QSC Audio PL380, PL340, PL325 Service Manual

PowerLight3 Series

▲▲

▲ PL325

▲▲

▲ PL340

▲▲

▲ PL380

▲▲

QSC Audio Products, LLC Costa Mesa, CA 92626 USA

www.qscaudio.com

Technical Service Manual

*TD-000274-00*

Rev. A

PowerLight 3 Series

Technical Service Manual

PL325

PL340

PL380

QSC Audio Products, LLC

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)

PL3 Series Service Manual 1 TD-000274-00 Rev. A

PL325 and PL340 amplifier, rear view

PL380 amplifier, rear view

2 QSC Audio Products, LLC

Table of Contents

Tables and Figures ........................................................................................................................................................................ 4

Specifications................................................................................................................................................................................ 6

1. Introduction ................................................................................................................................................................................ 7

1.1 Restriction of Hazardous Substances Directive (RoHS) ............................................................................................................................................. 7

1.2 Service bulletins and updates .................................................................................................................................................................................... 7

1.3 The well-equipped service bench .............................................................................................................................................................................. 7

1.4 Working with surface-mount components................................................................................................................................................................. 8

1.5 PL380 Service Fixture ............................................................................................................................................................................................... 10

2. Technical Descriptions and Testing .................................................................................................................................... 14

2.1 PL380 Circuit Description ......................................................................................................................................................................................... 14

2.2 PL380 Major Circuit Blocks ...................................................................................................................................................................................... 16

2.3 PL380 Test Procedure ............................................................................................................................................................................................... 21

2.4 PL340 Test and Calibration Procedure ..................................................................................................................................................................... 24

2.5 PL325 Test and Calibration Procedure ..................................................................................................................................................................... 27

3. Troubleshooting ....................................................................................................................................................................... 31

3.1 PL380: Symptoms, causes, and remedies ................................................................................................................................................................ 31

3.2 PL325 and PL340: Symptoms, causes, and remedies .............................................................................................................................................. 39

PLC Power Supply Restoration Kit ................................................................................................................................................................................. 43

4. Printed Circuit Boards............................................................................................................................................................ 53

4.1 PL325 and PL340 Amplifier ...................................................................................................................................................................................... 53

Main module circuit board assembly ............................................................................................................................................................................................................ 53

Input module circuit board assembly ............................................................................................................................................................................................................ 56

4.2 PL380 Amplifier ........................................................................................................................................................................................................ 58

5. Schematic Diagrams .............................................................................................................................................................. 64

5.1 PL325 Schematics .................................................................................................................................................................................................... 64

5.2 PL340 Schematics .................................................................................................................................................................................................... 72

5.3 PL380 Schematics .................................................................................................................................................................................................... 80

6. Replacement parts .................................................................................................................................................................. 89

6.1 Semiconductor package descriptions and pinouts .................................................................................................................................................. 89

6.2 PL325 parts and assemblies ..................................................................................................................................................................................... 93

PL325 Power Amplifier (120V) (QSC part # FG-032500-00) ......................................................................................................................................................................... 93

PL325 Power Amplifier (100V) (QSC part # FG-032500-01) .......................................................................................................................................................................... 93

PL325 Power Amplifier (230V) (QSC part # FG-032500-02) ......................................................................................................................................................................... 93

Chassis Assembly PL325 (120V) (QSC part # WP-032500-00) ...................................................................................................................................................................... 93

Chassis Assembly PL325 (100V) (QSC part # WP-032500-01) ...................................................................................................................................................................... 94

Chassis Assembly PL325 (230V) (QSC part # WP-032500-02) ...................................................................................................................................................................... 95

PCB Assembly PL325 (120V) (QSC part # WP-032501-00) ............................................................................................................................................................................ 95

PCB Assembly PL325 (100V) (QSC part # WP-032501-01) ............................................................................................................................................................................ 98

PCB Assembly PL325 (230V) (QSC part # WP-032501-02) .......................................................................................................................................................................... 101

Input Assembly PL325 (QSC part # WP-032502-00) ................................................................................................................................................................................... 105

6.3 PL340 parts and assemblies ................................................................................................................................................................................... 106

PL340 Power Amplifier (120V) (QSC part # FG-034000-00) ........................................................................................................................................................................ 106

PL340 Power Amplifier (100V) (QSC part # FG-034000-01) ........................................................................................................................................................................ 106

PL340 Power Amplifier (230V) (QSC part # FG-034000-02) ........................................................................................................................................................................ 106

Chassis Assembly PL340 (120V) (QSC part # WP-034000-00) .................................................................................................................................................................... 106

Chassis Assembly PL340 (100V) (QSC part # WP-034000-01) .................................................................................................................................................................... 107

Chassis Assembly PL340 (230V) (QSC part # WP-034000-02) .................................................................................................................................................................... 108

PCB Assembly PL340 (120V) (QSC part # WP-034001-00) .......................................................................................................................................................................... 108

PCB Assembly PL340 (100V) (QSC part # WP-034001-01) .......................................................................................................................................................................... 112

PCB Assembly PL340 (230V) (QSC part # WP-034001-02) .......................................................................................................................................................................... 115

Input Assembly PL340 (QSC part # WP-034002-00) ................................................................................................................................................................................... 118

6.4 PL380 parts and assemblies ................................................................................................................................................................................... 120

PL380 Power Amplifier (120V) (QSC part # FG-038000-00) ........................................................................................................................................................................ 120

PL380 Power Amplifier (100V) (QSC part # FG-038000-01) ........................................................................................................................................................................ 120

PL380 Power Amplifier (230V) (QSC part # FG-038000-02) ........................................................................................................................................................................ 120

PL3 Series Service Manual 3 TD-000274-00 Rev. A

Table of Contents (continued)

Chassis Assembly PL380 (120V) (QSC part # WP-038000-00) .................................................................................................................................................................... 120

Chassis Assembly PL380 (100V) (QSC part # WP-038000-01) .................................................................................................................................................................... 121

Chassis Assembly PL380 (230V) (QSC part # WP-038000-02) .................................................................................................................................................................... 122

PCB Assembly PL380 (120V) (QSC part # WP-038001-00) through January 2008 ..................................................................................................................................... 122

PCB Assembly PL380 (100V) (QSC part # WP-038001-01) through January 2008 ..................................................................................................................................... 126

PCB Assembly PL380 (230V) (QSC part # WP-038001-02) through January 2008 ..................................................................................................................................... 130

PCB Assembly PL380 (120V) (QSC part # WP-038001-00) from February 2008– ....................................................................................................................................... 134

PCB Assembly PL380 (100V) (QSC part # WP-038001-01) from February 2008– ....................................................................................................................................... 138

PCB Assembly PL380 (230V) (QSC part # WP-038001-02) from February 2008– ....................................................................................................................................... 142

Tables and Figures

PL325 and PL340 amplifier, rear view ..................................................................................................................................................................... 2

PL380 amplifier, rear view ....................................................................................................................................................................................... 2

Table 1.1. Load resistor bank switch truth table ..................................................................................................................................................... 7

Figure 1.1. Load resistor bank ................................................................................................................................................................................. 7

Figure 1.2. Use two irons......................................................................................................................................................................................... 8

Figure 1.3. Soak up solder ....................................................................................................................................................................................... 8

Figure 1.4. Apply new solder ................................................................................................................................................................................... 8

Figure 1.5. Place component ................................................................................................................................................................................... 8

Figure 1.6. Solder one end of the component ......................................................................................................................................................... 8

Figure 1.7. Solder other end .................................................................................................................................................................................... 8

Figure 1.8. The PL380 service fixture .................................................................................................................................................................... 10

Figure 1.9. Locations of Test Point A and Test Point B.......................................................................................................................................... 10

Figure 1.10. Jumpers to be removed from Test Point B ........................................................................................................................................ 11

Figure 1.11. Connecting the hookup leads ............................................................................................................................................................ 11

Figure 1.12. Schematic diagram of the PL380 service fixture .............................................................................................................................. 12

Figure 1.13. Proper IGBT drive signals (with chassis ground reference ) ............................................................................................................. 13

Figure 1.14. Proper FET gate drive signals with FETs installed (with chassis ground reference) ....................................................................... 13

Figure 2.1. Signal with 250 kHz switching noise .................................................................................................................................................. 21

Figure 2.2. Signal with switching noise filtered out ............................................................................................................................................. 21

Figure 2.3. Burst sine wave signal for 2Ω power testing ..................................................................................................................................... 21

Figure 2.4. Noise and distortion residual with bias properly set .......................................................................................................................... 25

Figure 3.1 IGBT gate drive waveforms .................................................................................................................................................................. 32

Figure 3.2 FET gate drive waveforms .................................................................................................................................................................... 33

Figure 3.3 Dead time between one pulse turning off and the other turning on should be about 20–30 ns at the 5 V level. ............................. 33

Figure 3.4 The clock drive logic signals................................................................................................................................................................. 33

Figure 3.5 The power supply sync pulse. .............................................................................................................................................................. 34

Figure 3.6 Triangle wave at comparator inputs (pin 2 of U8 and U28). ................................................................................................................ 35

Figure 3.7 Triangle wave with supply rails energized. .......................................................................................................................................... 35

Figure 3.8. Switching pulse at node N401 (pin 3 of U19) ..................................................................................................................................... 39

Figure 3.9: Switching signal with dead time at nodes N397 and 398 (pins 11 and 14 of U19). .......................................................................... 40

Figure 3.10. Typical crossover residual from distortion analyzer output .............................................................................................................. 41

Figure 3.11. Identify damaged transistors by measuring resistance across the collector and emitter. .............................................................. 43

Table 3.1. Clamping voltage troubleshooting ....................................................................................................................................................... 44

Table 3.2. Troubleshooting clamp malfunctions ................................................................................................................................................... 45

Figure 3.12. The overcurrent detection circuit for power supply cutback. ........................................................................................................... 46

Table 3.3. Troubleshooting clamp transistors ....................................................................................................................................................... 46

4 QSC Audio Products, LLC

Tables and Figures (continued)

Table 3.4. Troubleshooting short circuit cutback clamping ................................................................................................................................... 47

Figure 3.13. Output signal and positive rail steps. ............................................................................................................................................... 48

Figure 3.14. Output signal and negative rail steps. .............................................................................................................................................. 48

PL325 and PL340 main circuit board, with major circuit sections shown ............................................................................................................ 53

PL325 and PL340 main circuit board, top layer ..................................................................................................................................................... 54

PL325 and PL340 main circuit board, bottom layer, mirror image ........................................................................................................................ 55

PL325 and PL340 input board, top layer ............................................................................................................................................................... 56

PL325 and PL340 input board, bottom layer, mirror image ................................................................................................................................... 57

PL380 main circuit board, with major circuit sections shown .............................................................................................................................. 58

PL380 main circuit board, detailed view ............................................................................................................................................................... 59

PL380 input board, main circuit blocks shown ...................................................................................................................................................... 59

PL380 main circuit board, top layer ....................................................................................................................................................................... 60

PL380 main circuit board, mid1 layer .................................................................................................................................................................... 61

PL380 main circuit board, mid2 layer .................................................................................................................................................................... 62

PL380 main circuit board, bottom layer ................................................................................................................................................................ 63

PL325 schematic notes .......................................................................................................................................................................................... 64

Schematic sheet INPUT, PL325 ............................................................................................................................................................................. 65

PL325 wiring diagram ............................................................................................................................................................................................ 66

PL325 schematic guide .......................................................................................................................................................................................... 67

Schematic sheet AMP CH-A, PL325...................................................................................................................................................................... 68

Schematic sheet AMP CH-B, PL325 ...................................................................................................................................................................... 69

Schematic sheet PROTECT/CONTROL, PL325 ...................................................................................................................................................... 70

Schematic sheet POWER SUPPLY, PL325 ............................................................................................................................................................. 71

PL340 schematic notes .......................................................................................................................................................................................... 72

Schematic sheet INPUT, PL340 ............................................................................................................................................................................. 73

PL340 wiring diagram ............................................................................................................................................................................................ 74

PL340 schematic guide .......................................................................................................................................................................................... 75

Schematic sheet AMP CH-A, PL340...................................................................................................................................................................... 76

Schematic sheet AMP CH-B, PL340 ...................................................................................................................................................................... 77

Schematic sheet PROTECT/CONTROL, PL340 ...................................................................................................................................................... 78

Schematic sheet POWER SUPPLY, PL340 ............................................................................................................................................................. 79

PL380 schematic notes .......................................................................................................................................................................................... 80

PL380 wiring diagram ............................................................................................................................................................................................ 81

PL380 schematic guide .......................................................................................................................................................................................... 82

Schematic sheet AMP CH-A, PL380 through January 2008 ................................................................................................................................. 83

Schematic sheet PROTECT/CONTROL, PL380 ...................................................................................................................................................... 84

Schematic sheet AMP CH-B, PL380 through January 2008 ................................................................................................................................. 85

Schematic sheet POWER SUPPLY, PL380 through January 2008 ......................................................................................................................... 86

Schematic sheet INPUT, PL380 through January 2008 ........................................................................................................................................ 87

Schematic sheet AMP CH-A, PL380 from February 2008– ................................................................................................................................... 88

Schematic sheet AMP CH-B, PL380 from February 2008– ................................................................................................................................... 89

Schematic sheet POWER SUPPLY, PL380 from February 2008–........................................................................................................................... 90

Schematic sheet INPUT, PL380 from February 2008– .......................................................................................................................................... 91

PL3 Series Service Manual 5 TD-000274-00 Rev. A

Specifications

PL325 PL340 PL380

Stereo Mode (both channels driven)

8Ω / EIA 1 kHz / 1% THD 4Ω / EIA 1 kHz / 1% THD 2Ω / EIA 1 kHz / 1% THD

Bridge Mono Mode 8Ω / EIA 1 kHz / 1% THD 4Ω / EIA 1 kHz / 1% THD

Typical Distortion

(20 Hz–20 kHz, 3 dB below clip, or

20 Hz–5 kHz, 10 dB below clip, or

20 Hz–20 kHz, 20 dB below clip)

Maximum Distortion

(20 Hz–20 kHz, 1 dB below rated power)

Frequency Response (8Ω) 20 Hz–20 kHz, ±0.2 dB 20 Hz–20 kHz, ±0.2 dB 20 Hz–20 kHz, ±0.2 dB

Noise (20 Hz–20 kHz, 32 dB gain) -106 dB -105 dB -104 dB

Dynamic Headroom (4Ω) 2 dB 2 dB 2 dB

Damping Factor (8Ω) 500 500 200

Output Circuitry Class H (2-tier) Class H (2-tier) Class D

Input Sensitivity @ max gain

(26 dB setting) (32 dB setting)

Maximum Gain (1.2 V setting) 34.5 dB 36.4 dB 39.1 dB

Input Impedance >10 kΩ, balanced or unbalanced >10 kΩ, balanced or unbalanced >10 kΩ, balanced or unbalanced

Maximum Input Level

(1.2 V setting) (32 dB setting) (26 dB setting)

Controls and LEDs—Front Panel Common: AC Power Switch, Power LED (blue), Br Mono LED (yellow), Par LED (orange)

Controls and LEDs—Rear Panel Common: Input Mode Parallel LED (orange), Stereo LED (green), Br Mono LED (yellow); Sensitivity 26

Input Connectors Common: HD15 DataPort (inputs parallel with XLR inputs)

Output Connectors Each Channel: 5-Way Binding Posts, Neutrik Speakon® (upper has both channel outputs)

Amplifier and Load Protection Short circuit, open circuit, thermal, RF protection. On/off muting, DC fault shutdown, active inrush current limiting

AC Power** / Cordset

120 V, 50–60 Hz

230 V 50 Hz

Dimensions Height: 2 RU 3.5" (8.9 cm); Width: 19" (48.3 cm); Depth: 15.63" (39.7 cm) from front mounting rails

Weight Net / Shipping 22 lb (10 kg) / 31.5 lb (14.3 kg) 22 lb (10 kg) / 31.5 lb (14.3 kg) 24 lb (11 kg) / 33.5 lb (15.2 kg)

Agency Approvals UL, CE, RoHS / WEEE compliant, FCC Class B (conducted and radiated emissions)

* Burst mode testing required due to AC service current limitations ** Representative of current draw with tyical music program material with occasional clipping

All specifications are subject to change without notice.

500 W 850 W 1250 W

1700 W 2500 W

8Ω

0.002–0.01%

4Ω

0.005–0.01%

2Ω

0.02%

4–8Ω 0.05% 0.05% 0.20%

3.28 V (+12.5 dBu)

1.60 V (+6.3 dBu)

11 V (+23 dBu)

14.6 V (+25.5 dBu) 25 V (+30 dBu)

Each Channel: Signal -35 dB LED (green), -20 dB LED (green), -10 dB LED (orange), Clip/Prot LED (red), Gain Control (21 detents, 1 dB steps)

LED (orange), 32 dB LED (green), 1.2 V LED (yellow) Each Channel: LF Filter Switch off–30 Hz (yellow LED)–50 Hz (red LED); Clip Limiter Switch off–on (yellow LED)

Each Channel: Male XLR, Female XLR, 3-Pin Terminal Block

8.5 A / NEMA-15

7.5 A / Euro 16 A

800 W 1250 W 2000 W

2600 W 4000 W

0.002–0.01%

0.005–0.01%

0.02%

3.92 V (+14 dBu)

1.96 V (+8.1 dBu)

11 V (+23 dBu)

14.6 V (+25.5 dBu) 25 V (+30 dBu)

12 A / NEMA-15 7 A / Euro 16 A

1500 W 2500 W 4000 W*

5000 W 8000 W*

0.01–0.03%

0.03–0.06%

0.10%

5.27 V (+16.7 dBu)

2.67 V (+10.7 dBu)

10 V (+22 dBu) 22 V (+29 dBu) 25 V (+30 dBu)

18 A / L5-30 (30 A twist-lock) 11 A / Euro 16 A

6 QSC Audio Products, LLC

1. Introduction

SW1 SW2 SW3

SW4

8Ω 8Ω 8Ω 8Ω

R1, R2: 8 , 1500W non-inductiveΩ R3, R4: 8 , 1000W min. non-inductiveΩ

R1 R2

R3 R4

1.1 Restriction of Hazardous Substances Directive (RoHS)

All PowerLight 3 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 noncompliant 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 noncompliant product or solder, you can no longer use it with RoHS-compliant products or solder.

1.2 Service bulletins and updates

Contact QSC Technical Services to make sure you have the most up-to-date service bulletins, schematics, and parts lists for PowerLight 3 Series amplifiers. Service materials 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.

At the time of this manual’s publication, one service bulletin, PL30001 amplifiers.

(PL380 and Standby),

had been released for the PowerLight 3

1.3 The well-equipped service bench

To properly service PowerLight 3 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

• Low-distortion audio sine wave generator

• Audio distortion analyzer

• Switching amplifier measurement filter, such as Audio Precision AUX-0025

• Bank of non-inductive load resistors, configurable as 8 ohms (min. 1500 watts capacity), as 4 ohms (min. 3000 watts), and 2 ohms (min. 4000 watts). See Figure 1.1.

• Variable AC voltage source, such as a Variac or Powerstat variable transformer; For PL325 and PL340: rated current capacity of up to 25 A (for 120 V models) or 12 A (for 230 V models) For PL380: rated current capacity of up to 50 A (for 120 V models) or 25 A (for 230 V models)

• Philips and flat screwdrivers

Figure 1.1. Load resistor bank

Table 1.1. Load resistor bank switch truth table

SW1 SW2 SW3 SW4

∞Ω (no load)

0Ω (short circuit)

OFF•••

8Ω

4Ω

2Ω

ON OFF OFF OFF

ON ON OFF OFF

ON ON ON OFF

ON • • ON

PL3 Series Service Manual 7 TD-000274-00 Rev. A

1.3 The well-equipped service bench (continued)

• Soldering iron with a fine tip (25–60 W recommended)

• RoHS-compliant rosin-core solder

• Long-nose pliers

• Diagonal cutters

• Wire strippers

• PL380

Automated test equipment, such as an Audio Precision workstation, is very useful for servicing QSC amplifiers. Contact QSC Technical Services to obtain applicable AP test files.

1.4 Working with surface-mount components

Figure 1.2. Use two irons

Solder braid

PowerLight 3 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 are used in the amplifiers’ 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.

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 1.2).

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 1.3).

Insertion 1 With a soldering iron and RoHS-compliant solder, melt just enough solder onto one pad to create a small

mound (Figure 1.4).

Figure 1.3. Soak up solder

Solder

Figure 1.4. Apply new solder

Tweezers

Figure 1.5. Place component

Solder

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 1.5).

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

nent in place.

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

5 Fully solder the tacked end of the component to its pad (Figure 1.7).

Figure 1.6. Solder one end of the component

Solder

Figure 1.7. Solder other end

8 QSC Audio Products, LLC

1.4 Working with surface-mount components (continued)

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 component 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 RoHS-compliant 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 RoHS-compliant 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.

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.

PL3 Series Service Manual 9 TD-000274-00 Rev. A

1.5 PL380 Service Fixture

With its class D output section, the PL380 amplifier differs from all previous QSC stand-alone models, which up to now have used linear output circuitry in either a class AB configuration or a class AB-based class G or H one.

To the end user, these differences should not be apparent, except that he or she may notice that such a high-power amplifier does not generate much heat and appears to consume much less electricity than might be expected. The PL380 amplifier should behave sonically like a high-quality, high-power audio amplifier.

Being a class D amplifier, the PL380 uses pulse-width modulation to allow output transistors that are either fully on or fully off to produce varying output voltages. To reduce noise, the clock for the output sections’ modulators is synchronous with the power supply’s clock. However, that interdependence makes testing and troubleshooting one section of the amplifier without the other impossible. This is the reason for the PL380 service fixture (Figure 1.8). It is necessary for many of the procedures described in Chapter 2’s section on the PL380 test procedure, and in Chapter 3’s sections on PL 380 troubleshooting.

Figure 1.12 shows the schematic for the PL380 service fixture. The fixture is available for purchase from QSC Technical Services.

Figure 1.8. The PL380 service fixture

abnormal situations such as defective op amps or other circuitry that could cause abnormally high or low current demand.

• Monitors the +5-volt suppy. The 5-volt regulated supply

powers the clock and logic circuitry. Because it is derived from other higher voltage DC supplies, the presence of the voltage on the screw terminal indicates that they also are functioning.

• Verifies clock switching. The “Sync Sig” terminal should

carry a 250 kHz pulse train signal. Its presence verifies that the clock oscillator and divider circuits are operating. Because of the cables connecting the fixture to the amp, the pulse train will tend to be messy, with significant ringing. Therefore, the signal is useful only to verify the operation of the amp’s circuitry.

Hooking up the service fixture

These steps describe how to set up the service fixture and connect it to the PL380.

Prepare the service fixture

1. Set the service fixture on the right side of your test bench work area, with the screw terminals and hookup leads facing toward you. Setting the fixture this way makes it nearly impossible to connect the hookup leads the wrong way.

Functions of the service fixture

• Substitutes for the amplifier’s housekeeping supply. The housekeeping supply

powers the clock, power supply switching, and modulation circuitry. The fixture allows you to operate and check these key areas of the amplifier’s circuitry even without its being connected to the AC mains.

It also allows you to operate the amplifier for testing and troubleshooting at low AC mains voltages that would be less likely to cause damage if a fault exists.

• Monitors the ±15-volt rail currents. The terminal strip on the service fixture provides precision voltages that are analogous to the currents drawn by the positive and negative 15volt supply rails. The voltages are scaled to 1 volt = 1 ampere. This is useful for detecting

10 QSC Audio Products, LLC

Figure 1.9. Locations of Test Point A and Test Point B

TESTPOINTA

Test Point A

Test Point B

TESTPOINT B

1.5 PL380 Service Fixture (continued)

2. Turn off the service fixture’s power switch and connect it to the AC mains. The service fixture has a universal power supply that can operate on any AC voltage from 100 to 240 volts.

Prepare the amplifier

3. Disconnect anything connected to the amplifier’s inputs and outputs.

4. Disconnect the amplifier from the AC mains and allow at least five minutes for the internal voltages to bleed down.

5. Remove the amplifier’s bottom cover.

6. Set the amplifier on the test bench next to the service fixture, with its open side up and front panel facing

Figure 1.10. Jumpers to be removed from Test Point B

you.

7. Locate the two test points on the amplifier’s main circuit board. Test Point A is located near the output board, while Test Point B is in front of the power transformer (Figure 1.10).

8. Remove the two jumpers from Test Point B (Figure 1.11). Set them aside to be re-installed later.

Pin 1

TESTPOINT B

Remove jumpers on pins 3–4 and 5–6 before connecting service fixture. Replace jumpers for normal operation.

Initial tests with the service fixture

These initial tests with the service fixture will allow you to determine whether the amplifier’s clock and control circuitry are working properly, with no risk of damaging high-power devices or circuits.

Tools and equipment needed

• Oscilloscope (2 channels minimum) and probes

• Digital multimeter (frequency counter is a plus)

Service fixture tests

1. Start-up sequence—Turn on the service fixture’s power

switch and watch the amplifier’s LED display on the front panel. The clip/protect indicator LEDs should light briefly and then go out. (If they stay on, then it is usually because an audio signal is being put into the amplifier inputs. Disconnect any input signals for this stage of testing.)

The amplifier should start with its normal sequence. You should hear a small relay click after about two seconds, followed by a large relay click about two seconds after that.

2. Regulated +5 volts—Check for the regulated 5 volts DC at

the “+5V REG.” terminal (reference to the fixture’s ground

Do not connect amplifier

to AC mains yet.

Connect the fixture and amplifier

9. Connect the service fixture’s two hookup leads to the corresponding

Test Point A

test points. Make sure you orient the housings correctly (as shown in Figure 1.12) when you plug them onto the test point headers.

As the lengths of the leads suggest, the long one goes to Test Point A and the short one to Test Point B .

Figure 1.11. Connecting the hookup leads

PL3 Series Service Manual 11 TD-000274-00 Rev. A

TESTPOINTA

Test Point B

TESTPOINT B

Remove two jumpers from Test Point B. Connect long lead to Test Point A and short lead to Test Point B.

To AC mains 100–240 VAC 50–60 Hz

QTY ITEM NO. PART NO. DESCRIPTION VENDOR

fixture

diagram of the PL380 service

Figure 1.12. Schematic

12 QSC Audio Products, LLC

1.5 PL380 Service Fixture (continued)

terminal). The voltage is derived from other low-voltage power supplies, so its presence indicates that the other supplies are probably good.

If the voltage is not present, then check the other supplies and associated circuitry to find the fault. For more detailed troubleshooting, see Section 3.1.

3. Power supply sync signal—Connect an oscilloscope to the

sync signal terminal on the service fixture. You can use the ground terminal on the fixture for the scope probe’s ground clip.

You should see a steady 250 kHz pulse train. The purpose of this terminal is simply to verify that the clock and divider circuits are working; the signal here is not clean and has a significant amount of ringing, but that is not important.

If the signal is not present, then check the clock oscillator circuit (74HC4060 ripple counter U1, Y1, C3, C8, R4, R10, et al; on the schematic, see sheet the dividers and complementary logic generators (74HC74 flipflops U4:1, U4:2, U7:1, and U7:2; same sheet, zones C-7 and C-

6), and the Schmitt inverters U5:1 and U5:2. For more detailed troubleshooting, see Section 3.1.

4. Low-Voltage Supply Current—Measure the voltages on the

+15 V and -15 V current monitoring points. These terminals present voltage analogs of the current demand on the lowvoltage supply rails, scaled to 1 volt = 1 ampere. For example, a voltage of 0.55 volt on the -15 V terminals would indicate that

0.55 A was being drawn from that supply rail.

Typically, the currents drawn should be approximately 400 mA on the negative rail and 500 mA on the positive when the amplifier is muted (during the start-up sequence, for example). When the output FET gates are being driven, the current should increase to about 500 mA on the negative rail and 800 mA on the positive one.

If the currents drawn appear to differ substantially from these figures, there may be a fault in the low-voltage circuitry.

5. IGBT drive—The service fixture allows you to use an

oscilloscope to check the drive signals to the gates of the power supply IGBTs, Q68 and Q69. Figure 1.13 shows what the signals should look like. For more details on the IGBT drive waveforms, please see the section in Chapter 3.

Troubleshooting Serious Failures

“AMP CH-A,”

zones C-8 and D-8),

Troubleshooting Serious Failures

Figure 1.13. Proper IGBT drive signals (with chassis ground reference )

Figure 1.14. Proper FET gate drive signals with FETs installed (with chassis ground reference)

6. FET drive—The service fixture also makes it possible to check

and verify the drive signals to the gates of the output FETs, Q10, Q11, Q58, and Q59 (see Figure 1.14). However, to get at the FET gates you must remove the amplifier module (the main circuit board assembly) from the chassis.

For more details on the FET drive waveforms, please see the

Troubleshooting Serious Failures

section in Chapter 3.

PL3 Series Service Manual 13 TD-000274-00 Rev. A

2. Technical Descriptions and Testing

2.1 PL380 Circuit Description

The PL-380 uses an unregulated, open-loop switchmode power supply that provides DC power to a pair of high-power Class D amplifier channels. A master clock circuit synchronizes and regulates the switching frequencies of all power supply and audio amplification sections. Various input circuits, controls and displays, protection systems, and output monitoring connect to the various subsystems.

Power Supply

There are three distinct power supply domains in this amplifier.

• The main power supply provides ±185 volts at high current to

the main rails of the output section.

• The auxiliary power supply is derived from the main supply,

providing ±16-volt rails that are used by the cooling fan and various low-voltage circuit blocks. Note that this supply tracks the voltage and on/off status of the main supply.

• The "housekeeping" supply (labeled on the schematic as a

“keep-alive” supply) uses a separate low-power flyback system to provide power to functions that need to be active during standby.

The main power supply uses a large, center-tapped primary-side reservoir capacitor bank (C209, C210, C213, C214, C216, and C217) charged to DC voltages of about ±165 V by an offline (direct from the AC mains) rectifier, BR1. The rectifier is wired as a voltage doubler for 120 V AC operation, and as a full-wave bridge for 230 V, and thus either configuration results in about the same DC voltage on the reservoir.

The main switching devices, D68 and D69, are mounted along with the rectifiers on one of the two smaller heat sinks and operate at a frequency of 125 kHz to alternately couple the positive and negative reservoir voltages (reference to the center tap of the reservoir, node PRICAPCT) through T2:1, which are the primary windings of the main power transformer, an E55-core isolation type. Capacitors C231 and C232, together with the inductance of the primary, form a series resonant tank that shapes the voltage waveform across the primary into a pseudo-sinusoid instead of a square wave.

The transformer’s secondary voltages are rectified into a bank of eight capacitors (Channel 1: C247–C250; Channel 2: C253, C254, C259, and C260), which store nominal DC voltages of about ±185 V. The secondary HF (fast) rectifiers D82–D85 are mounted on a second, smaller heat sink.

The auxiliary supply uses single-turn taps on the main transformer T2, with surface-mount rectifiers and small electrolytic capacitors, to produce a bipolar pair of DC rails of approximately ±16 V.

The housekeeping flyback supply operates independently of the

high-power main supply, and powers certain circuit functions that allow the main supply to operate.

Turn-On Sequence

The AC power switch spans points W17 and W18 and controls power only to the housekeeping supply, which has a much smaller DC reservoir (C190 and C191) than the main does. When switched on, this supply starts in about a half second and provides secondaryside DC power to the crystal-controlled clock circuit (U1, Y1, and associated components) and main supply controller IC (U49), resulting in 125 kHz drive to the main power supply switches.

After a short delay, the inrush-current relay, K2, closes and charges the main supply’s reservoirs through large NTC resistors, R262 and R266, which prevent drastic inrush current surges. During this time, the primary and secondary reservoirs charge smoothly in unison because the power supply switches are already operating. After about one second, the main power relay, K1, closes and couples the AC rectifiers directly to the primary reservoir, bypassing the inrush resistors and providing full power. There is an additional 1-second delay to allow all internal voltages to settle before the amplifier comes out of muting.

Turn-off behavior

When power is removed, the amplifier mutes promptly, but the power supply switches continue operating until primary and secondary DC voltages have discharged to about 20%. There is an OR-diode from the main reservoir to the housekeeping reservoir that maintains voltage to the aux supply during this period. When this DC voltage drops below about 45 V, the “housekeeping” supply stops switching and the amp shuts off fully. This sequence of delays and controlled inrush current will occur on all starts and restarts, without causing adverse transients.

AC Cordsets

The 230 V version of the PL380 uses a 20 A/250 V rated Neutrik Powercon® AC inlet, and the provided cordset uses 1.5 mm² × 3 wiring with a 16 A/250 V plug. The 120 V version uses a fixed 12/3 jacketed cable with a 30 A twist-lock plug. (We expect to use the 30 A 120 V PowerCon connector when it becomes available.)

Due to the high efficiency of Class D circuitry, there is a greater difference between peak operating current and average operating current at 1/8 of maximum power, so there is an extra margin of current carrying capacity in the AC components.

Amplifier Channels

Each of the two amplifier channels uses two large switching FETs operating as a half-bridge at a clock frequency of 250 kHz. The FETs

14 QSC Audio Products, LLC

2.1 PL380 Circuit Description (continued)

are mounted under the single large heat sink, together with HF diodes that clamp the output voltage when their respective FETs turn off.

At idle (i.e., no audio signal), the duty cycle of the drive signal is 50/ 50 and the output voltage is zero with respect to the secondary reservoir center tap. As with all Class-D (PWM) amplifiers, turning on the positive side switch for longer intervals and the negative side for shorter ones will push the output voltage proportionally towards the positive rail, and turning on the low-side switch for longer intervals will lower the output voltage towards the negative rail. When either switch’s “on” time reaches 100%, the amplifier has reached the equivalent of clipping and the output voltage will be equal to that supply rail’s voltage.

Both switches must be operated in alternation, with very exact synchronization to prevent cross-conduction (both switches on simultaneously) or excessive “dead time” (both switches off). A complex, optically coupled, separately powered gate drive circuit for each FET receives timing signals from the modulator and provides several amps of drive current to rapidly charge and discharge the FET gates. If there is any significant disorder in this circuitry, leading to both FETs turning on at once, immediate failure is likely.

Output Filter

The pulse width modulated voltage must be filtered before it can be connected to external loudspeakers. The main low-pass filter uses a toroidal inductor and high-quality film capacitor (L1 with C64, and L3 with C171) on each channel output, along with additional trap components to further reduce switching interference at 250 kHz to approximately 55 dB. Some distortion analyzers may still have difficulty reading the distortion with this much interference present. Any available HF filters should be engaged to remove as much of this interference as possible.

Output Connections

A twisted pair of wires from each channel couple the high-current output signal to the speaker connectors, which use parallel 30 A binding posts and 30 A Neutrik Speakon connectors. The output peak voltage swing can reach 200 V, and output current can peak at 80 A, but internal limiter circuits and the normal dynamics of music program will prevent long-term currents in excess of the connector ratings. Technicians should be aware of the amplifier’s potentially hazardous output voltages when making bench connections.

Input Connections

Input connections are balanced XLR in parallel with “euro-block” screw terminals and the DataPort™ inputs. There are six input panel switches: a three-position input sensitivity switch (1.2 V input, 32 dB, or 26 dB), which combines with the 21-detent front panel

gain controls to regulate overall gain as desired; a three-position Parallel-Stereo-Bridge Mono switch; two three-position lowfrequency filter (high-pass) settings, and two clip limiter enable switches. LEDs of different colors indicate certain switch selections, so the end user can easily check the setup with a quick glance.

QSC DataPort

The PL380 uses the same type of HD-15 connector used on other QSC DataPort amplifiers to connect to QSControl devices such as Basis processors. Two changes for the PowerLight 3 series are:

• Input signals from the DataPort are directly parallel with the XLR and euro-block inputs, allowing the signals to be patched to other amps for greater flexibility.

• The sensitivity switch settings are visible to the Basis processor.

Protection Systems

As with all audio amplifiers, full rated power is only required for brief peaks in the audio program, and typical use rarely exceeds 1/8 of full power, when averaged over some time. Therefore the amplifier must allow high peak power to flow for short periods of time but also provide longer-term protective systems that limit this power to reasonable levels. The amplifier’s protection relies on peak clamps for certain instantaneous overstresses, with analog gain reduction in each channel to reduce long-term overloads, and as a last resort, muting of the amplifier if stresses continue to build up. The limiter thresholds and time constants are matched to the thermal behavior of the systems being protected.

In brief, protection systems are provided for:

• Peak clamping of output current, with internal gain reduction and/or muting to reduce long-term output current to a reasonable value. This protection occurs when outputs are shorted, and gain reduction will also be observed within two seconds, when attempting full power into 2 ohms, and after about 5 seconds into 4 ohms.

• AC current limiting. An additional system measures power supply current and reduces gain of both channels as necessary to keep currents within the carrying capacity of the overcurrent protective devices. This prevents nuisance tripping of internal and external circuit breakers.

• Over-temperature protection. A precision temperature sensing IC is mounted in the heat sink close to each channel’s output devices, and it controls a DC “thermal bus” for each channel, whose voltage is used to increase fan speed, cause gain reduction (thermal limiting) and, if necessary, amplifier muting, to keep the temperature of the heat sink below 85º C.

• High Frequency Limiting. Certain internal parts are subject to overload if operated at full powers near 20 kHz, and therefore

PL3 Series Service Manual 15 TD-000274-00 Rev. A

2.2 PL380 Major Circuit Blocks (continued)

frequency-sensitive limiting will prevent full power operation above about 15 kHz. A backup system mutes the amplifier quickly in the event of runaway oscillations.

• Clip Limiting. While clipping is not harmful to the amplifier, the

clip limiter will minimize distortion when it occurs. A userselectable switch engages the clip limiter on each channel.

2.2 PL380 Major Circuit Blocks

The following notes cover the same areas noted before in further detail, with references to voltages, part numbers, and PCB locations. All locations and directions are described though the amplifier is placed upside down with the cover removed, with the front panel facing the observer. This section should be read with the PL380 schematic at hand for reference.

Power Supply

Unless otherwise noted, this section refers to the schematic sheet

SUPPLY, PL380.

AC Entry

AC power enters the amplifier through an chassis-mounted cordset or AC inlet, circuit breaker, and line filter located on an auxiliary PCB in the right rear corner (L15 and L16; Y-caps C234 and C235; and Xcaps C294, C295, and C293, which are discharged by R348, R349, and R350 within 2 seconds after power is disconnected). Sleeved wires couple the two sides of the AC voltage to selected terminals on the main PCB: J19 to J20 (all voltages), and J15 to J21 (230 V) or J15 to J22 (120 V). These lines lead to power control relays discussed below (schematic zone D-8).

Housekeeping Supply

A smaller fuse, F1, protects the inrush limiting resistors in the event of a downstream load fault. A small amount of AC power is taken after this fuse to the power switch via W17 and W18, returning to a housekeeping reservoir C190 and C191 (zone C-8), charged by D64 through a current-limiting resistor bank that comprises R261, R263, R264, and R267. This reservoir supplies U41, an integrated TOP244VN flyback switcher that produces auxiliary supply voltages through transformer T1. The components immediately surrounding U41 provide feedback, over- and undervoltage sensing, and flyback clamping.

The circuitry on the secondaries of T1 (zone B-6) provide a +10 V DC supply (for future low-power accessory circuits), and unregulated ±25 V voltages. The 25-volt rails are reduced by regulators U42, U44, U45, and U50 to produce clean ±15 V and ±5 V bipolar supply rails. These regulated rails are labeled +15_TOP, +5_TOP, , and

-15_TOP, -5_TOP. Providing power for the clock and audio switching circuitry is another set of ±5 V rails, labeled +5:SW and -5:SW, which are decoupled through L5, L6, C226, and C227 to filter out switching noise. All circuitry connected to these rails will be

powered as soon as U41 starts operating, which normally occurs any time the amplifier is turned on.

The +5:SW rail powers the crystal-controlled clock and divider circuitry (schematic: see sheet which is centered around U1 and U4–U7. The divider sends a sync pulse train to U49 (schematic: see sheet D-2), a PWM switch-mode supply controller that delivers switching pulses with controlled dead time to U46. A specialized gate drive IC, U46 in turn provides a gate drive signal through transformer T4, to the pair of isolated-case switching transistors Q68 and Q69. T4’s two secondary windings have opposing polarities so that the gate drive pulses will alternately switch the transistors on and off.

The result of the switching is a 125 kHz alternating current through the primary of the power transformer, T2. The transformer has a turns ratio of 10:11, so the secondary voltage is about 10% higher than the primary voltage. Thus, the energy from the primary reservoir capacitors—C209, C210, C213, C214, C216, and C217 (zone D-5)—couples through to the transformer secondary, where it is rectified and stored in the secondary reservoirs comprising C247– C250 and C253, C254, C259, and C260 (zone C-3). These are the ±185-volt rails for the two channels’ output sections.

Ordinarily, at startup there is little or no voltage on the primary reservoir, and therefore, little current flows through the switches. The aux supply powers a sequence of delays that causes the inrush and main relays to close progressively, ramping the entire main supply up to full voltage in a controlled manner.

AMP CH-A, PL380,

Power Supply, PL380,

zone C-7 and D-7),

zone

Power Supply On-Off Sequencing

The relay control circuitry is shown in zone C-5 of the schematic sheet

PROT/CNTRL, PL380

sequence must occur to make the main power supply turn on:

1. The AC voltage detector (schematic: see sheet

PL380”

zone B-7 and B-8) connects to the incoming AC line through resistors R258 and R259. It must sense that the amplifier has AC mains voltage coming in to the power supply.

If it does, the AC voltage signal will turn transistor Q66 on, which turns Q67 off. This allows current to flow through the LED side of the optocoupler, U40, and pulls the AC-ON bus high to +5 V. Resistor R265 adds a small amount of hysteresis, so that the turn-off threshold is slightly lower than the one for turn-on.

2. In the relay control circuitry, the AC-ON bus turns transistor Q34 off. The IGBTs in the power supply should be receiving gate drive pulses, which would cause bus IGBT-SW (a safety interlock to prevent the relays from closing if the main power switches are inactive) to charge capacitor C104. The bus AC-OFF-LO is a remote control line that permits C104 to be remotely discharged to shut down the main supply.

3. When the capacitor charges to greater than 3.3 V, the compara-

. When the amplifier is turned on, the following

“Power Supply,

16 QSC Audio Products, LLC

2.2 PL380 Major Circuit Blocks (continued)

tor U24:1 will swing low, turning on Q35 and Q37. These actuate relay K2 (schematic: sheet couples AC through large NTC resistors R262 and R266 to the main AC rectifier BR1. The resistors limit inrush current as the primary reservoirs charge.

4. Transistor Q35 also charges C107 through R142. The voltage on pin 7 of U24:2 reaches +5 V (the threshold set by bus +5V:LIN) in about one second, at which point the comparator will trip low and turn on transistor Q43. This turns on relay K1, through bus 50A-RY, to bypass the inrush-limiting resistors. The main supply will have now now reached full operating voltage and at normal AC line voltage, the secondary DC rails should measure ±185 V.

5. The final timing delay is established by C108. When Q43 turns on to energize K1, transistor Q48 turns off. This allows C108 to charge through R163, D35, R166, and D43, controlling the rate of charge of capacitors C111 and C114. When C111 and C114 reach +5 V thresholds (set by the +5_TOP bus), which takes about one second, comparators U24:3 and U24:4 respectively switch the RUN-A-LO and RUN-B-LO buses low, which enables the audio switching on channels 1 and 2. This allows the channels to pass audio signals.

6. When AC power is removed, including when the amplifier is switched off, the AC detector circuit mentioned earlier shuts down the AC-ON bus within several AC cycles. This lets Q34 quickly discharge C104, which triggers a rapid discharge of all the other timing capacitors as well. This ensures that the delay intervals are all reset when the amplifier is turned on again. The RUN-A-LO and RUN-B-LO buses disable the audio switching, which immediately mutes the audio.

Meanwhile, the housekeeping supply operated by U41 keeps running, powered through D70 by energy stored in the primary reservoir, and so the clock and power supply switching continue until the reservoir voltage drops to about 45 V. Through this after-shutoff switching action, the primary and secondary reservoirs discharge together. This ensures that inrush current is managed properly when the amplifier is switched on again.

7. The red Clip LEDs on the faceplate will light as long as auxiliary power is present and the amp is in muting. On amplifiers manufactured in January 2008 or earlier, it is normal for the clip LEDs to remain lit for about 15–20 seconds after the amplifier is switched off. On amplifiers manufactured in February 2008 or later, the clip LEDs will flash briefly at turn-off and then remain dark.

Supply, PL380

, zone C-6), which

Amplifier Channels

NOTE: Most components are duplicated in the two amplifier channels, so these descriptions will limit themselves to channel 1’s circuitry (schematic: see sheet

“Amp Ch-A, PL380”

) unless there is

an actual difference to be noted.

High Current Switching

The amplifier channel uses two large switching FETs, Q10 and Q11, operating as a half-bridge at 250 kHz. They are mounted under the single large heat sink, together with HF diodes D14 and D15 and steering diodes D12:1 and D12:2, which clamp the output voltage when their respective FET turns off.

Power for Gate Drives

Each FET is driven by a gate drive circuit that consumes appreciable power and also must be referenced to its respective FET’s source terminal. Therefore, each gate drive circuit has an unregulated 25volt power supply that suitable for its location in the overall channel circuit. FET Q11’s source is connected to the fixed -185 V rail; therefore, power for its gate drive circuitry comes from the LOSIDE GATE-A supply. The source terminal on the high-side (positive) FET, Q10, is connected through D12:1 to the switched PWM audio bus SW-A, so its power supply, HI-SIDE GATE-A, floats with respect to the audio circuitry.

Gate Drives and Signal Isolation

In the high-side gate drive power supply, the regulator U20 drops the unregulated 25 volts down to 12 volts for the gate driver IC, U17, so it can produce 12-volt gate drive pulses. Regulator U15 reduces the 12 volts to 5 volts for the optocoupler U12, which isolates the gate drive circuitry from the ground-referenced modulator and logic circuitry. The components D4, C29, and R43 delay the negativegoing transitions out of U9:2 to control the dead time.

Close Synchronization of FET Switching

Each FET of the pair switches on within tens of nanoseconds after the other switches off. These transitions must be coordinated closely to avoid both overlapping conduction—which would cause destructive “shoot-through” currents—and excessive dead time, which would increase audio distortion. Gate drive circuit disorders that cause an FET to stay on too long are likely to result in both FETs’ destruction.

Output Current Clamping

The PWM signals also separately enable current sources that sample each FET’s source terminal voltage during its “on” state to determine the current through the FET. Transistor Q8 is the current source associated with Q10; when it is switched on by U6:6, it forward-biases D16 and D17, resulting in a signal voltage that corresponds with the current through the FET. Transistor Q12 converts the voltage into a current on the OC-LIM-A bus.

The negative FET, Q11, has a similar current sensing circuit comprising Q9, U6:5, D18 and D19, Q13, and their associated components. This circuit also feeds bus OC-LIM-A.

The Overcurrent Feedback network (schematic: see sheet

Amp Ch-A,

PL3 Series Service Manual 17 TD-000274-00 Rev. A

2.2 PL380 Major Circuit Blocks (continued)

PL380,

zone B-1) passes the current through resistor R87 to produce a signal voltage, which is compared to ±5 V reference voltages. If the signal exceeds about ±6.2 volts the network sends a correction signal through R82 to bus OC-FB-A. This is a feedback bus for the modulator, and the correction signal reduces the amount of modulation to limit the FET current to about 70 A peak when the FETs are cold, and less at higher temperature.

PWM Modulator

Comparator U8 is the pulse-width modulator that converts the audio signal into a PWM stream that controls the output FETs. At its noninverting input, pin 2, a triangle wave develops from C21 integrating the 250 kHz square wave current from bus CLK-A through R34. The triangle wave is averaged about ground potential, so that with no audio signal the comparator produces two 50/50 pulse trains that switch between the negative and positive rails. One output, Q, is the complement of the other, Q. If these pulse trains were averaged by an integrator, the resulting voltages would be zero. PWM negative feedback from SW-A through R35 and R36, as well as audio negative feedback from OUT-A through R29 and R24, help keep any DC offset to minimal amounts.

Audio signal voltage from U3:2, through R30, combines with the triangle wave voltage, thereby changing the points, with respect to the triangle wave, at which pin 2 crosses zero volts. This varies the duty cycles of the Q and Q so they are proportional to the audio signal voltage.

Amplifier Muting

The amplifier mutes protectively during turn-on and turn-off, as well as when various overstresses occur. It mutes by disabling the PWM pusle trains to the output FETs; when both FETs are off, the channel passes no audio and power dissipation is minimal.

When bus RUN-A-LO is low, the output of U6:1 is high. It feeds flip flop U7:2’s D input, and thus when the flip flop’s Q output goes high it will allow U9’s four NAND gates to pass the PWM stream from AMP_TRIG_A. RUN-B-LO, U6:2, U7:1, and U30 do the same for channel 2.

Output Filter Network

A passive low-pass network comprising L1 and C64 integrate the PWM stream on SW-A back into a fairly smooth audio waveform with the switching-frequency components reduced about 40 dB. The resulting audio spectrum deliverable to the loudspeaker load is reasonably flat to beyond 20 kHz.

Inductor L2 and capacitor C71 form a parallel-resonant 250 kHz trap that adds about another 15 dB of attenuation of the switching noise. A zobel network, which comprises C78 and two high-power resistors, R85 and R86, further stabilizes the output impedance. The resistors, mounted under the large heat sink, are protected against excessive high frequency output signals by the protective limiter described next.

Protective Limiter

A limiter circuit reduces stresses on the amplifier channel when certain conditions are detected. For example, a fast-rising stress like excessive output current will be clamped first by the overcurrent feedback but will trigger the limiter if it is sustained. Slowerchanging stresses like high temperature trigger limiting as the first defense, and then muting if the limiter control voltage is driven to its limit. Muting eliminates all significant stress and heat generation. When the clip limiter function is engaged, the limiter also reduces the amount of distortion that occurs during clipping.

The gain reduction cell of the limiter circuit is op amp U3:1 and dual LM13600M transconductance op amps U2:1 and U2:2, which act as parallel variable negative feedback loops about U3:1. Normally, the control current via R7 and R8 is zero, and so the transconductance op amps are cut off and have no effect on the gain of U3:1.

Bus LIMITER-A is normally held at -5 V by R27. Several control circuits—detecting excessive FET current, excessive temperature, zobel network overload, and excessive power supply current—can pull this voltage upward if necessary. For example, excessive highfrequency voltage at bus ZOB-A, across R85 and R86 in the zobel network, will cause Q4 to pull the LIMITER-A bus high.

As the LIMITER-A bus voltage gets pulled more positive, Q1, D1, and Q2 conduct, feeding up to 2 mA of control current via R7 and R8. The transconductance op amps U2:1 and U2:2 then will pass signal, increasing the effective negative feedback around U3:1 and reducing its gain.

This ciruict offers up to 40 dB of gain reduction. If the control current exceed 2 mA, though, the voltage across R19 will push A-LIM to a negative voltage and turn on Q3, which pulls bus A-MUTE-LO low, muting the amplifier. Muting promptly relieves overloads such as overcurrent, and the amp will cycle in and out of muting quickly. A shutdown due to excess temperature will usually take 30 to 60 seconds to return to normal, during which muting will continue.

Thermal Sensing

A precision temperature-to-voltage converter, U10 is located in a heat sink hole packed with thermal grease near channel 1’s power devices. The sensor’s output voltage is amplified to a more usable level by U11:1. The resulting thermal bus voltages A-THERM and B-THERM change linearly from 6.84 V at 0º C to 3.41 V at 80º C, and are used for several purposes: fan speed control, protective muting.

Fan Speed Control

In the fan speed control circuitry (schematic: see sheet

Control, PL380”

buffered respectively by Q41 and Q38. When either channel’s thermal bus voltage drops to below 4.4 V (at approximately 55° C), its transistor, Q38 or Q41, will turn Q44 on. This will turn on Q47, gradually increasing the fan voltage from 10 V. Higher temperatures will further increase the fan voltage. The maximum fan voltage available, which would be at temperatures at or about the point of

zone A-7), buses A-THERM and B-THERM are

“Protect/

18 QSC Audio Products, LLC

2.2 PL380 Major Circuit Blocks (continued)

thermal shutdown, is about 30 V DC. Feedback elements R160, R157, Q45, and R153 provide stabilizing feedback to regulate the fan voltage. R152 provides the reference current that sets the cold or low-speed fan voltage.

• Fan failure or blockage will ultimately result in amplifier muting once the heat sink temperatures reach about 80° C. This will remove most sources of heat dissipation and prevent further overheating.

• Q36 and Q39 diode-OR the THERM-A and THERM-B voltages into a common bus called A/B_THERM.

Thermal Limiting and Muting

In the thermal sensing circuit (schematic: see sheet

PL380,”

zone A-5), Q7’s emitter connects to a 4.48 V reference, defined by R41, R42, and the +5V:LIN supply bus. When the heat sink reaches about 75° C, the voltage on THERM-A decreases to turn on Q7, which pulls the LIMITER-A control bus voltage positive, activating the limiter circuitry to reduce the heat dissipation. If this fails to arrest temperature rise, the amplifier will mute when the heat sink reaches about 80° C.

Zobel Network Protection

The RC network on the output must be protected against dissipations that exceed the 200-watt rating of the resistors. When the voltage on ZOB-A is excessive, it will turn on Q4, which pulls the voltage positive on LIMITER-A.

Prolonged high frequency overloads will trigger muting, but if the instability is due to component fault or abnormal load conditions, runaway oscillations will promptly trigger muting via C224, R383, R384, and Q83 pulling low bus A-MUTE-LO.

Prolonged Overcurrent Limiting

Overcurrent feedback via OC-FB-A into the modulator instantly clamps peak currents, but the amplifier also needs to prevent prolonged operation at such high levels. The output of the current sensing network, OC-LIM-A, reaches a signal-rectifier circuit Q5 and Q6, which pulls the LIMITER-A bus toward positive after several seconds of peak output current.

Power Supply Current Limiting

Excessive long-term average power supply current triggers the bus PS-LIM, which turns on Q79 on channel 1 and Q76 on channel 2. Both channels limit simultaneously.

Clip Detection and Limiting

During normal switching, the modulator output is coupled via U6:4 through high frequency rectifiers D6 and D7 into C35, producing a positive voltage on CLIP-A. When the channel clips, the modulation has reached 100% and switching therefore stops. The voltage on CLIP-A drops to zero, which triggers the Clip LED, and sends a 4 V flag to the DataPort.

When the clip limiter switch on the rear panel is set to “off,” it

“Amp Ch-A,

connects CL-A-ON to ground; when set to “enable” the clip limiter, it is open and allows the falling voltage at CLIP-A to let Q72 turn on and put limiter control current directly into R7 and R8. This bypasses the relatively slow limiter bus, and acts rapidly on C228 and C257 to produce prompt limiting of clip events.

DC Fault Shutdown System

The DC shutdown system (schematic: see sheet

PL380”

zones A-4 and A-5) looks for significant DC offsets on either channel and triggers a power supply shutdown in such an event. Large DC voltages normally indicate a serious component fault such as a shorted output device, and limiting the energy to the load and through the output devices may prevent further damage.

Capacitor Overvoltage Protection

Prolonged output voltage of a given polarity may in rare occasions result in current being drawn from one rail and flowing to the other rail (thru the catch diodes) long enough to pump the off-side rail to an excessively high voltage. The capacitor overvoltage protection circuit (schematic: see sheet B-3) divides the rail voltages down to compare them with ±5 V references. If either rail exceeds 225 V, the corresponding polarity of the amplifier signal will be clamped by the op amps and diodes to prevent further overvoltage.

both

rails exceed 225 V (corresponding to an approximately 22% high AC mains line with no signal present), both clamps will operate, probably quenching the signal altogether. However, the amplifier should continue switching, which will maintain a load on the supply capacitors. The power supply’s primary-side overvoltage shutdown may also occur in this sort of situation.

“Protect/Control, PL380”

“Protect/Control,

zones A-3 and

Peripheral Signal-Processing Circuits

Many additional circuits, powered by the ±15 V rails, provide for user adjustments, status displays, gain control, input buffering etc. A brief orientation and review of these circuits follows.

Amplifier Balanced Inputs with Gain Adjustment

Op amps U21:1 and U22:1 (schematic: see sheet are the input stages for channels 1 and 2, respectively. They are arranged as differential amplfiers with precision 0.1% resistors to maintain a very high common-mode noise rejection ratio.

A three-position switch allows the selection of different gain structures, and an auxiliary set of contacts light corresponding indicator LEDs and also provide an indicating voltage for remote monitoring of the switch position.

Low Frequency Filters

The signal is padded by R322 and R323 to prevent full-scale signals’ overloading the input of op amp U21:2, which with components C290, C291, C292, R334, and R335 make a switchable 30 Hz or 50 Hz, two-pole input filter. R327 and R337 restore the gain lost by R322 and R323. Channel 2 has an identical arrangement.

“Inp-Displ, PL380”

)

PL3 Series Service Manual 19 TD-000274-00 Rev. A

2.2 PL380 Major Circuit Blocks (continued)

Gain Controls

The front-panel gain control, configured as a variable attenuator, is fed through a blocking capacitor, C95. The signal returns to and is buffered by op amp U51:2. Channel 2 has an identical arrangement.

Bridge-Mono and Parallel Signal Routing

The “parallel inputs” position of switch SW3 connects together the corresponding input terminals of channels 1 and 2. The “stereo” position completely separates inputs.

The “bridge mono” position routes the output from channel 1’s gain control buffer into channel 2’s gain buffer, replacing that channel’s normal input signals.

Switchable Clip Limiting

As described earlier under Clip Detection and Limiting, switches SW2 and SW5 defeat clip limiting when closed and enable it, along with a yellow LED indicator, when open.

Front-Panel Controls and Displays

Ribbon cable J7A-J7B runs from the input board to the front panel board, carrying signals to and from the gain controls and various status LEDs.

LEDs LD16 (orange) and LD17 (yellow) repeat the rear-panel status LEDs that display the mode switch settings.

LEDs LD7 and LD8 (red) display clipping, limiting, and protect activity.

Circuitry connected to LD1–LD3 and LD4–LD6 provide a stepped display of the output signal level, triggering at -35 dB, -20 dB, and

-10 dB. Op amps U23:2 and U23:1 convert the output signal into a quasi-logarithmic form that maps to the linear step circuits that switch on the successive LEDs.

“DataPort” Connector and Monitoring Signals

The DataPort is a QSC-specific connector scheme that passes lowvoltage (line level) signals to and from external monitoring devices such as the Basis series. The DataPort may also host a plug-in accessory that receives power from the +15 V line and sends processed signals to the amplifier. In brief:

• Vmon-A and Vmon-B represent the output voltage, scaled

down 50:1 (100 V at the speaker = 2 V at the Vmon output). DC voltages riding under these signals show the state of the bridgeparallel and gain/sensitivity switches.

• The +15 V DC line is fused at 1 A by surface mount fuse F4.

•“Stby” carries a voltage representing the main rails, scaled to

about +12 V peak. Pulling this line to ground will shut down the main power supply via the STBY-LO bus.

• Imon-A and Imon-B represent the output current, scaled to

approx 50 A = 2 V. DC voltages riding under these signals represent temperature information for each channel.

• Clip-A and Clip-B rise to 4.2 V during clipping and muting, and

to 1.7 V during limiting.

• The “IDR” line has a resistor and diode combination that is

unique to this model, allowing remote identification.

20 QSC Audio Products, LLC

2.3 PL380 Test Procedure

Test Equipment required:

• Distortion analyzer with built-in signal generator

The generator must have total harmonic distortion plus noise (THD+N) of no higher than 0.01%, and the analyzer must be capable of resolving to that level as well. We suggest either the

HP-Agilent 339A Two, ATS-1,

Audio Precision System One can be obtained by sending an email request to tech_support@qscaudio.com.

• High power load resistor bank (8, 4, and 2 ohms)

• 20 MHz oscilloscope and digital multimeter

• Switching amplifier measurement filter

Suggested:

• Variable autoformer (Variac, Powerstat, etc.), 0–140 V AC,

50 A for 100 V/120 V models or 0–260 V AC, 30 A for 230 V model

• Burst sinewave generator (if the analyzer doesn’t have burst

output mode)

Suggested:

or the

Audio Precision System One, System

ATS-2

. The PL380 test procedures written for

Audio Precision AUX-0025)

HP 33120A Waveform Generator

Notes:

• The PL380 is a class D amplifier; as such, it uses a process that results in some residual 250 kHz switching noise in the amplifier output signal; this noise could produce misleading results in measurement of power, percent THD, and noise. We recommend using a low-pass filter such as the Audio Precision AUX-0025 to reduce the noise prior to the analyzer. Figures 2.1 and 2.2 show the PL380 amplifier’s output signals with and without the AUX0025 filter.

• Measuring 2-ohm power requires a burst sine wave signal (Figure 2.3). This is to avoid excessive power supply current demand under testing, which would trigger the protection circuit and quickly reduce the output power to a safe level (approxi-

mately 2800 watts with a continuous sine wave signal).

The circuit design of the PL380 is fine-tuned and requires no internal trim pots. Therefore, no adjustment or calibration is required during testing. If the amplifier does not meet specifications in this test procedure, it requires service. Follow the troubleshooting guide in the service manual to repair the amplifier as needed.

Setup

1. Set switches and controls as follows:

• AC switch (front panel)

• Gain controls (front panel) to minimum, or fully counter clockwise.

• Mode switch (rear panel) on

• Sensitivity switch (rear panel) on

• Clip limiter switches (rear panel)

• High-pass filter switches (rear panel)

2. Connect a test load(s) to the output terminals of the amplifier.

3. Connect the distortion analyzer input to the AP AUX-0025 filter’s channel A output.

4. Connect the AP AUX-0025 filter’s channel A input to the channel 1 output terminals of the amplifier.

5. Set up and connect a dual-channel oscilloscope to the following test points:

• Channel 1—Set vertical sensitivity to 2 V/div; connect a

10× probe to the channel speaker output.

• Channel 2—Set vertical sensitivity to 0.1 V/div; connect a

1× scope probe to the distortion analyzer output.

6. Connect the analyzer’s test signal output to the amplifier’s channel 1 XLR input.

7. Set the analyzer signal to a 1.0 V rms 1 kHz sine wave.

8. Turn on the analyzer’s 80 kHz filter.

off

stereo

32 dB

off

Figure 2.1. Signal with 250 kHz switching noise

PL3 Series Service Manual 21 TD-000274-00 Rev. A

Figure 2.2. Signal with switching noise filtered out

Figure 2.3. Burst sine wave signal for 2Ω power testing

2.3 PL380 Test Procedure (continued)

9. Plug the amplifier’s power cord into a variable autoformer (set to 0 V) and set up an ammeter to monitor AC line current.

Power On and Mute Delay Test

1. Turn the amplifier on and slowly increase the AC voltage to 120 V (120 V model) or 240 V (230 V model) while monitoring the AC line current. At full voltage, the idle AC current should be about 2.5 A (120 V model) or 1.25 A (230V model).

2. Verify that the fan is operating at low speed.

3. Turn the power switch off and on a few times to verify the 3second power-on muting delay and the instant power-off mute; listen to how the power relays engage about 3 seconds after turn-on but disengage instantly at turn-off. NOTE: When AC power is turned off, the amplifier mutes the audio immediately, but the power supply switches continue operating until the primary and secondary DC voltages have discharged to about 20% of normal. Below this point, the auxiliary supply itself stops switching and the amplifier becomes fully quiescent. On amplifiers made in January 3008 or earlier, it is normal for the Clip and Power LEDs to remain lit for about 15 to 20 seconds after the amplifier shuts off. On later amplifiers, the Clip LED will flash at the moment of turn-off, and they and the Power LED will shut off.

3. Turn down the signal by 10 dB. The -10 dB LED should be dim.

4. Turn down the signal another 10 dB (-20 dB to the reference level). Both the -10 dB and -20 dB LEDs should be off.

5. Turn the amp gain control to minimum. All three signal LEDs should be off. Repeat this test with the other channel.

Bridge Mode Test

1. Turn off the power switch.

2. Set the mode switch to the bridge position. The bridge LEDs on the front and rear panels should light when you turn the amplifier back on.

3. Connect the load to the two red output binding posts (channel 1 positive and channel 2 negative).

4. Select an 8 ohm load resistance and apply a 2.7 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input. Adjust the amp gain control to obtain 5000 watts output (200 V rms).

Quickly verify that the THD is below 1%, and then turn the gain control down all the way to prevent excessive current stress on the AC line and power stress on the test load.

5. Turn the amplifier off and set its mode switch back to stereo. Connect a separate load resistance to each channel’s output.

Channel Output Test

1. Look for amplified signal on scope channel 1. Check for a noisy or contaminated gain potentiometer by looking for general instability on the distortion waveform (scope channel 2) while you rotate the gain control.

2. Set the amp gain control to maximum (fully clockwise) and verify the output level of 32 dBV (40 V rms), with a 1 V input.

3. Select 8-ohm load and set the analyzer signal output at

2.7 V rms, 1 kHz. Confirm that this amplifier is producing 1500 watts (109.5 V rms), with less than 1% THD. Repeat steps 1 through 3 with the other channel.

Signal Indicators Test

1. Disconnect the load resistors.

2. With a 2.7 V rms, 1 kHz input signal into the channel, turn the gain control to maximum. The three signal LEDs (Signal, -20 dB, and -10 dB) should be lit. Set the analyzer’s 0 dB reference to this level.

Frequency Response Test

1. Connect an 8-ohm load resistance to channel 1’s output. Apply a

2.7 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input and turn its gain control all the way up. The output voltage should be 108 V rms.

2. Reduce the signal by 10 dB to 34.1 V rms (150 watts output). Set the analyzer’s 0 dB reference to this point.

3. Check the frequency response from 20 Hz to 20 kHz by sweeping or spot-checking frequencies between these extremes. Verify that the output voltage’s amplitude at each frequency is 0 dB, ±0.20 dB.

4. Repeat steps 1 through 3 for channel 2.

4 Ohm Power vs. Distortion Test

1. Connect a 4-ohm load resistance to channel 1’s output. Apply a

2.7 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input .

22 QSC Audio Products, LLC

2.3 PL380 Test Procedure (continued)

2. Adjust the gain control to obtain 2500 watts output power (100 V rms). Verify that the THD is below 1%. Check the output power at 20 Hz and 10 kHz; it should also be 2500 watts, with THD less than 1%.

3. Turn the gain control down 3 dB to obtain 1250 watts (70.7 V rms). Verify that the THD is below 0.02%. Check the output power at 20 Hz and 20 kHz; it should be 1250 watts, with THD less than 0.06%.

4. Repeat steps 1 through 3 for channel 2.

2 Ohm Power and Short Circuit Current Test

1. Connect a 2-ohm load resistance to channel 1’s output. Apply a

2.3 V rms, 1 kHz sine wave burst signal (3 out of 10 cycles) to amp channel 1’s XLR input.

2. Adjust the gain control to obtain 4000 watts of burst output power (at least 252 V p-p, undistorted, during the bursts), as shown in Figure 2.3. NOTE: adjust the variable autoformer to maintain the AC line voltage at 120 V or 240 V during this test.

3. Change the test signal to a 1.6 V rms, 1 kHz continuous sine wave.

4. Verify amplifier output power of 2000 watts (63.2 V rms), with less than 0.1% THD. While the amplifier is producing power into the load, apply a short circuit across the output.

3. Measure AC line current; should be about 13–21 A (120 V model) or 6.5–12 A (230 V model). As the amplifier gets hot, the current draw will increase slowly. That is normal.

4. Block the fan’s intake and verify that the fan speed ramps up from low speed to high.

5. After a while, the thermal protection should engage; the AC line current should drop significantly to about 2.5 A (120 V model) or

1.3 A (230 V model).

6. Remove the short from the outputs and unblock the fan’s intake.

7. Select the 8-ohm loads and allow the amplifier to fully recover from thermal protection; verify that output signals are still present on each channel and that the fan speed ramps down from high speed to low.

Output Noise Test

1. Connect an 8-ohm load resistance to channel 1’s output. Apply a

2.7 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input.

2. Turn the amplifier gain controls to maximum. Set the analyzer’s 0 dB reference to this level.

3. Disocnnect the input signal from the amplifier input and measure the residual noise level produced into the load. The noise signal should be at least 104 dB below the 0 dB reference.

4. Repeat steps 1 through 3 for channel 2.

5. After a couple of seconds, the amplifier will start cycling in and out of protective cutback, alternately drawing low current and then high current. Measure the AC line current at its maximum; it should be no greater than 13 A (120 V model) or 7 A (230 V model).

6. Remove the short. The channel’s output signal should immediately resume into the 2-ohm load as before, with no delay or hangup.

7. Repeat steps 1 through 6 for channel 2.

Turn On/Turn Off Transient Test

1. Connect a loudspeaker to the output of each amplifier channel.

2. Disocnnect any inputs from the amplifier.

3. Turn on the amplifier and listen for any transient or thump noises during turn-on delay sequences.

4. Turn off the amplifier; on amplifiers made in February 2008 or later, all LEDs should turn off immediately, while on those made in January 2008 and earlier the clip and power LEDs will stay lit for some time. Listen for any transient or thump noise at turn-off.

Thermal Test

1. Set both gain controls to minimum and apply a 2.7 V rms, 1 kHz sine wave signal to both channels’ inputs. NOTE: You can do this by setting the mode switch to parallel.

2. Apply a short circuit across the output of each channel and turn both gain controls to maximum. The clip LEDs should light.

PL3 Series Service Manual 23 TD-000274-00 Rev. A

Final Check

This completes the amplifier test procedure for this model. Inspect the amplifier for mechanical defects. Inspect the solder connections. Reassemble the amplifier and verify its operation prior to returning the product to service.

2.4 PL340 Test and Calibration Procedure

Test Equipment required:

• Distortion analyzer with built-in signal generator

The generator must have total harmonic distortion plus noise (THD+N) of no higher than 0.01%, and the analyzer must be capable of resolving to that level as well. We suggest either the

HP-Agilent 339A Two, ATS-1,

Audio Precision System One can be obtained by sending an email request to tech_support@qscaudio.com.

• High power load resistor bank (8, 4, and 2 ohms)

• 20 MHz oscilloscope and digital multimeter

• Variable autoformer (Variac, Powerstat, etc.), 0–140 V AC, 30–

50 A for 100 V/120 V models or 0–260 V AC, 20–35 A for 230 V model

or the

Audio Precision System One, System

ATS-2

. The PL340 test procedures written for

Setup

1. Set switches and controls as follows:

• AC switch (front panel)

• Gain controls (front panel) to minimum, or fully counter clockwise.

• Mode switch (rear panel) on

• Sensitivity switch (rear panel) on

• Clip limiter switches (rear panel)

• High-pass filter switches (rear panel)

off

stereo

off

32 dB

off

Power On and Mute Delay Test

1. Turn the amplifier on and slowly increase the AC voltage to 50 V (120 V model) or 100 V (230 V model) while monitoring the AC line current. The power indicator LED should be at half-brightness, and the current draw should not exceed 0.3 A (120 V model) or 0.15 A (230 V model). Continue increasing the voltage to 100 V (120 V model) or 200 V (230 V model), and pause for about three seconds until the mute/ protect circuit disengages. Continue to 120 V (120 V model) or 240 V (230 V model). At full voltage, the idle AC current should be about 1.0 A (120 V model) or 0.5 A (230V model).

2. Verify that the fan is operating at low speed.

3. Turn the power switch off and on a few times to verify the 6second power-on muting delay and the instant power-off mute. The power-on delay should proceed like this:

• The blue Power LED at half brightness for three seconds

• Power and Clip LEDs at full brightness for another three

seconds

• Amplifier unmutes; Power LED remains bright while Clip

LEDs turn off All LEDs should turn off immediately when the amplifier is turned off.

2. Connect a test load(s) to the output terminals of the amplifier.

3. Set up and connect a dual-channel oscilloscope to the following test points:

• Channel 1—Set vertical sensitivity to 2 V/div; connect a

10× probe to the channel speaker output.

• Channel 2—Set vertical sensitivity to 0.1 V/div; connect a

1× scope probe to the distortion analyzer output.

4. Connect the analyzer’s test signal output to the amplifier’s channel 1 XLR input.

5. Set the analyzer signal to a 1.0 V rms 1 kHz sine wave.

6. Turn on the analyzer’s 80 kHz filter.

7. Plug the amplifier’s power cord into a variable autoformer (set to 0 V) and set up an ammeter to monitor AC line current.

Channel Output Test

2. Set the amp gain control to maximum (fully clockwise) and verify the output level of 32 dBV (40 V rms), with a 1 V input.

3. Select 8-ohm load and set the analyzer signal output at

1.96 V rms, 1 kHz. Confirm that this amplifier is producing 800 watts (80 V rms), with less than 1% THD. Repeat steps 1 through 3 with the other channel.

24 QSC Audio Products, LLC

2.4 PL340 Test and Calibration Procedure (continued)

Signal Indicators Test

1. Disconnect the load resistors.

2. With a 1.96 V rms, 1 kHz input signal into the channel, turn the gain control to maximum. The three signal LEDs (Signal, -20 dB, and -10 dB) should be be lit. Set the analyzer’s 0 dB reference to this level.

3. Turn down the signal by 10 dB. The -10 dB LED should be dim.

4. Turn down the signal another 10 dB (-20 dB to the reference level). Both the -10 dB and -20 dB LEDs should be off.

5. Turn the amp gain control to minimum. All three signal LEDs should be off. Repeat this test with the other channel.

Figure 2.4. Noise and distortion residual with bias properly set

Bridge Mode Test

1. Turn off the power switch.

2. Set the mode switch to the bridge position. The bridge LEDs on the front and rear panels should light when you turn the amplifier back on.

3. Connect the load to the two red output binding posts (channel 1 positive and channel 2 negative).

4. Select an 8 ohm load resistance and apply a 1.96 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input. Adjust the amp gain control to obtain 2600 watts output (144 V rms). Verify that the THD is below 1%.

5. Turn the amplifier’s gain controls to minimum.

6. Turn the amplifier off and set its mode switch back to stereo. Connect a separate load resistance to each channel’s output.

Bias (Crossover) Adjustment

NOTE: The bias should not need readjusting unless the amplifier is overheating or draws excessive idle current.

1. Let the amplifier cool down to room temperature.

2. Turn off the analyzer’s 80 kHz filter.

2. Set the analyzer signal sine wave output to 0.2 V rms at 20 kHz and set the amplifier gain controls to full gain. Put the signal into the input of one channel.

3. Put an 8-ohm load on the channel’s output.

4. Adjust the bias trimpot VR43 (channel 1) or VR166 (channel 2) for a total THD+N figure of 0.07% or slightly less. Figure 2.4 shows what the residual crossover spike should look like when the bias

is properly set.THD+N. This adjustment must be done quickly, before the amplifier starts to warm up significantly. If the amplifier begins to feel warm to the touch before you complete the bias adjustment, you must turn it off and allow it to cool down to room temperature before trying again.

5. Turn the channel’s gain control to minimum. Verify that the AC line current is no more than 1.0 A (120 V model) or 0.5 A (230 V model).

6. Turn off the amplifier to let it cool down, then repeat the procedure for the other channel, if needed.

7. Turn the analyzer’s 80 kHz filter back on.

Frequency Response Test

1. Connect an 8-ohm load resistance to channel 1’s output. Apply a

1.96 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input and turn its gain control to full gain. The output voltage should be about 78 V rms.

2. Reduce the signal by 10 dB to 25 V rms (80 watts output). Set the analyzer’s 0 dB reference to this point.

3. Check the frequency response from 20 Hz to 20 kHz by sweeping or spot-checking frequencies between these extremes. Verify that the output voltage’s amplitude at each frequency is 0 dB, ±0.20 dB.

4. Repeat steps 1 through 3 for channel 2.

PL3 Series Service Manual 25 TD-000274-00 Rev. A

2.4 PL340 Test and Calibration Procedure (continued)

4 Ohm Power vs. Distortion Test

1. Connect a 4-ohm load resistance to channel 1’s output. Apply a

1.96 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input .

2. Adjust the gain control to obtain 1250 watts output power (70.7 V rms). Verify that the THD is below 1%. Check the output power at 20 Hz and 10 kHz; it should also be 1250 watts, with THD less than 1%.

3. Turn the gain control down 3 dB to obtain 625 watts (50 V rms). Verify that the THD is below 0.02%. Check the output power at 20 Hz and 20 kHz; it should be 625 watts, with THD less than

0.02%.

4. Repeat steps 1 through 3 for channel 2.

2 Ohm Power and Short Circuit Current Test

1. Connect a 2-ohm load resistance to channel 1’s output. Apply a

1.96 V rms, 1 kHz sine wave signal to channel 1’s XLR input.

2. Adjust the gain control to obtain 2000 watts of output power (63.2 V rms). Verify that the THD is below 1%. NOTE: adjust the variable autoformer to maintain the AC line voltage at 120 V or 240 V during this test.

3. Turn the channel gain down 3 dB, to obtain output power of 1000 watts (44.7 V rms). While the amplifier is producing power into the load, apply a short circuit across the output.

4. Measure the AC line current; it should be no greater than 7 A (120 V model) or 4 A (230 V model).

3. Measure AC line current; should be about 12 - 14 Aac for both channels. As the amplifier gets hot, there will be some current drift upwards. That’s normal.

4. Block the fan’s intake and verify the fan speed will ramp up from low to high speed.

5. Run the test until thermal protection engages; the AC line current will drop significantly to 1A.

6. Remove the short from the outputs and remove the block from the fan’s intake.

7. Select 8 ohm load and allow the amplifier to fully recover from thermal protection; verify output signal of each channel and fan speed ramps down from high to low speed.

Output Noise Test

1. Connect an 8-ohm load resistance to channel 1’s output. Apply a

1.96 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input.

2. Turn the amplifier gain controls to maximum. Set the analyzer’s 0 dB reference to this level.

3. Disconnect the input signal from the amplifier input and measure the residual noise level produced into the load. The noise signal should be at least 105 dB below the 0 dB reference.

4. Repeat steps 1 through 3 for channel 2.

Turn On/Turn Off Transient Test

1. Connect a loudspeaker to the output of each amplifier channel.

5. Remove the short. The channel’s output signal should immediately resume into the 2-ohm load as before, with no delay or hangup.

6. Repeat steps 1 through 5 for channel 2.

2. Disconnect any inputs from the amplifier.

3. Turn on the amplifier and listen for any transient or thump noises during turn-on delay sequences.

4. Turn off the amplifier; all LEDs should turn off immediately. Listen for any transient or thump noise at turn-off .

Thermal Test

1. Set the amplifier gain controls to minimum and apply a

1.96Vrms, 1 kHz input signal to both channels. Note: set the Input Mode switch in the PARALLEL position if only one input is available.

2. Apply a short across the output of each channel and turn the amplifier gain controls to maximum. Clip LEDs should be on.

26 QSC Audio Products, LLC

Quality Review

2.5 PL325 Test and Calibration Procedure

Test Equipment required:

• Distortion analyzer with built-in signal generator

The generator must have total harmonic distortion plus noise (THD+N) of no higher than 0.01%, and the analyzer must be capable of resolving to that level as well. We suggest either the

HP-Agilent 339A Two, ATS-1,

Audio Precision System One can be obtained by sending an email request to tech_support@qscaudio.com.

• High power load resistor bank (8, 4, and 2 ohms)

• 20 MHz oscilloscope and digital multimeter

• Variable autoformer (Variac, Powerstat, etc.), 0–140 V AC, 30–

50 A for 100 V/120 V models or 0–260 V AC, 20–35 A for 230 V model

or the

Audio Precision System One, System

ATS-2

. The PL325 test procedures written for

Setup

1. Set switches and controls as follows:

• AC switch (front panel)

• Gain controls (front panel) to minimum, or fully counter clockwise.

• Mode switch (rear panel) on

• Sensitivity switch (rear panel) on

• Clip limiter switches (rear panel)

• High-pass filter switches (rear panel)

off

stereo

off

32 dB

off

Power On and Mute Delay Test

2. Verify that the fan is operating at low speed.

3. Turn the power switch off and on a few times to verify the 6second power-on muting delay and the instant power-off mute. The power-on delay should proceed like this:

• The blue Power LED at half brightness for three seconds

• Power and Clip LEDs at full brightness for another three

seconds

• Amplifier unmutes; Power LED remains bright while Clip

LEDs turn off All LEDs should turn off immediately when the amplifier is turned off.

2. Connect a test load(s) to the output terminals of the amplifier.

3. Set up and connect a dual-channel oscilloscope to the following test points:

• Channel 1—Set vertical sensitivity to 2 V/div; connect a

10× probe to the channel speaker output.

• Channel 2—Set vertical sensitivity to 0.1 V/div; connect a

1× scope probe to the distortion analyzer output.

4. Connect the analyzer’s test signal output to the amplifier’s channel 1 XLR input.

5. Set the analyzer signal to a 1.0 V rms 1 kHz sine wave.

6. Turn on the analyzer’s 80 kHz filter.

7. Plug the amplifier’s power cord into a variable autoformer (set to 0 V) and set up an ammeter to monitor AC line current.

Channel Output Test

2. Set the amp gain control to maximum (fully clockwise) and verify the output level of 32 dBV (40 V rms), with a 1 V input.

3. Select 8-ohm load and set the analyzer signal output at

1.6 V rms, 1 kHz. Confirm that this amplifier is producing 500 watts (63 V rms), with less than 1% THD. Repeat steps 1 through 3 with the other channel.

PL3 Series Service Manual 27 TD-000274-00 Rev. A

2.5 PL325 Test and Calibration Procedure (continued)

Signal Indicators Test

1. Disconnect the load resistors.

2. With a 1.6 V rms, 1 kHz input signal into the channel, turn the gain control to maximum. The three signal LEDs (Signal, -20 dB, and -10 dB) should be be lit. Set the analyzer’s 0 dB reference to this level.

3. Turn down the signal by 10 dB. The -10 dB LED should be dim.

4. Turn down the signal another 10 dB (-20 dB to the reference level). Both the -10 dB and -20 dB LEDs should be off.

5. Turn the amp gain control to minimum. All three signal LEDs should be off. Repeat this test with the other channel.

Bridge Mode Test

1. Turn off the power switch.

2. Set the mode switch to the bridge position. The bridge LEDs on the front and rear panels should light when you turn the amplifier back on.

3. Connect the load to the two red output binding posts (channel 1 positive and channel 2 negative).

4. Select an 8 ohm load resistance and apply a 1.6 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input. Adjust the amp gain control to obtain 1700 watts output (116.5 V rms). Verify that the THD is below 1%.

5. Turn the amplifier’s gain controls to minimum.

5. Turn the channel’s gain control to minimum. Verify that the AC line current is no more than 1.0 A (120 V model) or 0.5 A (230 V model).

6. Turn off the amplifier to let it cool down, then repeat the procedure for the other channel, if needed.

7. Turn the analyzer’s 80 kHz filter back on.

Frequency Response Test

1. Connect an 8-ohm load resistance to channel 1’s output. Apply a

1.6 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input and turn its gain control to full gain. The output voltage should be about 64 V rms.

2. Reduce the signal by 10 dB to 20 V rms (51 watts output). Set the analyzer’s 0 dB reference to this point.

3. Check the frequency response from 20 Hz to 20 kHz by sweeping or spot-checking frequencies between these extremes. Verify that the output voltage’s amplitude at each frequency is 0 dB, ±0.20 dB.

4. Repeat steps 1 through 3 for channel 2.

6. Turn the amplifier off and set its mode switch back to stereo. Connect a separate load resistance to each channel’s output.

Bias (Crossover) Adjustment

NOTE: The bias should not need readjusting unless the amplifier is overheating or draws excessive idle current.

1. Let the amplifier cool down to room temperature.

2. Turn off the analyzer’s 80 kHz filter.

2. Set the analyzer signal sine wave output to 0.16 V rms at 20 kHz and set the amplifier gain controls to full gain. Put the signal into the input of one channel.

3. Put an 8-ohm load on the channel’s output.

4. Adjust the bias trimpot VR43 (channel 1) or VR166 (channel 2) for a total THD+N figure of 0.07% or slightly less. Figure 2.4 shows what the residual crossover spike should look like when the bias

28 QSC Audio Products, LLC

4 Ohm Power vs. Distortion Test

1. Connect a 4-ohm load resistance to channel 1’s output. Apply a

1.6 V rms, 1 kHz sine wave signal to amp channel 1’s XLR input .

2. Adjust the gain control to obtain 850 watts output power (58 V rms). Verify that the THD is below 1%. Check the output power at 20 Hz and 10 kHz; it should also be 625 watts, with THD less than 1%.

3. Turn the gain control down 3 dB to obtain 425 watts (41 V rms). Verify that the THD is below 0.02%. Check the output power at 20 Hz and 20 kHz; it should be 425 watts, with THD less than

0.02%.

4. Repeat steps 1 through 3 for channel 2.

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