QSC RMX-2450 Service manual

RMX™ Series

▲▲

▲ RMX 850

▲▲

▲ RMX 1450

▲▲

▲ RMX 2450

▲▲

Technical Service Manual

*TD-000098-00*

TD-000098-00

Rev. B

RMX Series

Technical Service Manual

RMX 850

RMX 1450

RMX 2450

QSC Audio Products, Inc.

Technical Services Group

Phone: 1-800 QSC AUDIO (1-800-772-2834) USA only

+1 (714) 957-7150 Fax: +1 (714) 754-6173 Postal: 1665 MacArthur Blvd.

Costa Mesa, California 92626 USA E-mail: tech_support@qscaudio.com Web: http://www.qscaudio.com (product information and support)

http://www.qscstore.com (parts and accessory sales)

Document # TD-000098-00, Rev. B. Released February 2002.

RMX Series Technical Service Manual 1

RMX Series Performance Specifications

RMX 850 RMX 1450 RMX 2450

OUTPUT POWER

in watts

FTC: 20 Hz–20 kHz @ 0.1% THD, both channels driven

8Ω per channel 185 260 450 4Ω per channel 280 400 650

EIA: 1 kHz @ 0.1% THD, both channels driven

8Ω per channel 200 280 500 4Ω per channel 300 450 750

1 kHz @ 1% THD, typical, both channels driven

2Ω per channel 430 700 1200

Bridge Mono:

8Ω, 20 Hz–20 kHz, 0.1% THD 530 800 1300 8Ω, 1 kHz, 0.1% THD 600 900 1500 4Ω, 1 kHz, 1% THD, typical 830 1400 2400

DYNAMIC HEADROOM 2 dB @ 4Ω DISTORTION

SMPTE-IM < 0.01% < 0.01% < 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 VOLTAGE GAIN 31.6× (30 dB) 40× (32 dB) 50× (34 dB) INPUT SENSITIVITY, V RMS

full rated power @ 8Ω 1.15v (+3.4 dBu) 1.15v (+3.4 dBu) 1.23v (+4.0 dBu)

INPUT IMPEDANCE 10 KΩ unbalanced

20 KΩ balanced

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, AC coupling, triac crowbar (on each channel) OUTPUT CIRCUIT TYPE AB AB H

AB: Class AB complementary linear output H: Class AB 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)

Net: 35 lb. (15.9 kg) 40 lb. (18.2 kg) 44.5 lb. (20.2 kg)

POWER REQUIREMENTS Available for 120 or 220–240 VAC, 50/60 Hz POWER CONSUMPTION

(both channels driven)

Multiply currents by 0.5 for 230V units

@ 120 VAC

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SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE

2 QSC Audio Products, Inc.

Table of Contents

RMX Series Performance Specifications ........................................................................................................................................ 4

1. Introduction ........................................................................................................................................................................................ 6

1.1 Service bulletins ................................................................................................................................................................................ 6

1.2 The well-equipped service bench ..................................................................................................................................................... 6

1.3 Working with surface-mount components ....................................................................................................................................... 6

1.4 Series description ............................................................................................................................................................................. 8

1.5 Technical descriptions and theory of operation ................................................................................................................................ 8

2. Component identification and pinout ........................................................................................................................................... 10

3. Troubleshooting: Symptoms, causes, & remedies ..................................................................................................................... 12

3.1 Excessive current draw ..................................................................................................................................................................... 12

3.2 Protection, muting, and turn-on/turn-off delay problems ................................................................................................................ 12

3.3 Faults with signal present ................................................................................................................................................................. 13

3.4 Instability ........................................................................................................................................................................................... 13

3.5 Power supply & rail balancing problems .......................................................................................................................................... 14

4. RMX calibration procedures .......................................................................................................................................................... 14

4.1 Setting bias ....................................................................................................................................................................................... 14

4.2 Setting positive and negative current limits ..................................................................................................................................... 15

5. Servicing RMX amplifiers ............................................................................................................................................................... 16

5.1 Mechanical disassembly and re-assembly ....................................................................................................................................... 16

6. Replacement parts ............................................................................................................................................................................ 17

6.1 RMX 850 Replacement Parts ............................................................................................................................................................ 17

6.2 RMX 1450 Replacement Parts .......................................................................................................................................................... 20

6.3 RMX 2450 Replacement Parts .......................................................................................................................................................... 23

7. Schematics and diagrams ............................................................................................................................................................... 27

7.1 RMX Assembly/Disassembly Diagram 1 of 2

7.2 RMX Assembly/Disassembly Diagram 2 of 2

7.3 RMX 850 Schematic Diagram 1 of 3

7.4 RMX 850 Schematic Diagram 2 of 3

7.5 RMX 850 Schematic Diagram 3 of 3

7.6 RMX 1450 Schematic Diagram 1 of 3

7.7 RMX 1450 Schematic Diagram 2 of 3

7.8 RMX 1450 Schematic Diagram 3 of 3

7.9 RMX 2450 Schematic Diagram 1 of 3

7.10 RMX 2450 Schematic Diagram 2 of 3

7.11 RMX 2450 Schematic Diagram 3 of 3

7.12 RMX 850 Chassis Wiring Diagram ................................................................................................................................................. 38

7.13 RMX 1450 Chassis Wiring Diagram ............................................................................................................................................... 39

7.14 RMX 2450 Chassis Wiring Diagram ............................................................................................................................................... 40

(All models) ...............................................................................................................

Channel 1 ...............................................................................................................................

Channel 2 ...............................................................................................................................

Power Supply .........................................................................................................................

Channel 1 .............................................................................................................................

Channel 2 .............................................................................................................................

Power Supply .......................................................................................................................

Channel 1 .............................................................................................................................

Channel 2 ...........................................................................................................................

Power Supply .....................................................................................................................

RMX Series Technical Service Manual 3

1. Introduction

1.1 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” (RMX 2450 only); RMX0002, “RMX Turn-on Delay” (all RMX models); RMX0003, “RMX 2450 Turn-off Mute” (RMX 2450); RMX0004, “RMX 2450 AC Wire Routing” (RMX 2450); RMX0005, “RMX 2450 AC Wire Replacement” (RMX 2450); and RMX0006, “RMX 2450 IRFZ44N Field Effect Transistors (RMX 2450).

1.2 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)

• Variable AC voltage source, such as a Variac or Powerstat variable transformer, 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)

• Rosin-core solder (60/40 or 63/37)

• 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.

1.3 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 1 Use two soldering irons, preferably about 25 to 40 watts, with fine tips.

2 With 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 1.1).

3 Once the solder melts on both ends, grip the component between the two tips and lift it from the circuit

board.

4 Use solder braid and a soldering iron to remove the solder from the two pads (Figure 1.2).

4 QSC Audio Products, Inc.

Figure 1.1.

Solder braid

Figure 1.2.

Solder

Figure 1.3.

Tweezers

Figure 1.4.

Figure 1.5.

Solder

Figure 1.6.

Solder

Insertion 1 With a soldering iron and 60/40 or 63/37 eutectic-type solder, melt just enough solder onto one pad to

create a small mound (Figure 1.3).

2 Grasp 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 1.4).

3 Hold the component in place and take the soldering iron away. Let the solder harden to tack the

component in place.

4 Fully solder the other end of the component to its pad. Let the solder harden (Figure 1.5). 5 Fully solder the tacked end of the component to its pad (Figure 1.6).

Three-terminal components (transistors, etc.)

Removal 1 With a soldering iron and solder braid, remove as much solder as possible from the middle terminal of

the component.

2 With a soldering iron in each hand, hold one tip on the solder at the terminal at one end of the compo-

nent and the other tip on the terminal at the other end.

3 When 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.

4 Use solder braid and a soldering iron to remove the solder from the three pads.

Insertion 1 With a soldering iron and 60/40 or 63/37 eutectic-type solder, melt just enough solder onto one pad to create a small mound of solder.

2 Grasp 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.

3 Hold the component in place and take the soldering iron away. Let the solder harden to tack the component in place. 4 Fully solder the other terminals of the component to their pads. Let the solder harden. 5 Fully 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:

1 Use a soldering iron and solder braid to remove as much solder as possible from the pins of the component. 2 With 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.

3 Repeat the process until all the pins are free and you can remove the component. Insertion 1 With a soldering iron and 60/40 or 63/37 eutectic-type 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.

2 Grasp 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.

RMX Series Technical Service Manual 5

1.4 Series description

QSC’s RMX Series amplifiers are entry-level professional audio products, designed for good, basic performance and reliability at low price. The series comprises three models: the RMX 850, RMX 1450, and RMX 2450. Each one has two audio channels and is two rack spaces tall. See page 2 for complete specifications.

The RMX 850 and RMX 1450 have single-sided printed circuit boards. The RMX 2450 uses double-sided boards.

1.5 Technical descriptions and

theory of operation

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

Figure 1.7

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 1.7. Note that unlike many bipolar supplies for complementary 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 RMX 2450, the secondaries are tapped to provide an intermediate set of bipolar rails for the Class H output circuitry. Figure 1.8 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

Figure 1.8

To Channel 2 Center Tap

+Vcc

-Vcc

+Vcc

-Vcc

+110V

-110V

+55V

-55V

Ch. 1 Center Tap

0.047 µF

12 5W×2Ω

Channel 1

Channel 2

Channel 1

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 1.9) 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.

C101

180p-5%

^C_0805

+IN_A

-IN_A

R100

10.0K

^R_0805

R101

10.0K

^R_0805

R102

^R_0805

To LM13600 operational

transconductance amp

Figure 1.9

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 a 13600 dual operational transconductance amplifier (Figure 1.10). 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.

The next active device is another 5532 op amp, U101:2. Its output drives the driver transistors, which in turn drive the output transistors. 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

R105

10.0K

^R_0805

NE5532

U101:1

R106

10.0K

^R_0805

C103

180p-5%

C106

10-50NP

R112

10K

10K LINEAR 3B RIGHT ANGLE POT

R113

270

^R_0805

CCW

6 QSC Audio Products, Inc.

To input op amp U101:1

+14V

R108

7.50K ^R_0805

R107

U10:1

LM13600M

R103

R104

10.0K ^R_0805

150K

^R_1206

-14V

C105

39K

^R_0805

100

R111

^R_0805

100-25V

R110

270

^R_0805

R109

100

3906

Q100

From clip detection

820

R115

CLIP LIMIT SWITCH (Open to defeat clip limiter)

^R_1206

Figure 1.10

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 2.2K trimpot 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, the arrangement of the output transistors might look somewhat strange: the positive voltage swings are handled by PNP transistors, while the negative swings are handled by NPN devices. 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 approcahes 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.

DC protection

The RMX 2450 has 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 RMX 850 and RMX 1450 are AC coupled.

Class H

The RMX 2450 utilizes 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 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.

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.

RMX Series Technical Service Manual 7

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 RMX 2450) 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 forwardbias 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.

2. Component identification and pinout

NE5532AN Dual operational amplifier

OUTPUT A

INVERTING

INPUT A

NON-INVERTING

INPUT A

V-

V+

OUTPUT B

INVERTING

INPUT B NON-INVERTING

INPUT B

LM311 Voltage comparator

GROUND 1

NON-INVERTING

INPUT

INVERTING

INPUT

V-

V+

OUTPUT

BALANCE/

STROBE

BALANCE

4N29 Opto-isolator

LM13600 Dual operational transconductance amplifier

AMP BIAS INPUT A

DIODE BIAS A

NON-INVERTING

INPUT A

INVERTING INPUT A

OUTPUT A

BUFFER INPUT A

BUFFER OUTPUT A

V-

AMP BIAS INPUT B

DIODE BIAS B

NON-INVERTING

INPUT B

INVERTING INPUT B

OUTPUT B

V+

BUFFER INPUT B

BUFFER OUTPUT B

2N5064 Sensitive gate thyristor

KAG

8 QSC Audio Products, Inc.

2N3904 (NPN) and 2N3906 (PNP) Small-signal transistors

BCE

IRFZ44 TMOS power field effect transistor

MAC224 Triac

MT1

MT2

MJE15032 (NPN) and MJE15033 (PNP) Driver transistors

GDS

2SC5200 (PNP) and 2SA1943 (NPN) Power transistors

TOSHIBA

BCE

RMX Series Technical Service Manual 9

B C E

3. Troubleshooting: Symptoms, causes, & remedies

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.

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 RMX 2450, 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 (RMX 2450) 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.

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.

3.2 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.

The amplifier will not thermally shut down when it overheats

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.

10 QSC Audio Products, Inc.

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.

• R130 or R132 (Channel 1) or R230 or R232 (Channel 2) is open.

Excessive hum in loudspeaker when no signal is present (RMX 850 and RMX 1450)

Check R118, R119, R218, and R219.

Excessive current draw with signal present (RMX 2450)

• 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

• (RMX 850 and RMX 1450) 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.

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 (RMX 2450 only), D115, and D116

(Channel 1) or R157, R258, R298 (RMX 2450 only), D215, and D216 (Channel 2).

• (RMX 850 and RMX 1450) Check R118 and R119 (Channel 1) or R218 and R219 (Channel 2).

3.4 Instability

The customer complains of gain problems, spurious noises, or oscillations.

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 distortion (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 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).

RMX Series Technical Service Manual 11

+ 29 hidden pages

QSC RMX-2450 Service manual

Specifications and Main Features

Frequently Asked Questions

User Manual