn the gain control all the way down and remove the short
circuit so the channel drives the 2-ohm load. Turn the gain control
back up until the output clips. The voltage at which the signal
starts to clip should fall within the range shown in Table 1. If the
clipping is asymmetrical, that is, the signal clips on either the
positive or negative side first, adjust R139 to make it symmetrical.
6. Turn the gain control down. If the amp has begun to warm up
shut it off and let it cool a few minutes before proceeding with
Channel 2.
7. Repeat steps 1 through 5 for Channel 2. Use trimpots R239 and
R240 to adjust the current limiting in steps 11 and 12.
8. Turn both channels’ gain controls all the way down. Clamp the
ammeter onto one of the amp’s AC wires and check the amp’s
idle current. If the amplifier is still at about room temperature,
the idle current should match the value shown in Table 1.

RMX Series Technical Service Manual 17
Calibrations Adjust RMX850 RMX1450 RMX1850HD RMX2450
Channel 1 bias: DC voltage across R146 R131 0.16 V 0.14 V 0.07 V 0.07 V
Channel 2 bias: DC voltage across R246 R231 0.16 V 0.14 V 0.07 V 0.07 V
Output current into shorted load Channel 1: R139 & R140
Channel 2: R239 & R240 4–4.5 A 4–5 A 7.5–8.5 A 8.5–9 A
AC current when driving shorted load* Channel 1: R139 & R140
Channel 2: R239 & R240 3.75–4.5 A 4.5–5.5 A 4.5–5.5 A 5.5–6.5 A
Clipping voltage into 2 ohms (RMS)
Channel 1: Adjust R139
for symmetry
Channel 2: Adjust R239
for symmetry
26–29 V 33.5–37.5 V 42–44 V 44–49 V
Clipping voltage into 2 ohms (peak)
Channel 1: Adjust R139
for symmetry
Channel 2: Adjust R239
for symmetry
36.8–41 V 47.4–53 V
59.3 –62.2 V 62.2–69.3 V
Idle AC demand* (at ambient temperature;
higher when hot) 0.4 A,
±10% 0.4 A, ±10% 0.6 A, ±10% 0.6 A, ±10%
*Figures shown are for 120V amplifiers; multiply current by 0.5 for 230V or 1.2 for 100V.
Table 1: Bias and current limit adjustments

18 QSC Audio Products, LLC
5.1 Mechanical disassembly and
re-assembly
Replacing components will usually require removing the channel
modules and/or AC board from the amplifier chassis, especially on the
RMX850 and RMX1450, which have single-side printed circuit
boards. The RMX2450 has double-side boards; many of the through-
hole components on the upper side of the board can be unsoldered
and soldered from the top side of the board, so removing modules or
boards is not always necessary.
See the fold-out assembly guides on pages 25 and 26 in this
manual for assistance.
NOTE: As viewed from the front of the amplifier, the left channel
module is Channel 1 and the right is Channel 2 except in the
RMX2450; its right module is Channel 1, and its left is Channel 2.
Removing the channel modules
1. Disconnect the amplifier from AC power and allow at least 10
minutes for internal voltages to bleed down.
2. Using a Philips screwdriver, remove the screws that fasten the
top cover to the chassis. Also remove the top cover’s four
recessed screws that fasten it to the heat sinks. As you
remove screws, set them aside, but also make note of where
each type is used so you can properly re-assemble the
amplifier.
3. Lift the top cover up at the rear and carefully pull it toward the
back, removing the five hooks on the front edge from their slots
in the chassis.
4. Pull the gain control knobs straight off from the potentiom-
eter shafts.
5. Tip the amplifier up on its side and remove the four screws
that fasten the heat sinks to the chassis.
6. Set the amplifier back down and remove the screws that
mount the channel modules to the chassis standoffs. There
are six screws in the left module and five in the right one.
7. Remove the four screws that fasten the fan, fan shroud, and
fan guard to the chassis. Lift the fan shroud out from the
chassis; this will give you room to properly remove the
modules from the chassis.
8. Disconnect the wire and cable connections to the channel
modules. All of the connections are either detachable headers
or ¼-inch quick-connect tabs that are disconnected by pulling
them straight up. No unsoldering is necessary.
5. Servicing RMX amplifiers
9. Slide the channel modules toward the back so the potentiom-
eter shafts and front panel LEDs are clear of their holes in the
front panel. Lift the channel modules out from the chassis.
10. Re-assembly is the opposite of disassembly.
Removing the AC board
The AC board provides AC voltage selection, rectification of the
transformer secondary current, and a regulated DC supply for the
cooling fan. It seldom needs to be replaced unless it is physically
damaged itself. Most failures involving the AC board can be
repaired through replacement of individual components.
WARNING: Regulatory agencies require that any operating
voltage conversions from 120 volts to any other voltage be done
only by QSC’s factory service. Any other operating voltage
conversions may be done only by a QSC-authorized service center
or international distributor.
1. Disconnect the amplifier from AC power and allow at least 10
minutes for internal voltages to bleed down.
2. Remove the four screws that fasten the fan, fan shroud, and
fan guard to the chassis. Lift the fan shroud out from the
chassis.
3. Disconnect the wires that connect to the channel modules. All
of the large single wires attach to the channel modules with
¼-inch quick-connect tabs that are detached by pulling them
straight up. The remaining three black wires disconnect at the
left channel module with a detachable header.
If you are planning to replace the AC board with another,
carefully cut each of the transformer wires connecting to the
board just above its solder tab. You must leave enough slack
to allow connection to the new AC board. Remove the old
heat shrink tubing from the wires and strip the wire ends
about 0.25 inch or 6.3 mm.
4. Remove the five screws that attach the AC board to the
chassis standoffs. Lift the board out from the chassis.
5. Re-assembly is the opposite of disassembly. If you’re using a
new AC board, slide new pieces of heat shrink tubing over the
transformer wires before you solder them to the appropriate
tabs on the board; after soldering, cover the joints with the
tubing and use a heat gun or other heat source to shrink them
tightly.