QSC PowerLight3 Series, PowerLight PL325, PowerLight PL340, PowerLight PL380 Technical & Service Manual
Page 1
PowerLight3
Series
▲▲
▲ PL325
▲▲
▲▲
▲ PL340
▲▲
▲▲
▲ PL380
▲▲
Technical Service Manual
QSC Audio Products, LLC
Costa Mesa, CA 92626 USA
www.qscaudio.com
*TD-000274-00*
*TD-000274-00*
Rev. A
Page 2
Page 3
PowerLight 3 Series
Technical Service Manual
PL325
PL340
PL380
PL3 Series Service Manual 1
TD-000274-00 Rev. A
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)
Copyright © 2008 QSC Audio Products, Inc. All rights reserved.
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PL325 and PL340 amplifier, rear view
PL380 amplifier, rear view
2 QSC Audio Products, LLC
Page 5
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
Main module circuit board assembly ............................................................................................................................................................................................................ 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
Page 6
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
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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
Page 8
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)
dB
18 A / L5-30 (30 A twist-lock)
11 A / Euro 16 A
6 QSC Audio Products, LLC
Page 9
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
Page 10
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).
8 QSC Audio Products, LLC
Figure 1.6. Solder one end
of the component
Solder
Figure 1.7. Solder other
end
Page 11
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
Page 12
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
Page 13
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
Page 14
QTY ITEM NO. PART NO. DESCRIPTION VENDOR
12 QSC Audio Products, LLC
fixture
diagram of the PL380 service
Figure 1.12. Schematic
Page 15
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
Troubleshooting Serious Failures
in
in
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
Page 16
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
Page 17
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
Page 18
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
Page 19
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
Page 20
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
Page 21
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.
If
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
Page 22
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
Page 23
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
or
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
.
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
Page 24
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
Page 25
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.
Page 26
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
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
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
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
Page 27
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
Page 28
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.
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
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 .
Quality Review
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.
Page 29
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
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
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
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
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
Page 30
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.
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
1.6 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.
Page 31
2.5 PL325 Test and Calibration Procedure (continued)
2 Ohm Power and Short Circuit
Current Test
1. Connect a 2-ohm load resistance to channel 1’s output. Apply a
1.6 V rms, 1 kHz sine wave signal to channel 1’s XLR input.
2. Adjust the gain control to obtain 1250 watts of output power
(50 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 625
watts (35.4 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 6 A
(120 V model) or 3 A (230 V model).
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.
Thermal Test
1. Set the amplifier gain controls to minimum and apply a 1.6Vrms, 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.
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.
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.
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 .
Quality Review
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.
Output Noise 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.
PL3 Series Service Manual 29
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Page 32
This page is blank.
30 QSC Audio Products, LLC
Page 33
3. Troubleshooting
3.1 PL380: Symptoms, causes, and remedies
Prerequisites to troubleshooting
Safely turning on an unknown PL380 amplifier
Using the service fixture
Before connecting it to AC power, always first check the amplifier
using the PL380 service fixture. This allows you to check the timing
and control circuits safely before applying power. See Section 1.5,
PL380 Service Fixture,
Using a variable autoformer
When first checking the operation with AC power of an amplifier on
the bench, always turn your variable autoformer down to zero
before plugging the amplifier into it.
After you turn the amplifier on, gradually turn up the transformer to
65% of full operating voltage (78 V for a 120 V unit, or 150 V for a
230 V unit), then pause for about six seconds as you observe the
turn-on sequence of the amplifier; during this time the power supply
will begin switching, the relays will energize, idle current will reach
about 2 A (120 V unit) or 1 A (230 V), the power indicator LED will
light, and finally the clip LEDs will light for about 2 seconds and then
extinguish. Observing the amp through this sequence will help you
determine what, if anything, is wrong with it.
If you see or smell smoke, flames, or any other signs of short circuits
or excessive current draw, quickly turn the transformer back down to
zero. If no such problems occur, it is usually safe to turn it up to the
amplifier’s full operating voltage for further testing or troubleshooting. Idle current at this point should not exceed 2.5 A (120 V unit) or
1.3 A (230 V).
for details and procedures.
Relay Circuit, Inrush System.
• To prevent high surge currents that would otherwise flow into
the large primary- and secondary-side capacitors, the power
supply IGBTs start switching
main reservoir capacitors. The TOP switch “keep-alive” power
supply based around U41 starts up first, providing power for the
crystal divider clock circuit and IGBT drivers.
• Then the primary reservoir is charged up through the inrush
limiting relay, K2, and large NTC resistors R262 and R266
(schematic: see sheet
Because the IGBTs are switching, the secondary reservoirs
charges along with the primary reservoir. After a short delay, the
main relay, K1, closes to bypass the inrush resistors.
• After shut-off, the IGBTs continue switching until the keep-alive
supply discharges the primary reservoir to less than 25% of its
normal full voltage. Thus, the voltages of the secondary reservoir
capacitors track those of the primary reservoir, both up and
down.
•
Never
attempt to start up the amp with fully charged primary
capacitors and discharged secondary capacitors; high peak
currents will damage the IGBTs if the primary reservoir has a
voltage more than 70% higher than the secondary one. Normal
“hot restarts” are allowable because the IGBT switching will
keep the secondary linked to the primary voltage.
• NOTE: an interlock feature (schematic: see sheet
Control, PL380”
IGBTs fail to switch. This prevents the primary capacitors’ being
charged without also charging the secondary capacitors. If there
is no IGBT drive voltage on signal bus
zero and does not provide voltage to ramp up C104.
zone D-5 and D-6) halts the relay sequence if the
before
voltage is applied to the
“Power Supply, PL380,”
IGBT-SW,
zone D-6).
“Protect/
R133 remains at
AC current draw
At idle, the current draw should be approximately 2 to 2.5 A (120 V)
or 1 to 1.3 A (230 V).
• The main heat sink will get noticeably warm but not hot,
eventually causing the fan speed to increase slightly.
• Power supply heat sinks should be cool at idle.
On/off muting
• During the normal startup sequence, use loudspeakers connected to the amp’s outputs to confirm silent on-off muting.
There should be at most only a small click audible in the
loudspeaker during on-off muting.
PL3 Series Service Manual 31
TD-000274-00 Rev. A
• If the IGBTs have been removed,
capacitors
the amplifier on.
120V/240V wiring (schematic: see sheet
zones C-6, C-7, D-6, and D-7)
• For 120V operation, wire jumper W1 (zone D-6) must be in place,
and terminal J22 would connect to J15 from the line filter.
• For 220–240V operation, W1 is not installed and terminal J21
connects to J15.
WARNING: Never connect J21 to J15 with W1 in place, because if
power is applied it will short-circuit the AC from the line filter and
cause damage to inrush components, the line filter, etc.
before
re-installing or replacing the IGBTs or turning
always
discharge the primary
“Power Supply, PL380,”
Page 34
3.1 PL380: Symptoms, causes, and remedies (continued)
SAFETY WARNING
The primary and secondary reservoir capacitors are not
covered. They store dangerously high voltages during
operation and for some time after power is turned off.
Do not touch them until they are fully discharged.
Classifying Failures
Most amplifier failures divide generally into two categories:
1. Serious—Problems that keep the amplifier from running or that
cause catastrophic failure; these must be diagnosed using the
service fixture. Typical indicators of serious amplifier failure
include blowing a circuit breaker or fuse and/or emitting a
burning smell or smoke.
2. Minor—The amplifier runs without excessive losses, but with
incorrect or reduced performance.
Troubleshooting Serious Failures
NOTE: The audio circuits of channels 1 and 2 are identical. For
simplicity, the PL380 troubleshooting instructions describe only
channel 1’s components. See the schematic diagrams to identify the
corresponding components on channel 2.
What to look for
Symptoms may include one or more of these:
• The amplifier draws excessive current; it may trip circuit breakers
or blow fuses.
• The amplifier’s switch-mode power supply (for example, the
IGBTs and their control circuit) has failed.
• One or more output FETs and/or its control circuits have failed.
Possible situations
Overvoltage failure: If the amplifier has been operated with an
excessively high AC voltage (>140 V for 120 V models, or >280 V for
230 V models), these components may be damaged and require
replacement:
• Capacitors C190 and C191 may have vented.
• The TOP switch supply U41 and diode D66 may have been
damaged.
• The primary winding of transformer T1 may be open.
Figure 3.1 IGBT gate drive waveforms
• Check for possible shorts in AC rectifier BR1 (schematic: see
sheet
“Power Supply, PL380,”
Q68 and Q69 (zone D-4), or the clamp diodes D86 and D87 (zone
D-4).
• Also check the line filter components and wiring, especially the
120 V/240 V configuration (for 120 V, J22 connects to J15 and
uses a jumper at W1; for 240 V, J21 connects to J15, with no W1).
Failed switch-mode power supply: If you suspect that the
switching supply IGBTs Q68 and Q69 (QSC part # QD-000315-00) are
damaged:
• Always remove and replace both IGBTs as a pair whenever one
or both fails. Do this even if one appears to be okay. If either
IGBT is damaged, check the gate coupling resistors R293 and
R294 for possible open circuits; check diode D106 for a possible
short. Confirm that the windings of T4 are intact. U46 may also
be damaged.
Note: The PL380 uses a special type of IGBT with internal isolation.
No mounting film or insulation is required. Use only thermal grease
between the IGBT and the mounting surface of the heat sink. Use
only the exact replacement part, IXGR60N60C2 (QSC part number
QD-000315-00).
• Inspect and repair damaged or burnt traces, if needed. For best
results, always replace R293, R294, R290, R291, U46, and D106
when replacing blown IGBTs. On amplifiers made after January
2008, there also are fuses to replace, F5 and F6. With the AC
voltage completely shut off, check gate drives at each IGBT by
using the service fixture to power the control circuits via TEST
PORT-A and TEST PORT-B. Confirm that the IGBT gate drive
waveform is correct, as shown in Figure 3.1.
zone D-6), or the IGBT switches
Excessive current draw: If the amplifier draws excessive current
at low AC voltage:
32 QSC Audio Products, LLC
Check the drive waveform at the gate of each IGBT (schematic: see
sheet
“Power Supply, PL380,”
zone D-4), Q68 and Q69. Confirm that
Page 35
3.1 PL380: Symptoms, causes, and remedies (continued)
Figure 3.4 The clock drive logic signals.Figure 3.2 FET gate drive waveforms
the wave shapes are normal and match each other well. Each gate’s
pulse train should be a rounded square wave reaching ±15 V. Note
how the waveform flattens slightly as the voltage passes through
zero (this controls the dead time between the IGBTs’ alternating
turn-off and turn-on).
If the pulse train waveforms are not correct or not present, see
“Troubleshooting switch-mode supply control circuit problems.”
Failed output FETs: If you suspect that the output switching FETs
Q10 and Q11 (channel 1; QSC part # QD-000318-00) or Q58 and Q59
(channel 2; same part number) may be blown or shorted:
• Check each FET in circuit by measuring the resistance between
its gate and its source. Shorted devices will measure less than 1
ohm.
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.
• When replacing FETs, also check their associated gate drive
resistors, IC driver, diode, etc. For example, if Q10 is shorted,
check D10, R60, R61, and U17.
With the AC voltage completely shut off, check the gate drive signal
at each FET by using the service fixture to power the control circuits
via TEST PORT-A and TEST PORT-B. Confirm that the signals on the
FET gates are normal square waves at 250 kHz, alternating between
0 and +12 V (see Figure 3.2), before replacing the heat sink.
CAUTION: Voltage—even residual amounts—on the main supply
rails can cause high current surges into a grounded scope probe
when it is connected to the FET sources. To avoid this, use only
isolated scope probes to look at gate drive signals when the main
power supply is operating.
With FETs in place (powered by the service fixture), each gate drive
should display a similar square-wave shape, with pulses alternating
between zero and +12 V, with a slightly rounded leading edge and
steeper falling edge as shown in Figure 3.2.
If there is any signal into the modulator, you should also observe
some pulse-width modulation (PWM).
You should observe a slight “dead time” between one gate’s shutoff
and the other’s turn-on (see Figure 3.3). The pulse shutting off
should pass thru 5 V about 20–30 ns before the pulse turning on
does (the two traces should cross each other near zero volts). This
helps ensure that both FETs on a channel output never turn on
simultaneously.
Check the gate drive resistors and the turn-off diode if the slopes or
dead time are wrong.
If the waveforms are not correct or not even present, see “Troubleshooting Power FET Control Circuit Problems.”
PL3 Series Service Manual 33
TD-000274-00 Rev. A
Page 36
3.1 PL380: Symptoms, causes, and remedies (continued)
is the same 250 kHz frequency as the clock drive signal but
comprises short positive pulses. See Figure 3.5.
• With the AC shut off and the amplifier powered by the service
fixture, check the drive signal at the gate of each of the two
IGBTs. See Figure 3.1. They should appear as somewhat
rounded square waves from gate driver transformer T4
(schematic: see sheet
measuring about ±14 V peak. Each waveform should have a
slight but visible flattening as it crosses zero, which forms a
short dead time where both IGBTs are off (an IGBT turns on at a
VGS of about 7 V).
• The sources of the drive signals are on the primary side of
transformer T4. Trace T4 input back to U46 (schematic: see zone
D-2), which may be damaged after an IGBT breakdown. Verify
Figure 3.5 The power supply sync pulse.
Troubleshooting switch-mode supply control circuit
problems
Troubleshooting the power supply control circuitry requires that the
service fixture be connected to the amplifier.
Excessive current draw from the service fixture: The current
draw should be about 800 mA positive and 500 mA negative with all
the gate drives operating (measure using the service fixture’s
current monitoring feature). The current draw should be approximately +620 mA and -380 mA when the amp is muted (no gate
drive).
• Abnormally high current draw often indicates a short-circuited
gate drive or a short on the output of one of the linear regulators,
which will get very hot and current limit. When investigating
possible short circuits, look for solder shorts or bad ICs.
• Solder shorts on the 5 V regulators will typically overload the
12 V or 15 V regulators feeding them.
Abnormally low current draw: This usually indicates some major
switching subsection, such as a gate driver, is not working. If none
of the switching subsections are working, check for clock problems.
Clock problems: If the clock is not working, there will be no drive
to the power supply IGBTs or the audio output FETs.
• Check logic circuitry (schematic: see sheet AMP CH-A, zone C-7).
Crystal Y1 should oscillate at 16 MHz, and you should be able to
trace the 1 MHz signal from U1 pin 7 to where it is divided to
500 kHz and 250 kHz in U4.
• The square waves in Figure 3.4 show what the clock drive logic
signals should look like on pins 2 and 5 of U4. The signals’
frequency and voltage are 250 kHz and 0 to 5 V.
• Components C15, R26, and R21 create a short pulse buffered by
U5:1, which serves as the power supply sync signal. Note that it
that the +15 V supply is present and so are drive signals from
U49, which should be synchronized to the 250 kHz pulse from the
SYNC line (see Clock, above). Check and if necessary, confirm
the values of, C275 and C277 (HF soft start time constant), Q70,
R297, R298 (reduces dead time), Q71, C276, D102, R300, and
R299 (resets C275, and also holds Q71 off if the TOP-SW supply
is not running).
NOTE: In normal operation, D102 and R299 couple negative
TOP-SW pulses into C276, turning off Q71 and enabling U49 to
ramp up to normal operation. The service fixture simulates the
effect of these pulses by pulling the base of Q71 to -0.6 V.
• DC Fault problems. If either channel develops a large DC
offset, the power supply is shut down to prevent load damage.
Sheet PROT/CNTRL, zone A-5, shows the DC shutdown system.
Each channel’s output is integrated via RC networks: R139 with
C105 (channel 2) and R140 with C106 (channel 1). Excessive
long-term positive or negative output voltages are rectified by
signal network D29, D30. D32 and Q40, Q42, Q46, and D33,
pulling down on AC-OFF-LO which discharges C104 (zone C-6),
thus shutting down the power supply. This behavior is also likely
if a low-voltage audio stage generates DC due to component
fault, being shorted to a rail, etc.
Power FET control circuit problems
The service fixture supplies operating power for all four FET gate
drive circuits, which can be inspected with the main supply voltage
shut off. CAUTION: Voltage—even residual amounts—on the main
supply rails can cause high current surges into a grounded scope
probe when it is connected to the FET sources. To avoid this, use
only isolated scope probes to look at gate drive signals when the
main power supply is operating.
With the FETs in place, AC power shut off, and the service fixture
supplying power, the drive signal at the gates (referenced to the
source) on each channel should be two similar but complementary
“Power Supply, PL380”
zone D-3),
34 QSC Audio Products, LLC
Page 37
3.1 PL380: Symptoms, causes, and remedies (continued)
0–12 V square waves, with a slightly rounded leading edge and
steeper falling edge (see Figure 3.2).
If there is any signal into the modulator, you should also observe
some pulse-width modulation (PWM).
You should observe a slight “dead time” between one gate’s shutoff
and the other’s turn-on (see Figure 3.3). The pulse shutting off
should pass through 5 V about 20–30 ns before the pulse turning on
does (the two traces should cross each other near zero volts). This
helps ensure that both FETs on a channel output never turn on
simultaneously.
Gate drive issues: Each FET is switched on and off by a specific
isolated gate drive signal that uses the source terminal as a
reference. The FET sources are at different potentials, and each
gate drive circuit requires its own 25 V power supply. Therefore,
each gate drive and supply has a voltage shifting scheme appropriate for its location in the circuit. For example, the gate drive for the
low-side (negative) FET, Q11 (schematic: see sheet
PL380,”
zone C-3), uses the -185 V rail (at the FET source), not
ground, as a reference. Therefore, the gate drive power derives from
capacitively coupling power pulses from the ground-referenced
housekeeping transformer tap via C262 and C263 (schematic: see
sheet
“Supply, PL380,”
and D95 and place about 25 V on C261 and C284. For channel 2, the
voltage at LO-SIDE GATE-B derives from an identical circuit
(schematic: zone C-2).
The source on each high-side (positive) FETs, Q10 on channel 1 and
Q58 on channel 2, must alternate between the positive and negative
rail voltages. The gate drive circuitry, therefore, is powered via a
coupling transformer, T3 (schematic: see sheet
zone A-4), a low-capacitance 1:1:1 gate drive transformer whose
primary is coupled to the 16 V housekeeping supply tap, with each
secondary connected to rectifiers shown (schematic: zone A-3) that
provide 25 V of floating power to each high-side gate drive.
zone C-3). The pulses are rectified by D94
“Amp Ch A,
“Supply, PL380,”
• With your oscilloscope, check and confirm the optocoupler input
signals, with respect to ground.
On channel 1, optocouplers U12 and U13 should receive a 5 V
PWM logic signal at their inputs
corresponding devices are U31 and U32.
Confirm that U12 and U13 are delivering a 5 V PWM output to
inputs of U17 and U18 (etc). Confirm that U17 and U18 are
delivering 12 V gate drive.
• Undervoltage lockout—Voltage dividers formed by R342 and
R343 (on U17) and R340 and R341 (on U18) hold the respective
“enables”
voltage. Confirm that the values are correct.
(pin 3)
low until the 12 V supply has neared full
(pin 2)
. On channel 2, the
Figure 3.6 Triangle wave at comparator inputs (pin 2 of U8 and U28).
One FET gate drive does not work
The Channel 1 gate drive circuitry is shown on schematic sheet
“Amp Ch A, PL380,”
B, PL380,”
If one FET gate drive is working but not the other, you only have to
troubleshoot the defective one. You can use the one that works as a
reference, though.
• With your voltmeter, check and confirm the gate drive supply
zones D-3, D-4, C-3, and C-4.
voltages, measured with respect to the reference
MC7812CT; pins 2, 3, 6, and 7 of the LM78L05)
There should be approximately +25 V (unregulated) at the inputs
(pin 1)
of U19 and U20 (channel 1) or U38 and U39 (channel 2),
and +12 V on the outputs
The outputs
and U34 (channel 2) should be +5 V.
PL3 Series Service Manual 35
TD-000274-00 Rev. A
zones D-4 and C-4. Channel 2’s is on
(pin 3)
.
(pin 1)
of regulators U14 and U15 (channel 1) or U33
“Amp Ch
(pin 2 of the
of each regulator.
Figure 3.7 Triangle wave with supply rails energized.
Page 38
3.1 PL380: Symptoms, causes, and remedies (continued)
Both gate drives do not work
First check the supply voltages and signal flow as described above
in “One FET gate drive does not work.” Then continue with these
steps:
• Check the mute logic signals at U7. A logic low at pin 5
(“AMP_TRIG_B”) or 9 (“AMP_TRIG_A”) will mute the PWM
signals to their respective channel’s output devices. A logic high
(approx. +5 V) will enable the PWM signals.
• Check for PWM pulse streams at pins 3, 6, 8, and 11 on U9
(channel 1) and on U30 (channel 2).
• Confirm that the audio signal to the modulator input is not DClocked, which could drive the modulator output to full positive or
full negative and result in no PWM switching.
Modulator issues
Even during muting, the clock will drive a 250 kHz square wave to
the modulators. On channel 1, clock signal CLK-A goes through R34
and C19 into C21, creating a triangle wave for the inputs of the
LT1016 ultra-fast precision comparator, U8. Likewise, on channel 2,
clock signal CLK-B goes through R204 and C126 into C127, creating
the triangle wave for comparator U28’s inputs.
The waveform shown in Figure 3.6 is what you should see at pin 2
of U8 and U28 with no power on the main rails.
Modulator waveform with normal amplifier voltage
With normal supply rail voltages, after muting ends you should
observe a distinct change in the triangle wave to a “double slope”
waveform (see Figure 3.7) as feedback occurs thru R35 and R36.
This may not be visible with zero main supply voltage.
• When the amplifier’s muting period ends, verify that there pulsewidth modulated logic signals on all outputs of U9 and U30 (pins
3, 6, 8, and 11). Note that the polarities are reversed for positive
and negative gate drives.
• Often, the customer complains that the amplifier runs too warm
in idle or exhibits excessive distortion, turn-on noise (excessive
thump), or instability.
Possible Situations
• Idle current exceeds 2.5 A (120 V) or 1.2 A (230 V) because of
insufficient dead time
• Excessive distortion
• Turn-on noise (DC Offset) problems
• Instabilities
• Abnormal clipping behavior
• Zobel network problems
• Anomalies in frequency response and gain
• Current limiting
• Output current clamping
• Main rail overvoltage clamping
• Fan does not run, or stays on high speed
Idle current exceeds 2.5 A (120 V) or 1.2 A (230 V) because of
insufficient dead time: On channel 1, R43, D4, and C29 on the
positive-going pulse transitions and R44, D5, and C30 on the
negative-going ones set the dead time by slightly delaying the
transition of each gate drive pulse.
On channel 2, R215, D48, and C137 set the dead time on the
positive-going pulse transitions, while R217, D47, and C139 do the
same for the negative-going transitions.
Too little dead time will increase idle current and heating. Too much
dead time will reduce idle current slightly but cause excess THD,
especially into loads of about 2 to 4 ohms per channel. The normal
midband THD, at 10% of full power (approx. 32% of the voltage the
amplifier starts to clip at) with a continuous sine wave, should be
about 0.05% into 4 ohms and 0.1% into 2 ohms.
Determine which channel draws excessive current:
on
Troubleshooting Minor Failures
NOTE: The audio circuits of channels 1 and 2 are identical. For
simplicity, the PL380 troubleshooting instructions describe only
channel 1’s components. See the schematic diagrams to idnetify the
corresponding components on channel 2.
What to look for
Symptoms may include one or more of these:
• The amplifier operates, but poorly or incorrectly.
36 QSC Audio Products, LLC
• Disable the fan by disconnecting the plug from header J5.
• Let the amplifier run at idle for 5 minutes.
• After that, turn the power switch off and measure the temperature on the main heat sink at the front and rear.
If the front section of the heat sink is warmer than the rear,
check channel 2’s dead time circuit (R215, D48, and C137; R217,
D47, and C139). If the rear section of the heatsink is warmer
than the front, check channel 1’s dead time circuit (R43, D4, and
C29; R44, D5, and C30).
Page 39
3.1 PL380: Symptoms, causes, and remedies (continued)
Excessive distortion: Each channel should exhibit midband
(approx. 1 kHz) THD of 0.01–0.02% at output levels between about
50 W and 500 W (about 20–63 V rms) into 8 ohms, increasing to
about 0.2% just below clipping and above about 5 kHz. A slight
oscillation or “glitch” in the distortion trace at about 1 to 2 dB below
clipping is normal but should not exceed 0.2%. Running signals on
the opposite channel should affect THD only slightly.
• Try each channel individually at full power (testing both channels
simultaneously will draw too much power, causing limiting).
Switching in a 4-ohm load should cause only a slight reduction in
the voltage at which the amplifier clips. After several seconds,
the gain may cut back to reduce power supply current below
about 35 A (120 V) or 18 A (230 V). Typical midband THD at 10 dB
below full power will reach 0.04–0.05%.
• Switch the load to 2 ohms. You should observe about 4000 W
(approx. 89 V rms or 126 V peak) power for about 1 to 1.5
seconds, with an AC draw of about 60 A (120 V) or 30 A (230 V).
The gain should then reduce fairly quickly to about 2500 W
(approx. 70 V rms or 100 V peak), with an AC draw of about 35 A
(120 V) or 18 A (230 V). Typical midband THD at 10 dB below full
power will reach 0.1%.
• Moderate distortion problems could be caused by incorrect dead
time.
Turn-on noise (DC offset) problems: On channel 1, resistors R23,
R309, and R17 (schematic: see sheet
provide feedback from the modulator output to op amp U3:2 during
muting, keeping the feedback voltages approximately where they
belong so there is no large jump when muting ends.
On channel 2, the equivalent components are resistors R194, R368,
and R189 and op amp U27:2 (schematic: see sheet
PL380”
zone B-7 and C-7).
If these resistors are missing or incorrect, the channel output may
have DC offsets in excess of ±50 mV, as well as turn-on thumps.
Instabilities are indicated by continuous or intermittent bursts of
high-frequency oscillation or noise in the audio signal. Check these
items:
• Disconnect the load, disable the clip limiter, and drive the
channel into clipping with a 1 kHz signal. You may observe slight
ringing on the clipped waveform, but no gross oscillations or
cutback.
• Put a 6 kHz square wave signal into the channel input and with
the load still disconnected, adjust the gain or signal level to get
about 50 to 60 volts on the output. Observe the output signal on
the oscilloscope.
Switch an 8 ohm load resistance onto the output, then 4 ohms
and 2 ohms. You should observe only slight changes of leading
edge and overshoot as the loads are changed.
“AMP CH-A, PL380”
“AMP CH-B,
zone B-7)
With no load, the output may show one or two cycles of damped
ringing after the square wave leading edge. If you increase the
signal or the channel gain to nearly the point of clipping, the
peaking may become excessive, resulting in the amp limiting or
muting to protect against severe overshoot.
• The amplifier’s frequency response is set by C2 and C200
(schematic: see sheet
set to produce flat frequency response out to 20 kHz. The phase
shift of the 2-pole output filter will result in about 10% peaking
on the square wave’s leading edges.
• Stability compensation components include C10 with R13, C16
with R32, C13 with R25, R22, and C17 (schematic: see sheet
“AMP CH-A, PL380”
must also be correct.
Abnormal clipping behavior: At just below clipping, you may
notice some switching behavior that looks like a slight oscillation on
the output. This is normal for the class of modulator used in the
PL380 amplifier, and it should not increase THD beyond about
0.25%. It also should not increase greatly when the channel clips
with no load, which is the most severe test of the amplifier’s
stability.
If the switching behavior becomes chaotic with no load or the THD
climbs severely at higher powers, first check all the stability
components: C10 with R13, C16 with R32, C13 with R25, R22, and
C17 (schematic: see sheet
check the soft-clip circuit, D68 and D108 (zone C-6), and the
reference threshold system Q80, Q81, R369–R372, R148, and R391
(schematic: see sheet
Zobel network: An RC damping network (schematic: see sheet
“AMP CH-A, PL380”
(0.47 µF) with 100-watt resistors R85 and R86 (20 ohms, mounted on
the heat sink). This maintains a 10-ohm load on the amplifier at high
frequencies, but the resistors could still be overloaded by excessive
HF output energy. If this happens, as the power dumped into the
resistors exceeds their wattage ratings, bus ZOB-A triggers Q4 via
R33, and pulls up on the LIMITER-A bus, reducing the overload. This
constrains long-term, 20 kHz power to about 900 W into 8 ohms,
which should not interfere with any reasonable audio usage.
A second, fast-acting muting scheme with C224, R383, R384, and
Q83 (schematic: see sheet
mutes the amp within 15 ms if it detects runaway HF behavior, such
as no-load operation above 40 kHz.
If the zobel resistors fail, the amplifier may function okay into test
loads but will limit prematurely at all frequencies because voltage
will pass through C78 and trigger the limiter. The fast muting is
meant to protect the zobel resistors.
Frequency response and gain issues: The amplifier’s frequency
response is set by C2 and C200 (schematic: see sheet
“AMP CH-A, PL380”
zone B-7). Components R35, R36, and C21
“AMP CH-A, PL380”
“AMP CH-B, PL380”
zone A-2) across the output, comprising C78
“AMP CH-A, PL380”
zone B-8), which are
zone B-7). Also
zone C-7).
zones A-6 and A-7)
“AMP CH-A,
PL3 Series Service Manual 37
TD-000274-00 Rev. A
Page 40
3.1 PL380: Symptoms, causes, and remedies (continued)
PL380”
zone B-8), which are set to produce flat frequency response
out to 20 kHz. The zobel network components noted above will also
have a slight effect on frequency response.
Gain problems with otherwise normal THD can be traced to the
input section and sensitivity switch (schematic: see sheet
DISP”
zone C-5), the gain control buffers (zones D-2 and B-2), and
the main PCB re-referencing stage (schematic: see sheet
A, PL380”
Gain problems accompanied by abnormal THD may be caused by
problems with the limiter or problems with feedback networks
(schematic: see sheet
R24 or R35 and R36. The amplifier will probably also be unstable.
Current limiting problems: Confirm that each channel output can
be short-circuited, while passing a signal, without failing. The peak
current through a short circuit when the amplifier is cold is about
80 A.
With output signals up to about 4 V before the short circuit is
applied, each channel shorted should draw about 13 A (120 V
model) or 7 A (230 V model) of AC. If both channels are shorted at
the same time, the current draw should be about 26 A (120 V model)
or 13 A (230 V). After a few seconds, as limiting engages, the AC
draw caused by each shorted channel should decrease to about 8–
9 A (120 V) or 4–4.5 A (230 V). With higher output signals, the
amplifier may also periodically go into protective muting when it is
shorted.
Connect 8-ohm load resistors to the amplifier outputs. Use low dutycycle tone bursts to drive it into slight clipping. Observe the output
signals on an oscilloscope. Switch the loads to 4 ohms and then 2.7
ohms; you should see very slight peak reductions. Switch the loads
to 2 ohms; you should see only moderate peak reduction (this is the
onset of current limiting).
Current limiting should look like rounded clipping; any highfrequency oscillations should be minimal. If you have 1-ohm loads
available you can readily observe current limiting; the maximum
short-term output current should be about 70–80 A.
Output current clamping problems: High-voltage current
sources Q8 and Q9 are enabled shortly after each FET is turned on
by U6:6 and U6:5, using a short delay set up by R49, D8, and C38.
The current pulls up on D16 and D17 (zone D-3) and D18 and D19
(zone C-3) after the FET has fully turned on. The resulting signals
through C67 and C68 create currents in Q12 and Q13 that track FET
current. These currents combine into an overcurrent feedback
network (zone B-1), which compares this signal to ±5 V references
and sends excess current as feedback to the modulator. The
feedback prevents any further rise in modulation, therefore
clamping the output current to the desired limit (about 80 A peak;
less current at higher FET temperatures).
• Any missing or open components will prevent current clamping,
zone B-8).
“AMP CH-A, PL380”
which will probably allow FET failure if the outputs are shorted.
zone B-7 and B-8) R29,
“INP-
“AMP CH-
• Gross overclamping (distortion) could be caused by missing or
open components D16 or D17, D18 or D19, or R76 or R79.
• Mild overclamping could be caused by missing or open R87, and
excess THD could be caused by missing C79.
Main rail overvoltage clamping problems: A typical characteristic of class D operation is that as the audio signal swings positive
and negative, the FET switches draw power from the loaded power
supply rail and recycle some of this power to the opposite rail, via
the large catch diodes. For abnormal DC outputs or very low
frequency signals, this could pump the “off-side” rail up to abnormally high voltages. The capactor overvoltage protection circuitry
(schematic: see sheet
overvoltage on positive or negative rails via voltage dividers R171
with R172 and R173 with R174. If either voltage exceeds a 5 V
reference, its respective section of U25 will clamp the input signals
of both channels via diode network D37–D42.
This circuit will rarely trigger in actual use, since heavy low
frequency output signals also tend to cause the power supply to
sag, offsetting any “pumping.” Therefore, it is difficult to test it for
correct operation. However, if you observe positive or negative
clamping on both channels’ signals, check for defects in the voltage
divider or op amp U25.
The fan does not run, or stays on high speed: Thermal buses ATHERM and B-THERM are buffered by transistors Q41 and Q38,
respectively. The emitters drive into R147. When either thermal bus
voltage goes below 4.4 V, (at approx 55° C), its transistor puts
current into Q44’s base, in turn driving Q47 to increase the fan
voltage from 10 V. The voltage to the fan can range up to the full
amount available from the housekeeping supply rails (about 30 V).
Feedback elements R160, R157, Q45, and R153 stabilize and
regulate the fan voltage. R152 provides the low-speed reference
current.
• Fan failure or blockage will ultimately result in protective muting
as the heat sink temperature reaches about 80° C. Muting
removes most sources of heat dissipation and prevents further
overheating.
• Through their emitters, transistors Q39 (channel 1) and Q36
(channel 2) “or” the THERM-A and THERM-B buses into a
common bus called A/B_THERM.
• Note that the fan is powered from the main supply and runs for
some seconds after turn-off, but it stops during prolonged
periods in Standby mode.
• Incorrectly mounting the main heat sink can damage or short the
thermal sensors, causing incorrect thermal reading.
“PROT/CNTRL, PL380,”
zone B-3) detects any
38 QSC Audio Products, LLC
Page 41
3.2 PL325 and PL340: Symptoms, causes, and remedies
When first checking the operation of an amplifier on the bench,
always turn your variable autoformer down to zero before plugging
the amplifier in.
After you turn the amplifier on, gradually turn up the AC voltage to
25% of full operating voltage (30 V for 120 V model, or 60 V for
230 V) as you observe the amplifier’s current draw and confirm that
its bias supply is working normally such as the power LED should
come on between 30 and 35 V (120 V model) or 60 to 70 V (230 V),
with its usual, steady “half-bright” start-up level; this will help you
determine what, if anything, is wrong with it.
If you see or smell smoke, flames, or any other signs of short circuits
or excessive current draw, quickly turn the AC voltage back down to
zero. If the LED comes on at 20 V (120 V model) or 40 V (230 V
model) or not until 50 V (120 V model) or 100 V (230 V model), or
blinks, do not raise voltage past 60 V (120 V model) or 120 V (230 V
model) until you have measured the bias supply voltage. If no such
problems occur, it is usually safe to turn the AC up to the amplifier’s
full operating voltage for further testing. Note: the switching will
not start until the AC voltage reaches above about 90 V (120 V
model) or 180 V (230 V model).
Power Supply Failure
The customer complains of blowing circuit breakers or fuses, or
burning smell or smoke or no power indicator.
Symptoms may include:
• Fuses blow immediately
• Line circuit breakers trip at turn-on
• The amplifier emits smoke
• Perform a quick test of the bias supply as instructed in
shooting the TOP-210 Bias Supply Faults
.
Trouble-
Stays in protect; Power LED remains dim
Make sure that pin 2 (STBY) of the DataPort is not terminated to
ground by any device that might be plugged into the DataPort,
which would cause the amplifier to stick in standby mode.
Note: all measurements should be referenced to PRI_LO.
If node N408 (pin 1 of U13) is 0 V, check the following nodes:
• Node N446 (pin 6 of U13) should be 5 V. If not, check R338, R337,
and R336.
• Node N445 (pin 7 of U13) should be 8 V. If not, check R341, R342,
and L6. If these parts are okay, C124 may be leaking.
• Node N409 (pin 5 of U13) should be 8 V. If not, check R332, R325,
R326, R328, and C121.
• Node N418 (pin 9 of U13) should be 12 V. If not, check D58,
R323, R322, and R321. If these parts are okay, C107 or C108 may
be leaking.
• Node N410 (pin 8 of U13) should be 8.2 V. If not, check R325,
R326, and R328.
• Check Q94 for a possible shorted base-collector junction.
If node N408 (pin 1 of U13) is > 4 V, check the following nodes:
• Node N399 (pin 10 of U19) should be 0 V. If not, check U14:2.
• Node N401 (pin 3 of U19): 210–230 kHz (see Figure 3.8). If not,
check U14:1 and Q98.
• Nodes N397 and N398 (pins 11and 14 of U19): 105–115 kHz (see
Figure 3.9). If not, U19 or U18 is defective.
• The amplifier gives off a burning smell
• No power indicator or dim (half-bright)
Possible situations:
Blown IGBTs
Follow
Troubleshooting and Replacing Blown IGBTs
restore the IGBTs and their control circuit. Before applying AC
voltage, verify the following:
• No drivers or output transistors are shorted. If you find any
damaged, see
Transistors
• No rectifiers—D72–D75, D80, D81, D84, and D85—are shorted.
Replace any that are damaged.
PL3 Series Service Manual 39
TD-000274-00 Rev. A
Troubleshooting and Replacing Blown Output
.
instructions to
Figure 3.8. Switching pulse at node N401 (pin 3 of U19)
Page 42
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
• If the outputs and drivers are okay, check the current limit
circuitry. See
Hint: shorting across C86 will put the unit in mute protect. This
might stop the repeadted switching on and off and allow you to
troubleshoot for faulty part(s).
Troubleshooting Current Limit (Audio Power Stage).
Figure 3.9: Switching signal with dead time at nodes N397 and 398
(pins 11 and 14 of U19).
If the previous steps are okay, check U18, L6, and C144. L6 and C144
rarely go bad.
Protection, Fan, and Muting Problems
The customer complains that the amplifier stays in protect—power
supply alternates turning on and turning off; both clip indicators
remains lit; unit stuck in mute protect; or no mute delay.
Symptoms may include:
• Power supply alternately turns on and off; power LED and relay
repeatedly cycle on and off.
• Check for a faulty in DC protection circuit. See
DC Fault Shutdown
.
Troubleshooting
Amplifier stays in mute protection.
• If fan runs at high speed, C86 is leaking or the thermal sensor’s
lead has shorted to the heat sink. See
Tracking
.
• Defective U10.
Troubleshooting Thermal
No mute delay
• Defective U10 or shorted D56, Q13, or Q58.
• Channel 1: check Q16 and Q17; Channel 2: check Q61 and Q62.
Fan doesn’t run or stays on high speed
See
Troubleshooting Fan Speed
for in-depth troubleshooting.
Instability
The customer may complain of gain problems, spurious noise, or
oscillations, or running warm at idle.
Symptoms may include:
• Gain problems
• Amplifier stays muted and in protect; both clip LED indicators
remain lit.
• Fan doesn’t run or stays on high speed.
• No or brief mute delay.
Possible situations:
Power supply repeatedly turns on and off.
Usually this indicates shorted outputs or drivers, or bad or defective
parts in the current limit circuit. Follow the below steps to troubleshoot the unit:
• Check the output transistors and drivers. If any are damaged, see
Troubleshooting and Replacing Blown Output Transistors.
• Check for leaking capacitors C179, C180, C189, or C190.
40 QSC Audio Products, LLC
• Oscillations
Possible situations:
Intermittent output
• Check the gain pot.
Gain problems and oscillations
See
Troubleshooting Stability Feedback
Runs warm in idle
• Check the step FETs. If any is shorted, it will cause the amplifier
to run inefficiently. See
depth troubleshooting.
Troubleshooting Step Problems
.
for in-
Page 43
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
Faults With Signal Present
The customer complains that the amplifier passes a signal but
doesn’t run correctly.
Symptoms covered:
• Premature clipping (without load)
• Premature clipping (with load)
• Excessive distortion
• Runs too hot
• Unequal output in bridged mono mode
Fault: Premature clipping (without load)
• One of the step rails does not work. First inspect for broken or
damaged leads on the switching FETs and diodes Q41, Q38,
D21, and D22 (channel 1) and Q86, Q83, D47, and D46 (channel 2). If the FETs and diodes are okay, see
Problems
• Defective clip limiting. See
(Audio Power Stage).
for in-depth troubleshooting.
Troubleshooting Clipping, Limiting
Fault: Premature clipping (with load)
• Excessive current limiting into normal load. See
Current Limit (Audio Output).
Troubleshooting Step
Troubleshooting
• The bias may be set excessively high. Calibrate the bias on the
suspect channel as described in the test procedure. Figure 3.10
shows an acceptable crossover spike at the distortion analyzer
output.
Figure 3.10. Typical crossover residual from distortion analyzer
output
Fault: Unequal output level in bridged mono
mode
• Check for shorted or damaged (burnt) diodes and transistors:
D50, D48, Q46, and Q42.
Fault: Excessive distortion
Gain drops about 1 dB; a high-frequency oscillation is superimposed
on the audio signal.
• Inspect for damaged or burnt ground traces from the input and
display boards to the main board.
Troubleshooting Instructions
Power Supply: Troubleshooting TOP-210 bias
supply faults—overvoltage, undervoltage, or
no voltage
Excessive THD, greater than 0.1%.
• Make sure the main board, input, output, and display boards are
securely fastened to the chassis for proper grounding.
• See
Troubleshooting Stability Feedback.
Fault: Amplifier runs too hot
• Make sure the fan is working properly. It should speed up when
the amplifier gets warm. See
depth troubleshooting.
• Check the step FETs. If any is shorted, it will cause the amplifier
to run inefficiently. See
depth troubleshooting.
PL3 Series Service Manual 41
TD-000274-00 Rev. A
Troubleshooting Fan Speed
Troubleshooting Step Problems
for in-
for in-
NOTE: The term “bias supply” refers to the small housekeeping
power supply that powers the control and switching circuitry in the
amplifier’s power supply. It does not involve the bias current in the
amplifier output sections.
Quick bias supply test
120 V models: Start with the variable transformer set to zero. Turn
up the AC voltage slowly to about 25% of the amplifier’s full
operating voltage (30 V). If the bias supply is working normally, the
green
POWER
LED should come on between 30 and 35 V, with its
usual steady half-brightness.
If the LED comes on early (at about 20 V), or late (about 50 V or
more), or blinks,
do not raise the AC past 60 V
until you first
Page 44
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
measure the bias voltage. (The switching will not start until the AC
reaches 90 V, so stopping here can save the IGBTs from blowing.)
Measure the bias voltage at the + terminal C138; it should be about
+18–19 V.
230 V models: Start with the variable transformer set to zero. Turn
up the AC voltage slowly to about 25% of the amplifier’s full
operating voltage (60 V). If the bias supply is working normally, the
green
POWER
usual steady half-brightness.
If the LED comes on early (at about 40 V), or late (about 100 V or
more), or blinks,
measure the bias voltage. (The switching will not start until the AC
reaches 180 V, so stopping here may save the IGBTs from damage.)
Measure the bias voltage at the + terminal C138; it should be about
+18 to +19 V.
LED should come on between 60 and 70 V, with its
do not raise the AC past 120 V
until you first
Fault: Bias supply voltage much too high
• Diodes D63–D67 or resistor R349 are open or missing—the
feedback path to U16 may be broken.
Fault: No bias supply voltage
• U16 is missing or blown.
• T1 is missing or reversed or has an open primary. Check with an
ohmmeter: from pin 1 to pin 2 should measure about 15 ohms.
• Diode D62 may be open or missing.
• U16 may be overloaded (have an excessive current demand on
it). Check U13, U14, U18, and U19. Hints: with an ohmmeter, the
resistance across C139 (between +16V and PRI_LO) should
measure > 20 kΩ; between pins 16 (+VREF) and 1 (PRI_LO) of
U19 should measure > 3 kΩ.
Fault: Bias voltage errors
The exact voltage is controlled by the feedback through diodes
D63–D67 and resistor R349, as described:
• The + terminal of C138 is the “+18V” rail, with a typical voltage
of about 18.8 V.
• Diodes D63–D65 each subtract a “diode drop” (approximately
0.7 V) from the +18V rail.
• The + terminal of C139 is the “+16V” rail, with a typical voltage
of about 16.6 V.
• D66, a 10V zener diode, in series with diode D67, subtract about
11 V from +16.6 V.
• Resistor R349 subtracts about 0.5 V, bringing the net voltage at
pin 4 (feedback reference) of U16 to about 5.1V.
• U16 uses this feedback to maintain regulation of the +16V and
+18V supplies by adjusting the “on” time at pin 5 and thereby
adjust the flyback voltage out of T1.
• C142, R356, and R349 form a closed-loop stabilizing circuit that
keeps the regulated voltage from “hunting,” or varying unnecessarily.
• Transistor Q99 and resistor R374 reduce the voltage of the bias
supply by 33% when the AC voltage is turned off. This prevents
the Power LED from lighting at half brightness after turn-off as
U16 continues to run from the main reservoirs for some time
after shut down.
Resistors R375 and R376 sense the output of U13:3, the “Loss of
AC” comparator, and cause Q99 to turn on. If Q99 has shorted,
the bias voltages will remain 33% low when AC is turned on.
Fault: Replacing blown TOP-210
If U16 has blown, check T-1 for continuity after removing U16, as its
primary may be open. Between pins 1 and 2 should measure about
15 ohms. Replace diode D60 because it has been overstressed.
Power Supply: Troubleshooting and replacing
failed IGBTs
Probable causes
• Short circuits in control circuitry: The parts operate well
within their ratings and should hold up well in the field. The
usual cause of failure is when both IGBTs turn on at once,
shorting node PRI_HI to PRI_LO. This occurs when something
causes the drive signal to one part to remain on when the other
part is supposed to turn on. Short circuits from solder or other
debris are a possible obvious cause.
• Short circuits in the load: Although there is peak current
shutdown, shorts in the power amplifier transistors or secondary-side supply components can cause currents to increase too
quickly to prevent damage.
• Overvoltage on the bias supply: If the TOP-210 bias supply
fails to operate, no harm occurs, the unit simply does not
operate. However, open circuit (missing part) in several key
components can cause the bias supply voltage to be much too
high. This blows the 2110 and thus the IGBTs.
• Broken lead or open rectifier: D72–D75, D80, D81, D84, D85.
Troubleshooting failed IGBTs
The amplifier’s IGBTs are driven by an active, direct coupled
integrated circuit rather than a gate drive transformer. IGBT or driver
42 QSC Audio Products, LLC
Page 45
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
• U19, a SG3525AN pulse-width modulation controller; it may be
PLC Power Supply
Restoration Kit
The PLC power supply restoration kit contains most often
needed parts for restoring a faulty power supply in a PL325
amplifier.
damaged by high currents shorted through U18 or by overvoltage
on the supply rail.
• U14, a 556 dual timer; powered from the 5V reference voltage
output of U19, it may be damaged when the SG3525AN fails.
• U13 has fairly high supply voltage ratings and should rarely fail.
PLC Series Power Supply Restoration Kit (QSC
part number: SG-000060-00)
Part Number Description Reference Qty.
QD-000169-00 XISTOR IGBT TO-247AC 600V 55A Q96 and 97 2
QD-000042-00 DIODE RECT ULTRAFAST 400V 3A D70 and 71 2
QD-000108-30 DIODE SMT SWITCH 200V .2A 50NS D78 and 79 2
QD-000113-30 DIODE ZNR 10V 5% D66 1
RE-001003-30 RESISTOR SMT 10 OHM 1% 1206 R358 and 359 2
RE-003921-30 RESISTOR SMT 39.2 OHM 1% R349 1
RE-000210-NR THERMISTOR NTC 15A R324 1
IC-000134-00 IC CMOS HV DRVR IR2110 U18 1
IC-000024-00 IC REG PWM 40V 0.1A SG3525A U19 1
IC-000053-30 IC LIN SMT DUAL TIMER LM556 U14 1
IC-000054-30 IC LIN SMT QUAD COMP LM339AM U13 1
NA Technical Support CD NA 1
Needed for PL340
The PL340 amplifier uses the same parts except for the
IGBTs, which require a higher current rating.
Part Number Description Reference Qty.
QD-000315-00 IGBT,600V,75A,IXGR60N60C2,TO-247 Q96 and 97 2
failure should be rare (when
correctly assembled) but when
an IGBT fails, it usually damages
the following parts:
• Q96 and Q97 (IGBTs generally
fail in pairs)
• Gate drive coupling components D78, D79, R358, R359;
check them after removing
blown IGBTs.
Audio Output: Troubleshooting and replacing
damaged output transistors
Overview
When an output transistor fails, it will usually become a short circuit
across its base, emitter, and collector terminals. A short circuit in
one device often cause another to fail as well.
If an output transistor shorts:
• The driver transistor connected to it will probably also fail (Q26,
Q27, Q71, or Q72).
• Certain transistors tend to fail or short in pairs: Q39 and Q40,
Q36 and Q37, Q84 and Q85, or Q81 and Q82.
• Others may tend to fail or short in groups of four: Q28, Q29, Q34
and Q35; Q73, Q74, Q79 and Q80.
Measure
here
R
Measure
here
R
Measure
here
R
Measure
here
R
• U18, the IR2110 high-side
gate driver; damage occurs
from high current when lowside IGBT, Q97, shorts to
PRI_HI. Such current surges
typically also damage the
gate drive coupling components noted above.
Figure 3.11. Identify damaged transistors by measuring resistance across the collector and emitter.
PL3 Series Service Manual 43
TD-000274-00 Rev. A
If = approx. 0.4 : transistor is blown
R
If = 0.7 : transistor is good
R
Ω
Ωapprox.
Page 46
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
Troubleshooting hint: Identifying shorted transistors incircuit
Although the output transistors are arranged in two banks (one NPN
and the other, PNP) of four devices, you can identify a shorted
output transistor among them without first removing them from the
circuit. This requires an ohmmeter that can resolve to tenths of an
ohm reliably.
Measure the resistance across the collector and emitter of each
transistor in the bank. The ohmmeter’s polarity is not important. A
damaged transistor will measure about 0.4 Ω, while each good
transistor will measure about 0.7 Ω (see Figure 3.11). This test can
be done on both NPN and PNP transistors.
Remove and replace damaged transistors with genuine ones of the
same type. Do not substitute.
Troubleshooting hint: Checking emitter and base
resistors while transistors are removed
When you remove a failed output or driver transistor for replacement, check also for damage in the adjacent resistors. Replace any
that are physically damaged or do not measure within 10% of their
correct value.
• On each output transistor’s emitter is a pair of 0.47-ohm resistors
in parallel.
• Each bank of output transistors has a 15-ohm resistor from the
base to the supply rail (R85, R86, R208, or R209).
• Each driver transistor has a 200 ohm resistor (R81, R82, R204, or
R205) from its base to the supply rail.
Fault: Supply clamping diode shorted
This would draw excessive current from the power supply and
trigger DC protection. Check D19 and D20 on channel 1, or D44 and
D45 on channel 2.
Fault: Baker clamping circuit problem
A shorted driver transistor may cause damage to one of the Baker
clamp diodes.
• Channel 1: check D10–D14 and D16.
• Channel 2: check D35–D39 and D41.
Audio Output: Troubleshooting current limiting
Overview
The usual symptom of weak output current is premature clipping on
peaks of the audio signal. This could be caused by a malfunctioning
class H step, weak current limiting, or a dead output section.
Fault: No output on one polarity
If the positive or negative half of the output signal is missing, there
is an open in the part of the current splitter circuit that leads to the
dead output polarity. Check for missing or open components in these
locations:
• Channel 1, positive: Q19, Q20, R70, R381, D10, and D14;
negative: Q21, Q22, R71, R382, D11, and D12.
• Channel 2, positive: Q64, Q65, R193, R383, D35, and D39;
negative: Q66, Q67, R194, R384, D36, and D37
Table 3.1. Clamping voltage troubleshooting
Channel and polarity ☛ Channel 1 + Channel 1 - Channel 2 + Channel 2 -
normal voltage on PL325 6.0 V 6.0 V 6.0 V 6.0 V
normal voltage on PL340 6.5 V 6.5 V 6.5 V 6.5 V
check for missing resistor R60 R61 R183 R184
If voltage is too high
or missing transistor Q14 Q15 Q59 Q60
check for shorted transistor Q14, Q18, Q24 Q15, Q23, Q25 Q59, Q69, Q71 Q60, Q68, Q70
If voltage is about 0–0.3 V
or missing resistor R51, R72 R53, R75 R174, R195 R176, R198
If voltage is approx. 0.7 V check for missing resistor: R59 R62 R182 R185
If voltage is wrong check for wrong value R59, R60 R61, R62 R182, R183 R184, R185
44 QSC Audio Products, LLC
Measure across: C21 C22 C56 C57
Page 47
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
Table 3.2. Troubleshooting clamp malfunctions
Channel and polarity ☛ Channel 1 + Channel 1 - Channel 2 + Channel 2 -
Measure clamping voltage across: C21 C22 C56 C57
Check for low resistance R67, R73 R68, R74 R190, R196 R191, R197
Clamping
voltages collapse
too soon
Cutback
occurs
prematurely:
Check for high resistance R51, R72 R53, R75 R174, R195 R176, R198
Check for open or missing
diode
Check for shorted diode D10, D14 D11, D12 D35, D39 D36, D37
D9 D8 D34 D33
Clamping
voltages are
correct but
Check for missing, open, or unsoldered output
transistors or emitter resistors
current is still
weak
Check for stressed emitter resistors that may be
out of tolerance
R88, R91, R94, R97,
Fault: Premature clipping at about 60% of maximum
peak voltage, at any impedance
This indicates a malfunctioning step in the channel’s class H
circuitry. See
Troubleshooting a Step Problem
.
Fault: Constant premature clipping, worse at low load
impedances
If the amplifier can produce a full output voltage sine wave signal
into a load of 8 ohms (PL325: greater than 60 V rms or 85 V peak;
PL340: greater than 75 V rms or >110 V peak) but clips prematurely
into 4 or 2 ohms, it is safe to assume that the class H step circuitry
is all right but the output current capability is too low.
First, check the clamping voltages across C21 (Channel 1 +), C22
(Channel 1 -), C56 (Channel 2 +), C57 (Channel 2 -), as shown in
Table 3.1. At idle, all four voltages in the amplifier should be similar.
If one is significantly different from the others, check parts according to the following table
The exact voltages will vary with temperature, but the one that does
not match the others will indicate the weak cell. A voltage that is
too low will cause premature clamping on that polarity of the
output section.
If the voltage is correct and current is still low, also check for
missing or unsoldered output transistors or emitter resistors.
Q28, Q34, Q36,
Q39; R88, R91,
R94, R97, R102,
R107, R111, R113
R102, R107, R111,
R113
Q29, Q35, Q37,
Q40; R89, R92,
R95, R99, R103,
R108, R112, R114
R89, R92, R95, R99,
R103, R108, R112,
R114
Q73, Q79, Q81,
Q84; R211, R214,
R217, R220, R225,
R230, R234, R236
R211, R214, R217,
R220, R225, R230,
R234, R236
Q74, Q80, Q82,
Q85; R212, R215,
R218, R222, R226,
R231, R235, R327
R212, R215, R218,
R222, R226, R231,
R235, R327
Audio Power Stage: Troubleshooting current
limiting
Fault: Output collapses prematurely
Power supply cutback normally will cause the output signal to
collapse after several seconds at continuous level at or near full
power into low load impedances, such as 2 ohms per channel or a
4-ohm load in bridged mono. This is done to protect the power
supply circuitry from excessive long-term current or overheating
while still allowing full power for audio peaks and transients.
If the output collapses immediately instead of after several seconds,
it may indicate a premature triggering of the power supply cutback.
Cutback in one or both output sections at high temperature and
nearing maximum power into two ohms is also normal. But cutback
that occurs when driving loads of 4 ohms or higher per channel (or 8
ohms or higher in bridged mono), or when the amplifier is cold, into
2 ohms per channel or 4 ohms in bridged mono, indicates a problem
with the transistor power measuring circuitry (see Table 3.2).
Audio Power Stage: Troubleshooting power
supply cutback
Overview
As noted in the power supply description, the amplifier’s current
limit cuts back when necessary to protect the power supply circuitry.
Because the observed effect is a reduced maximum output voltage,
in response to prolonged operation above the long-term current
PL3 Series Service Manual 45
TD-000274-00 Rev. A
Page 48
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
Figure 3.12. The overcurrent detection circuit for power supply cutback.
limit, we commonly refer to this behavior as “power supply
cutback”, but we must remember that it is actually
limiting
in response to an overload signal sent from the power
supply. Full-power operation into 2-ohm loads (on both channels)
should produce a 50% cutback of current after several seconds.
If the amplifier fails to cut back both channels after about 3 seconds
when driving a 2-ohm load on each channel with a continuous test
tone at maximum output, the cutback signal is probably missing.
CAUTION: Prolonged operation under these conditions with no
cutback could blow the IGBTs or burn out capacitor C144. Conduct
testing for no longer than 6–10 seconds.
Check the output (secondary side) pins of the MOC8101 optocoupler U17 (see Figure 3.12). Confirm the presence of +6 V
(reference to amplifier’s ground) on pin 5. The voltage on pin 4
should normally be at about 0 V, and go high (+1 to +5 V) after about
3 seconds at full power.
amplifier current
Table 3.3. Troubleshooting clamp transistors
Channel and polarity ☛ Channel 1 + Channel 1 - Channel 2 + Channel 2 -
Clamping voltage
Measure voltage on C21 C22 C56 C57
decreases, but no
current cutback
Check clamp transistor Q18 Q23 Q63 Q68
If pin 4 on U17 does not go high, examine the IC itself. If it appears
undamaged, trace the circuitry driving U17 (CAUTION: this circuitry
is on the primary side, which uses PRI_LO as its reference). Check
for continuity through L6:2 to PRI_LO (from pin 3 to pin 4); also check
for missing or open R343, D61, Q95, or R347, all of which drive the
input of opto-coupler U17. A short in R346 or C131 will also prevent
voltage reaching U17.
If pin 4 of U17 goes high as it should yet both channels fail to cut
back, trace the voltage on the PS_OL bus to R273, which connects
to the MUTE+ bus. Continue tracing voltage on MUTE+ to Q16 and
Q61. If only one channel fails to cut back, look for missing Q16, R65,
or Q17 or Q61, R188, Q62.
Capacitor C131 controls how quickly the power supply cutback
reacts. If it is missing, the amplifier current limits will enter cutback
almost immediately at or above full power into 4 ohms per channel.
46 QSC Audio Products, LLC
Page 49
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
Table 3.4. Troubleshooting short circuit cutback clamping
Channel and polarity ☛ Channel 1 + Channel 1 - Channel 2 + Channel 2 -
Measure clamping voltage across: C21 C22 C56 C57
Clamping
voltage does
not decrease
50%
Check cutback transistor and
resistor
Check voltage sense resistor R73 R74 R196 R197
Check for shorted or low value
resistor
Q24, R67 Q25, R68 Q69, R190 Q70, R191
R72 R75 R195 R198
Audio Power Stage: Troubleshooting short
circuit cutback
Fault: Short circuit current doesn’t cut back
WARNING: Do not maintain a shorted load if cutback fails to occur
within 1 second.
It will be necessary to measure the output current with a DC current
probe, or by noting the voltage across a low value resistance with a
DC scope, in order to determine which output cell—the positive or
the negative one—is failing to cut back.
Failure to cut back could indicate either lack of clamping, or lack of
voltage cutback. Measure the voltage on the respective clamp
capacitor. If the voltage decreases, but current limiting does not cut
back, check the clamping transistor (see Table 3.3).
If the measured voltage on the clamp capacitor does not decrease
to about 50% during the short, check the circuitry that measures the
current during short circuit (see Table 3.4).
Audio Output Stage: Troubleshooting Stability
and Feedback
Fault: High frequency oscillations
Symptoms are severe oscillations accompanied by significant
current draw and gross distortion.
• Channel 1: C27 or R367 missing, open, or wrong value;
Channel 2: C62 or R368 missing, open, or wrong value.
• Channel 1: C25 or C26 missing, open, or wrong value;
Channel 2: C60 or C61 missing, open, or wrong value.
• Secondary reservoir capacitors missing or open (it would be very
unlikely that all are defective).
Symptoms are severe oscillation but no large current draw, with low
voltage gain.
• Channel 1: R22 open or missing;
Channel 2: R146 open or missing.
Fault: Marginal instability—may appear only as
excessive distortion
• Channel 1: C14 missing or open;
Channel 2: C49 missing or open.
• Channel 1: C16 missing or too large;
Channel 2: C50 missing or too large.
• Channel 1: C28 missing or open;
Channel 2: C63 missing or open.
• Channel 1: C25 or C26 missing or too large;
Channel 2: C60 or C61 missing or too large.
• C195 or C196 missing on the input board.
Fault: Excessive oscillation just below clipping into 2–
4 ohms per channel
NOTE: Into 2-ohm loads, about 0.1% oscillation at just below
clipping is normal.
• Channel 1: C17 missing or open; Channel 2: C52 missing or open.
Fault: Excessive switching noise
Switching noise may look like instability on an oscilloscope, but its
frequency will be much lower (110 kHz) than an oscillation typically
will be. It will be more visible on low-frequency signals (about
200 Hz and lower) and with lower load impedances.
• C129 or C134 missing on output board.
• Bad chassis-to-ground connections; check mounting screws on
the output and main boards.
PL3 Series Service Manual 47
TD-000274-00 Rev. A
Page 50
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
Fault: Incorrect gain
• Channel 1: R23 and R31 set the gain of the output stage;
Channel 2: R147 and R153 set the gain of the output stage.
• The two gain control potentiometers are each buffered by a
single op-amp stage, whose gain is set by a pair of resistors.
Channel 1: R11 and R16.
Channel 2: R137 and R139. Make sure Q48 is turned on (with the
amplifier not in bridged mono mode). Grounding R137 at Q48
should not affect gain; if it does, check R132, which turns Q48 on
or off.
• Each channel’s balanced input is a differential op-amp stage. The
gain of each is set by four matched resistors R8, R9, R12, and
R13 on Channel 1 and R129, R130, R135, R136. Confirm both the
+ and - sides of the balanced inputs are working; check R5 and
R6 on channel 1 and R123 and R124 on channel 2.
Audio Power Stage: Troubleshooting Clipping
and Limiting
Fault: Excessive clip sticking (too much distortion
during clip limiting)
• Channel 1: C14 is much too large in value;
Channel 2: C49 is much too large in value;
(these can also cause increased high frequency distortion).
signal peaks exceed 4 V during clipping, check these components:
• R38 (channel 1) or R161 (channel 2) may be missing or open.
• R38 or R39 (channel 1), or R161 or R162 (channel 2), may have
incorrect values.
• Q9 or Q10 (channel 1), or Q54 or Q55 (channel 2), may be missing
or open.
• R34 or R35 (channel 1), or R157 or R158 (channel 2) may be
missing or open.
• Q8 (channel 1), or Q53 (channel 2), may be missing or open.
If the op-amp output clamps at 3.5–4 V as expected, check these
parts surrounding U3:
• R32 (channel 1) or R154 (channel 2), may be missing or open.
• Channel 1: R38 is missing or open;
Channel 2: R161 is missing or open.
• Channel 1: R38 or R39 are of wrong values;
Channel 2: R161 or R162 are of wrong values.
• Channel 1: Q9 or Q10 is missing or open;
Channel 2: Q54 or Q55 is missing or open.
• Channel 1: R34 or R35 is missing or open;
Channel 2: R157 or R158 is missing or open.
• Channel 1: Q8 is missing or open;
Channel 2: Q53 is missing or open.
Fault: Clip limiting doesn’t work (both channels)
• See if U3 is missing or defective.
• Check U3 supply voltages at +13VCL and -13VCL. You can find
node +13VCL at the junction of R246 and C73, and -13VCL at
R247 and C74.
Fault: Clip limiting doesn’t work (only one channel)
With your oscilloscope, probe the op-amp output on pin 7 of U2
(channel 1) or U7 (channel 2), while the channel is clipping. If the
Figure 3.13. Output signal and positive rail steps.
Figure 3.14.
Output signal and negative rail steps.
48 QSC Audio Products, LLC
Page 51
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
• Q7 (channel 1) or Q52 (channel 2), may be missing or open.
• R28 (channel 1) or Q151 (channel 2), may be missing or open.
• Q6 (channel 1) or Q51 (channel 2), may be missing or open.
• R18 (channel 1), or R141 (channel 2), may be missing or open.
• R19 (channel 1), or R142 (channel 2), may be missing or open.
• On the rear panel DIP switches, SW1:1 (channel 1), or SW1:10
(channel 2), may not be making contact.
• Check each pin on U3.
Fault: Clip limiter oscillates
• C13 (channel 1), or C48 (channel 2), may be missing or open.
• R21 or R27 (channel 1), or R144 or R150 (channel 2), may be
missing or open.
Audio Power Stage: Troubleshooting Step
Problems
Fault: Step won’t turn on (premature clipping)
• First, inspect the step FETs—Q38 and Q41 on channel 1, or Q83
and Q86 on channel 2—as well as diodes D21 and D22 on
channel 1 or D46 and D47 on channel 2 for broken leads.
Replace any damaged components.
• Check the DC supply voltages on the LM311 step comparators,
U4 and U5 on channel 1 or U8 and U9 of channel 2. Measure
across pins 8 (V+) and 4 (V-).
The supply voltages should measure about 14 V on the positive
step driver comparators U4 and U8, and about 12 V on U5 and
U9, the negative step driver comparators.
If any step circuit will not switch to the high rail, the channel will
clip prematurely on that polarity, with any load or even with no load.
Make sure the clipping is not actually current cutback, which is
usually evident only when driving signal into 2-ohm loads.
With your oscilloscope, probe the output voltage and intermediate
rail voltages to confirm the clip point and the lack of step action.
Trace the circuit back from the step FET via the gate drive to the
drive circuit, to locate cracks, missing parts, etc.
Check the voltage of the nodes POSREF (it should be about 20 V
below the +MID-RAIL node) and NEGREF (it should be about 17.5 V
above the -MID-RAIL node). Look also for severe mismatches among
the comparator resistor ladders: R52 and R54–R56; R57, R58, R63,
and R64; R175 and R177–R179; R180, R181, R186, and R187.
Fault: FET does not stay fully turned on
This generally causes clipping and possible distortion problems at
low frequencies when the amp drives 2-ohm loads.
Use a 20 Hz sine wave signal and set it to just the onset of clipping
on the waveform peaks. Confirm that the step FETs remain fully on
for the entire time that the signal exceeds the step threshold and
shut off slightly after the signal descends below the threshold. If
they shut off prematurely, check for weak gate drive and determine
the cause.
• Weak positive step gate drive: check for about 14 V across C32
(channel 1) or C51 (channel 2).
Check capacitor C31 (channel 1) or C66 (channel 2).
Resistor R104 (channel 1) or R227 (channel 2) may be low in
value, and D18 (channel 1) or D148 (channel 2) may be missing or
open.
Also check R78, D15, and Q30 (channel 1) or R201, D40, and Q75
(channel 2).
• Weak negative step gate drive: check for about 12 V across C67.
Check R83, D17, and Q32 (channel 1) or R206, D42, and Q77
(channel 2).
Fault: FET turns on or off very slowly
This generally causes problems at high frequencies when driving 2ohm loads.
If the turn-on and turn-off slopes are both equally slow, check
capacitor C30 (channel 1) or C65 (channel 2) on the positive steps
and C29 (channel 1) or C64 (channel 2) on the negative.
If only one slope is slow, check these components and buffer
transistors:
• Positive step: R78, R79, D15, Q30, and Q31 (channel 1); R201,
R202, D40, Q75, and Q76 (channel 2).
• Negative step: R83, R84, D17, Q32, and Q33 (channel 1); R206,
R207, D42, Q77, and Q78 (channel 2).
Fault: Step stuck on (stays on high-voltage rail)
• Check for shorted FETs Q38 and Q41 (channel 1) or Q83 and Q86
(channel 2) and replace damaged FET if necessary.
If the positive step is stuck on, (as evidenced by a permanent high
voltage on the switched rail) the FET is certainly bad, since the
positive gate drive circuitry cannot indefinitely stay turned on due to
its bootstrapped supply.
If the negative step is stuck on, the cause could be a bad FET, or the
gate drive circuit could be holding the FET on, which can be
confirmed easily by measuring the gate voltage with no signal
present; the gate voltage should be very close to the negative high
rail voltage. If it is several volts positive with respect to the negative
PL3 Series Service Manual 49
TD-000274-00 Rev. A
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3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
high rail, then the gate drive is incorrectly turning the FET on.
Malfunctioning gate drive circuitry should be checked as descibed
under
Fault: Step won’t turn on (premature clipping)
.
Fault: Repeated FET failure
Repeated failure of a step FET is usually caused by thermal stress
because of a failure to switch fully on or off—that is, the FET
lingers in its linear region. Failures usually occur with 2-ohm loads,
when the FET’s dissipation is highest.
After replacing the FET, you should monitor the step waveform with
a large signal. Start with at light load, 8 ohms, to avoid a repeat
failure. Advance briefly to heavier loads—4 and then 2 ohms—
while closely watching the waveform. You will need to use an
isolated scope probe which allows voltage readings to be taken
with respect to the intermediate rails, or to FET sources.
Fault: Excessive step distortion (step glitch)
Close scrutiny of the distortion trace, and scope probing of the
switched waveform, will help determine the cause of excess step
distortion. The step should switch when the output voltage rises to
within 10 to 12 volts of the low rail voltage. This switching margin
should be fairly constant throughout the audio spectrum, from 20 Hz
to 20 kHz.
The switching event itself should be a fairly uniform up or down
ramp, changing at a rate of about 25 V/µs. It should therefore take
about 2 µs to complete its transition.
Fault: Step switching too close to the rail
The step switches well after the signal voltage has approached to
within 10 to 12 volts of the low rail voltage.
This will cause increased step glitch, especially at low load
impedances. If this happens at all signal frequencies, check the
reference voltages at the nodes POSREF and NEGREF.
• POSREF should be about 20 V below the +MID-RAIL node.
• NEGREF should be about 17.5 V above the -MID-RAIL node.
impedances. Two or three “false switches” at the step threshold on
very low frequency signals into 2-ohm loads is acceptable, but
prolonged bursts of chattering on mid or high frequencies may
cause FET failure.
To minimize chattering the step comparator has hysteresis, or
positive feedback, to slightly shift the threshold when it changes
state; when the comparator switches the step on, it shifts the “off”
threshold slightly toward zero.
• Positive step: Check the hysteresis resistor R66 (channel 1) or
R189 (channel 2).
• Negative step: Check the hysteresis resistor R69 and capacitor
C187 (channel 1) or R192 and C193 (channel 2).
Fault: Slow or fast switching
Slow switching reduces step glitch but puts more stress on the FET.
Fast switching, on the other hand, increases step glitch. The usable
range of step transition speed is about 17–27 volts/µs.
If the turn-on and turn-off slopes both appear to be equally off
speed, check the slope capacitors: C30 (positive) and C29 (negative
on channel 1, or C65 (positive) or C64 (negative) on channel 2.
If only one slope is slow, check these components and buffer
transistors:
• Positive step: R78, R79, D15, Q30, and Q31 (channel 1); R201,
R202, D40, Q75, and Q76 (channel 2).
• Negative step: R83, R84, D17, Q32, and Q33 (channel 1); R206,
R207, D42, Q77, and Q78 (channel 2).
Fault: Step FET oscillation
Certain FET types tend to oscillate at extremely high frequency
while ramping up and down. This injects interference into the
signal, effectively increasing the step glitch. Identifying such
tendencies is one of the evaluative tasks in qualifying or disqualifying specific FET types. If these problems appear in any QSC
amplifier, please report it to QSC Technical Services.
Confirm correct resistor values in the output voltage divider: R48–
R50 (channel 1) or R171–R173 (channel 2).
If the switching problem is present only on high frequencies, check
the value of the speed up capacitor C20 (channel 1) or C55 (channel
2) in the output voltage divider, or look for slow switching (see
Slow or fast switching
, below).
Fault: Step chattering
If the step repeatedly switches on and off in rapid succession, it
creates an oscillation burst which increases step glitch. The
tendency is usually greater with low-frequency signals and low load
50 QSC Audio Products, LLC
Fault:
Fault: Severe step oscillation
This happens generally on low frequencies with low load impedances, right at the step threshold.
• Positive step: Check the hysteresis resistor R66 (channel 1) or
R189 (channel 2).
• Negative step: Check the hysteresis resistor R69 and capacitor
C187 (channel 1) or R192 and C193 (channel 2).
Page 53
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
Audio Power Stage and Power Supply:
Troubleshooting DC Fault Shutdown
Normal circuit behavior
Any amplifier fault that causes a severely asymmetrical output, such
as premature clipping of one polarity, a missing step, etc., may
trigger DC fault shutdown. This is normal behavior of the DC fault
shutdown circuit.
Tracing the cause of false triggers
If amplifier is shutting down for no apparent cause, you must locate
the source of the false DC fault signal. Check the amplifier outputs
with a DC coupled oscilloscope to confirm the actual absence of a
DC offset. The circuit will trip on DC offsets exceeding about 4 V,
positive or negative.
The input of opto-coupler U15 can be safely disabled by shorting
pins 1 and 2. This will indicate if the false DC signal’s source is
before or after U15:
• If the amp comes out of shutdown when you disable U15’s input,
then look for faults in the DC sense circuitry, including the
comparator U10:1. Check the output of U10:1, pin 1. If it goes
low during DC shutdown, it is sending the false signal.
• If the amplifier stays in DC shutdown, then the fault lies in U15
or the power supply control circuitry, which is referenced to
PRI_LO.
Fault: Shutdown occurs as soon as power supply
switching starts
NOTE: Connect a 50-watt resistor in series with AC line to limit
fault current, in case the output transistors are shorted. For
120 volts the resistor should be 50 ohms, and 100 ohms for 230
volts.
Connect the DC-coupled oscilloscope to an amplifier channel’s
ouput. Briefly disable U15 as described under
false triggers
the other channel.
If the amplifier outputs look okay, check the comparator output at
pin 1 of U10:1. If it is low, check the voltages on pins 2 and 3:
• Pin 2: should be zero (no signal)
• Pin 3: should be about 2 V, set by R243, R244, and R245.
• Check R348 at U15.
to determine if there is actually a DC fault. Repeat for
Tracing the cause of
Fault: Shutdown occurs when output signal exceeds
4 V
• Check Q87, C7, R240, and D48 for open or missing components.
• Confirm that D48 is pulled low (-13 V), holding transistor Q87 on.
If not, check R117, R118, and Q42.
NOTE: This control voltage responds to the Bridge Mono mode
switch.
• Check R348 at U15.
• Check for open or bad connections at step diodes D21, D22, D46,
and D47.
Troubleshooting Thermal Tracking
Fault: Mounting problems with 10K NTC sensing
thermistor
Each channel’s thermal sensing for fan and bias tracking depends on
a 10K NTC thermistor, R30 (channel 1) or R155 (channel 2), that is
mounted in a hole in its heat sink. The hole is filled with thermal
grease to improve coupling. If the thermistor is not straight while
the heat sink is mounted, it may short out against the side of the
hole. The thermistor is mounted on a standoff that protrudes into
the hole, so it should not short-circuit if care is taken while installing
the heat sink.
The heat sinks are live, as they carry the output voltage of their
respective channels. If a thermistor short-circuits to its heat sink,
this output voltage is coupled to the it. If the short is to the
thermistor’s grounded lead, the thermistor might not be damaged,
although the channel output will then be shorted to ground. If the
other lead—the “live” end—shorts to the heat sink, a large signal
voltage could be put across the thermistor, which could damage it.
Fault: Short from the “live” end of the thermistor to the
heat sink
NOTE: This type of short circuit may occur unnoticed and the
amplifier will appear to operate normally. However, the amplifier
will exhibit poor bias tracking, and the fan speed may fluctuates
between low and high.
Inspect and replace affected NTC.
Fault: Thermistor is bent over and shorted to the driver
transistors
The thermistor may touch Q19 or Q26 (channel 1) or Q64, or Q71
(channel 2).
This will cause severe overcurrent to the affected output cell,
possibly damaging the parts in series with the shorted transistor. It
PL3 Series Service Manual 51
TD-000274-00 Rev. A
Page 54
3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
may also damage the power supply.
Replace the affected thermistor and driver transistors. Check these
components in series with the drive transistors:
• Channel 1
Q19 shorted: Check, R381, Q20, R70, D10, and D14.
Q26 shorted: Check
well as the opposing driver transistor, Q19.
• Channel 2
Q64 shorted: Check R383, Q65, R193, D35, and D39.
Q71 shorted: Check
well as the opposing driver transistor, Q64.
all
the output transistors on this channel, as
all
the output transistors on this channel, as
Troubleshooting Fan Speed
The customer complains of fan runs full speed or fan doesn’t run.
Symptoms covered:
• The fan is stuck at full speed.
• The fan doesn’t run.
Fault: The fan is stuck on high speed
• C85 may be leaking
• If the amplifier is in thermal protection, C81 may be leaking
• Q88, Q89, or Q91 failed
• Thermistor R30 or R155 shorted to heatsink
• R266 or R271 missing
Fault: The fan doesn’t run
Check the fan voltage; it should be 11 V when the amplifier is cold,
and up to 29 V when hot.
• If the fan voltage is okay, replace the fan.
• No voltage: check Q89–Q91, R264, and 265 for open or missing
components. Confirm that ±15 V supply voltages are present.
52 QSC Audio Products, LLC
Page 55
PL3 Series Service Manual 53
TD-000274-00 Rev. A
Top side
Top side
Secondary HF
rectifiers and
reservoir caps
Mute and
fan control
DataPort
signal
processing
Output Zobel
filters
Input
buffers and
HP filters
To J410
on input
board
Clip limiting,
loop gain
feedback,
bias, current
splitter
Utility ±15V
supplies
Ch. 1
negative
step
circuit
Ch. 1 current
limit and
mute control
Ch. 2 current
limit and
mute control
Ch. 2
positive
step
circuit
Ch. 2
negative
step
switches
Ch. 1
positive
step
circuit
Power
supply
section
Inrush
thermistor
and relay
Power
supply
control
IGBT switches
and spike
filter
AC rectifier
and main
reservoir
EMI line
filter
“Keep-alive”
supply
Ch. 1 negative
driver and
output
Ch. 1 positive
driver and
output
Ch. 2 positive
driver and
output
Ch. 2 negative
driver and
output
±15V supply
To J5A
on input
board
4. Printed Circuit Boards
4.1 PL325 and PL340 Amplifier
Main module circuit board assembly
Circuit board neighborhoods
PL325 and PL340 main circuit board,
with major circuit sections shown
Page 56
54 QSC Audio Products, LLC
Top side
Top layer
PL325 and PL340 main circuit
board, top layer
Page 57
PL3 Series Service Manual 55
TD-000274-00 Rev. A
Bottom side
(mirror image)
Silk screen layer is
shown for reference.
Bottom layer (mirror image)
Silk screen shown for reference
PL325 and PL340 main circuit
board, bottom layer, mirror
image
Page 58
56 QSC Audio Products, LLC
Input module circuit board assembly
Top layer
Top side
PL325 and PL340 input board,
top layer
Page 59
PL3 Series Service Manual 57
TD-000274-00 Rev. A
Bottom layer (mirror image)
Silk screen shown for reference
Bottom side
(mirror image)
Silk screen layer is
shown for reference.
PL325 and PL340 input board,
bottom layer, mirror image
Page 60
58 QSC Audio Products, LLC
Top layer
See input
module drawing
Ch.1
gain pot
Ch.2
gain pot
Ch. 1 & 2
temperature
signal amps
EMI filter
board
Fan
speed
Fan
speed
Lower Speakon
(Ch. 2) output
Upper Speakon
(Ch. 1 and 2) output
Binding
post outputs
TOP switch
keep-alive supply
AC rectifiers
and reservoir
capacitors
Inrush thermistors,
relays, and fuse
HF transformer
Aux supply—
fan and audio
Class D gate
drive power
±25V supplies
HF power
rectifiers and
reservoir caps
IGBT switches and
voltage spike filter
Loss of AC
detector
Power supply
control and gate drive
transformer
Ch. 2 output
section
Ch. 2 pos.
gate drive
Ch. 2 neg.
gate drive
Ch. 1 output
filter network
Ch. 1 output
MOSFETs
Ch. 1 output
section
Ch. 1 pos.
gate drive
Ch. 1 neg.
gate drive
Power supply
section
Ch. 1 temp
sensor
Ch. 2 temp
sensor
Ch. 2 output
filter network
Ch. 2 output
MOSFETs
4.2 PL380 Amplifier
Main module circuit board assembly
Circuit board neighborhoods
PL380 main circuit board, with
major circuit sections shown
Page 61
PL3 Series Service Manual 59
TD-000274-00 Rev. A
Capacitor voltage
protection
Capacitor voltage
protection
Fan speed
control
Fan speed
control
DC
shutdown
DC
shutdown
Service
plug-in
Service
plug-in
+15V
regulator
+15V
regulator
Ch 2
high-frequency
voltage mute
Ch 2
high-frequency
voltage mute
-15V, ±5V
supply
-15V, ±5V
supply
Relay timer &
stby/mute control
Relay timer &
stby/mute control
Ch 1
high-frequency
voltage mute
Ch 1
high-frequency
voltage mute
Ch. 1 limiter
control system &
protection limiter
Ch. 1 limiter
control system &
protection limiter
Ch. 2 feedback
compensation
& modulator
Ch. 2 feedback
compensation
& modulator
Limiter control:
PS current
Limiter control:
PS current
DataPort
Vmon, Imon,
Temp (Ch. 2)
DataPort
Vmon, Imon,
Temp (Ch. 2)
Ch. 2 overcurrent
feedback
Ch. 2 overcurrent
feedback
Ch. 2 gate drive
control & dead
time isolation
Ch. 2 gate drive
control & dead
time isolation
Ch. 1 gate drive
control & dead
time isolation
Ch. 1 gate drive
control & dead
time isolation
Ch. 1 overcurrent
feedback
Ch. 1 overcurrent
feedback
Ch. 1 clip
±14V power
Ch. 2 limiter
control system &
protection limiter
Ch. 2 limiter
control system &
protection limiter
Crystal-divider
clock circuit
Crystal-divider
clock circuit
Temperature
compensated
modulator
clamp reference
Temperature
compensated
modulator
clamp reference
Audio mute
Clean ±5V supply
Ch. 1 feedback
compensation
& modulator
Ch. 1 feedback
compensation
& modulator
Ch. 2 clip
Ch. 1 clip
limiter drive
Ch. 1 m/f XLR
inputs, buffer
& HP filter
Ch. 1 clip
limiter switch
Ch. 1 HP
filter switch
Mode
switch
Gain/
sensitivity
switch
Ch. 2 clip
limiter switch
Ch. 2 HP
filter switch
Ch. 2 m/f XLR
inputs, buffer
& HP filter
Ch. 1 & 2
pot buffers
Ch. 2 clip
limiter
drive
BMF
flag
±15V
supply
Ch. 2 gain/
sensitivity resistors
Ch. 1 gain/
sensitivity resistors
+5V supply
Ch. 1 & 2 log
amps, signal
LED drivers
PL380 main circuit board,
detailed view
PL380 input board, main
circuit blocks shown
Page 62
60 QSC Audio Products, LLC
Top layer
Top layer
PL380 main circuit
board, top layer
Page 63
PL3 Series Service Manual 61
TD-000274-00 Rev. A
Mid-1 layer (top view)
Silk screen shown for reference
Mid 1 layer
(top view)
Silk screen layer shown for reference
PL380 main
circuit board,
mid1 layer
Page 64
62 QSC Audio Products, LLC
Mid-2 layer
Silk screen shown for reference
Mid 2 layer
(top view)
Silk screen layer shown for reference
PL380 main
circuit board,
mid2 layer
Page 65
PL3 Series Service Manual 63
TD-000274-00 Rev. A
Bottom layer (top view)
Silk screen shown for reference
Bottom layer
(top view)
Silk screen layer shown for reference
PL380 main
circuit board,
bottom layer
Page 66
64 QSC Audio Products, LLC
ITEMNO.QTY PART NO. DESCRIPTION VENDOR
5. Schematic Diagrams
Check www.qscaudio.com for the latest schematic revisions.
5.1 PL325 Schematics
PL325 schematic notes
Page 67
PL3 Series Service Manual 65
TD-000274-00 Rev. A
Signal Path
Schematic sheet INPUT, PL325
Page 68
66 QSC Audio Products, LLC
PL325 wiring diagram
Page 69
PL3 Series Service Manual 67
TD-000274-00 Rev. A
ITEMNO.QTY PART NO. DESCRIPTION VENDOR
PL325 schematic guide
Page 70
68 QSC Audio Products, LLC
Signal Path
Schematic sheet AMP CH-A, PL325
Page 71
PL3 Series Service Manual 69
TD-000274-00 Rev. A
Signal Path
Schematic sheet AMP CH-B, PL325
Page 72
70 QSC Audio Products, LLC
Schematic sheet PROTECT/CONTROL, PL325
Page 73
PL3 Series Service Manual 71
TD-000274-00 Rev. A
Schematic sheet POWER SUPPLY, PL325
Page 74
72 QSC Audio Products, LLC
ITEMNO.QTY PART NO. DESCRIPTION VENDOR
5.2 PL340 Schematics
PL340 schematic notes
Page 75
PL3 Series Service Manual 73
TD-000274-00 Rev. A
Signal Path
Schematic sheet INPUT, PL340
Page 76
74 QSC Audio Products, LLC
PL340 wiring diagram
Page 77
PL3 Series Service Manual 75
TD-000274-00 Rev. A
ITEMNO.QTY PART NO. DESCRIPTION VENDOR
PL340 schematic guide
Page 78
76 QSC Audio Products, LLC
Signal Path
Schematic sheet AMP CH-A, PL340
Page 79
PL3 Series Service Manual 77
TD-000274-00 Rev. A
Signal Path
Schematic sheet AMP CH-B, PL340
Page 80
78 QSC Audio Products, LLC
Schematic sheet PROTECT/CONTROL, PL340
Page 81
PL3 Series Service Manual 79
TD-000274-00 Rev. A
Schematic sheet POWER SUPPLY, PL340
Page 82
80 QSC Audio Products, LLC
QTY ITEM NO. PARTNO. DESCRIPTION VENDOR
5.3 PL380 Schematics
PL380 schematic notes
Page 83
PL3 Series Service Manual 81
TD-000274-00 Rev. A
PL380 wiring diagram
Page 84
82 QSC Audio Products, LLC
PL380 schematic guide
Page 85
PL3 Series Service Manual 83
TD-000274-00 Rev. A
Signal Path (linear)
Signal Path (PWM)
CLIM-A
-14V
+14V
+5V:LIN
-14V
A-LIM
+5V:LIN +5V:LIN
A-MUTE-LO
-5V:LIN
LIMITER-A
-14V
+14V
IN-A-POS
+14V
IN-A-REF
OFFSET-A
-5V:SW
-5V:SW
OUT-A
CLIP-A
CL-A-ON
-5V:LIN CLIM-A
PS-LIM
+5V:LINOC-LIM-AZOB-A +14V A-THERM
VMON-A
+5V:SW +5V:SW
CLK-B
SYNC
RUN-B-LO
CLK-A
RUN-A-LO
OC-FB-A
SW-A
CLK-A
+5V:SW
+5V:SW
+5V:SW
VCL-P VCL-N
AMP_TRIG_A
+5V:SW
AMP_TRIG_B
+5V:SW
SW-A
A-HI-25
A-HI-S
+180
-14V
OUT-A
+14V
IMON-A
+14V
A-25V-LO
FB-A
-5V:LIN
OC-FB-A
-14V
+14V
OC-LIM-A
+5V:LIN
-180
ZOB-A
OUT-A
EMI_G
BR-POS
BR-NEG
EMI_G
Schematic sheet AMP CH-A, PL380 through January 2008
Page 86
84 QSC Audio Products, LLC
B-THERM
+5V:LIN
A-THERM
A/B_THERM
20A-RY
-15_TOP
+16V
-16V
OUT-A
OUT-B
-5_TOP
+5_TOP +180
IGBT-SW
AC-ON
+5_TOP
AC-OFF-LO
+15_TOP
+5V:LIN
50A-RY
A-MUTE-LO B-MUTE-LO
+5_TOP
A+B_MUTE
+5_TOP
+15_TOP
RUN-A-LO+5_TOP RUN-B-LO
AC-OFF-LO
-180
+14V
-14V
OFFSET-B
OFFSET-A
-5V:LIN
+180
+5V:LIN
-15_TOP
Schematic sheet PROTECT/CONTROL, PL380
Page 87
PL3 Series Service Manual 85
TD-000274-00 Rev. A
Signal Path (linear)
Signal Path (PWM)
-14V
CLIM-B
-14V
+5V:LIN
B-LIM
-5V:LIN
+5V:LIN
+14V
+14V
-14V
IN-B-POS
+14V
OFFSET-B
+5V:LIN
-5V:SW
B-MUTE-LO
+5V:LINZOB-B OC-LIM-B
OUT-B
-5V:SW
CLIP-B
-5V:LIN CLIM-B
CL-B-ON
+14V B-THERM
PS-CUR
PS-LIM
VMON-B
IN-B-REF
SW-B
+5V:SW
CLK-B
OC-FB-B
-5V:SW
+5V:SW
+5V:SW
+5V:SW
AMP_TRIG_B
VCL-N
VCL-P
+5V:SW
SW-B
B-HI-S
B-HI-25
+180
OUT-B
-14V
+14V
IMON-B
+14V
B-25V-LO
FB-B
OC-FB-B
-5V:LIN
-14V
+5V:LIN
+14V
OC-LIM-B
-180
OUT-B
ZOB-B
EMI_G
BR-POS
BR-NEG
EMI_G
Schematic sheet AMP CH-B, PL380 through January 2008
Page 88
86 QSC Audio Products, LLC
+18V_BIAS
PRI-LO
AC-ON
+5_TOP
+25_TOP
+15_TOP
PRI-LO
+5_TOP
PRI-Y
+5V:SW
-15_TOP
-25_TOP
-5V:SW
-5_TOP
SYNC
AC-ON
-25_TOP
+25_TOP
+5_TOP
SERV-1
TOP-HI
Y-GND
50A-RY
T-SW-5V
-15_TOP
-15_TOP
20A-RY
HOT
PRICAPCT
PRI-HI
PRI-Y
PRI-LO
+10V
PRI-HI
120/240
PRI-LO
PRI-Y
+180
PS-CUR
+180
A-25V-LO B-25V-LO
IGBT-SW
SYNC
VREF
+15_TOP+15-IG
SYNC
+15-IG
T-SW-5V
SERV-1
-16V
-14V
-16V
A-HI-S
A-HI-25
B-HI-25
B-HI-S
+14V
+16V
-180
-180
16V-SW
+16V
+5V:LIN
-5V:LIN
Power Supply, PL380
Schematic sheet POWER SUPPLY, PL380 through January 2008
Page 89
PL3 Series Service Manual 87
TD-000274-00 Rev. A
Signal Path
+XLRA
-XLRA
SEND-B
POT-B
-XLRA
+XLRA
LD-B2
LD-B
POT-A
SEND-A
-15IN
CL-A
CL-B
+15IN
LD-P
LOG-B
LOG-A
-XLRB+XLRB
+XLRA -XLRA
BR-S
BR-R
LD-P
+15IN
LD-B LD-B2
IM-B
CL-DPB
+10DP
VM-B
STBY-LO
CL-DPA
IM-A
LD-B
VM-A
-XLRB
+XLRB
-XLRB
+XLRB
+15IN
VM-B -15_AUD
-15IN
+15_AUD
-15_AUD
+15_AUD
LOG-B
+5V_IN
CL-B
CL-DPB
-15_AUD
+5V_IN
POT-B
SEND-B
BR-R
IN-B-POS
IN-B-REF
VMON-B
AC-OFF-LO
CLIP-B
IMON-B
IM-B
B-LIM
CL-B-ON
STBY-LO
VM-B
LOG-A
CL-A
-15_AUD
-15IN
+15_AUD
-15_AUD
+15_AUD
+5V_IN
+15_AUD
+5V_IN
BR-S
-15_AUD
CL-DPA
POT-A
SEND-A
-15IN
CLIP-A
IN-A-POS
+10DP +10V
-15_TOP
A+B_MUTE
+15_TOP
A/B_THERM
+15IN
A-LIM
VMON-A
CL-A-ON
IMON-A
IN-A-REF
VM-A
IM-A
+5V_IN
Schematic sheet INPUT, PL380 through January 2008
Page 90
88 QSC Audio Products, LLC
Components changed since prior
version are highlighted in yellow.
Signal Path (linear)
Signal Path (PWM)
CLIM-A
-14V
+14V
+5V:LIN
-14V
A-LIM
+5V:LIN +5V:LIN
A-MUTE-LO
-5V:LIN
LIMITER-A
-14V
+14V
IN-A-POS
+14V
IN-A-REF
OFFSET-A
-5V:SW
-5V:SW
OUT-A
CLIP-A
CL-A-ON
-5V:LIN CLIM-A
PS-LIM
+5V:LINOC-LIM-AZOB-A +14V A-THERM
VMON-A
+5V:SW +5V:SW
CLK-B
RUN-B-LO
SYNC
CLK-A
RUN-A-LO
OC-FB-A
SW-A
CLK-A
+5V:SW
+5V:SW
+5V:SW
VCL-P VCL-N
AMP_TRIG_A
+5V:SW
AMP_TRIG_B
+5V:SW
SW-A
A-HI-25
A-HI-S
+180
-14V
OUT-A
+14V
IMON-A
+14V
A-25V-LO
FB-A
-5V:LIN
OC-FB-A
-14V
+14V
OC-LIM-A
+5V:LIN
-180
EMI_G
ZOB-A
OUT-A
BR-POS
BR-NEG
EMI_G
Schematic sheet AMP CH-A, PL380 from February 2008–
Page 91
PL3 Series Service Manual 89
TD-000274-00 Rev. A
Components changed since prior
version are highlighted in yellow.
Signal Path (linear)
Signal Path (PWM)
-14V
CLIM-B
-14V
+5V:LIN
B-LIM
-5V:LIN
+5V:LIN
+14V
+14V
-14V
IN-B-POS
+14V
OFFSET-B
+5V:LIN
-5V:SW
B-MUTE-LO
+5V:LINZOB-B OC-LIM-B
OUT-B
-5V:SW
CLIP-B
-5V:LIN CLIM-B
CL-B-ON
+14V B-THERM
PS-CUR
PS-LIM
VMON-B
IN-B-REF
SW-B
+5V:SW
CLK-B
OC-FB-B
-5V:SW
+5V:SW
+5V:SW
+5V:SW
AMP_TRIG_B
VCL-N
VCL-P
+5V:SW
SW-B
B-HI-S
B-HI-25
+180
OUT-B
-14V
+14V
IMON-B
+14V
B-25V-LO
FB-B
OC-FB-B
-5V:LIN
-14V
+5V:LIN
+14V
OC-LIM-B
-180
OUT-B
ZOB-B
EMI_G
BR-POS
BR-NEG
EMI_G
Schematic sheet AMP CH-B, PL380 from February 2008–
Page 92
90 QSC Audio Products, LLC
Components changed since prior
version are highlighted in yellow.
+18V_BIAS
PRI-LO
AC-ON
+5_TOP
+25_TOP
+15_TOP
PRI-LO
+5_TOP
PRI-Y
+5V:SW
-15_TOP
-25_TOP
-5V:SW
-5_TOP
SERV-1
+5_TOP
SYNC
AC-ON
-25_TOP
+25_TOP
TOP-HI
Y-GND
50A-RY
T-SW-5V
-15_TOP
-15_TOP
20A-RY
HOT
PRICAPCT
PRI-HI
PRI-Y
PRI-LO
+10V
PRI-HI
120/240
PRI-LO
PRI-Y
+180
PS-CUR
+180
A-25V-LO B-25V-LO
IGBT-SW
SYNC
VREF
+15_TOP+15-IG
SYNC
+15-IG
T-SW-5V
SERV-1
-16V
-14V
-16V
A-HI-S
A-HI-25
B-HI-25
B-HI-S
+14V
+16V
-180
-180
16V-SW
+16V
+5V:LIN
-5V:LIN
Schematic sheet POWER SUPPLY, PL380 from February
2008–
Page 93
PL3 Series Service Manual 91
TD-000274-00 Rev. A
Components changed since prior
version are highlighted in yellow.
Signal Path
-XLRA
+XLRA
+5V_IN
CL-MASK
SEND-B
POT-B
-XLRA
+XLRA
LD-B2
LD-B
POT-A
SEND-A
-15IN
CL-A
CL-B
+15IN
LD-P
LOG-B
LOG-A
-XLRB+XLRB
+XLRA -XLRA
BR-S
BR-R
LD-P
+15IN
LD-B LD-B2
IM-B
CL-DPB
+10DP
VM-B
STBY-LO
CL-DPA
IM-A
LD-B
-XLRB
+XLRB
VM-A
-XLRB
+XLRB
+15IN
VM-B -15_AUD
-15IN
+15_AUD
-15_AUD
+15_AUD
LOG-B
CL-B
CL-DPB
-15_AUD
POT-B
SEND-B
BR-R
CL-MASK
IN-B-POS
IN-B-REF
VMON-B
AC-OFF-LO
CLIP-B
IMON-B
IM-B
B-LIM
CL-B-ON
STBY-LO
VM-B
LOG-A
CL-A
-15_AUD
-15IN
+15_AUD
-15_AUD
+15_AUD
+5V_IN
+15_AUD
BR-S
-15_AUD
CL-DPA
POT-A
SEND-A
CL-MASK
-15IN
CLIP-A
IN-A-POS
+10DP +10V
-15_TOP
A+B_MUTE
+15_TOP
A/B_THERM
+15IN
A-LIM
VMON-A
CL-A-ON
IMON-A
IN-A-REF
VM-A
IM-A
CL-MASK
Schematic sheet INPUT, PL380 from February 2008–
Page 94
92 QSC Audio Products, LLC
This page is blank.
Page 95
6. Replacement parts
A B
6.1 Semiconductor package descriptions and pinouts
Legend: A = anode; B = base; C = collector; D = drain; G = gate; K = cathode; S = source
IC-000024-00
PWM controller, SG3525AN
INV. INPUT
N.I. INPUT
SYNC
OSC. OUTPUT
C
T
R
T
DISCHARGE
SOFT-START
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V
REF
+V
OUTPUT B
V
GROUND
OUTPUT A
SHUTDOWN
COMP.
IC-000047-30
COMPARATOR, DUAL, LM393, SMT
OUTPUT A
INVERTING INPUT A
i
NON-INVERTING
INPUT A
c
GND
V+
OUTPUT B
INVERTING INPUT B
NON-INVERTING
INPUT B
TOP VIEW
IC-000025-00
MOSFET DRIVER, TC1427CPA
IC-000051-00
OPTO ISOLATOR, MOC8101
6
1
PL3 Series Service Manual 93
TD-000274-00 Rev. A
Page 96
6.1 Semiconductor package descriptions and pinouts (continued)
IC-000073-30
Dual transconductance op amp, LM13600M
AMP BIAS INPUT A
DIODE BIAS A
NON-INVERTING
INPUT A
INVERTING INPUT A
OUTPUT A
BUFFER INPUT A
BUFFER OUTPUT A
1
2
3
4
AB
5
V-
6
7
8
AMP BIAS INPUT B
16
DIODE BIAS B
15
NON-INVERTING
14
INPUT B
INVERTING INPUT B
13
OUTPUT B
12
V+
11
BUFFER INPUT B
10
BUFFER OUTPUT B
9
IC-000128-00
+5 V voltage regulator, LM78L05
SO-8 Plastic (M)
(Narrow Body)
V
OUT
GND
GND
NC NC
1
2
3
4
8
7
6
5
V
IN
GND
GND
IC-000106-30
D FLIP FLOP, DUAL, 74HC74, SMT
94 QSC Audio Products, LLC
QD-000315-00
600V 75A IGBT, 250W, IXGR60N60C2, TO-247
Page 97
6.1 Semiconductor package descriptions and pinouts (continued)
IC-000316-00
Counter, 14 stage, 74HC4060, SMT
IC-000342-00
TOP SWITCH, TOP244YN
Y Package (TO-220-7C)
Tab Internally
Connected to
SOURCE Pin
7 D
5 F
4 S
3 X
2 L
1 C
QD-000318-00
N-channel MOSFET 500V 85A, IXFN80N50Q2, SOT-227B package
PL3 Series Service Manual 95
TD-000274-00 Rev. A
IC-000371-30
MOSFET DRIVER, UCC37321D, SMT
Page 98
6.1 Semiconductor package descriptions and pinouts (continued)
IC-000446-30
OPTO COUPLER, , HCPL-0723, SMT
96 QSC Audio Products, LLC
Page 99
6.2 PL325 parts and assemblies
PL325 Power Amplifier (120V) (QSC part # FG-032500-00)
QSC Part # Description Qty. Reference Comments
CH-000579-00 COVER, BOTTOM, , PL3, 1
LB-000209-00 AGENCY, CERTIFICATION, CLASS B, , FCC 1
LB-000437-00 LABEL, WARNING, RAIN & MOISTURE, , 1
PM-000309-00 INSERT, SHIPPING, , PL380, , 2
PM-001401-00 BAG, SHIPPING, POLYPROPYLENE, SERIES 1, 20.5" X 12.5" X 24", 1
PM-001601-00 CARTON, SHIPPING, , PL380, 23.2" X 21.2" X 5.5", 1
PM-004001-00 PAD, SHIPPING, , EX/MXA, 21.2" X 23.2", 1
RD-000001-00 LABEL, SHIPG CRTN , THERMAL 0
SC-082051-PL PHILLIPS, PANHEAD SERATED BASE, #8, 0.313" L, BLACK, CR111 11
TD-000235-00 OWNERS MANUAL, PL3 SERIES, 1
TD-000254-00 MANUAL INSERT, CHINA ROHS, AMPLIFIERS, 1
TS-000004-WR 3 YEARS WARRANTY 3
WC-000341-00 AC CORD, POWERCON, 14 AWG, 3 CONDUCTOR, PL340, 72", 120V 1
WP-000001-00 ASSEMBLY, CATALOG KIT, , , 1
WP-000952-00 SHIP KIT, , PL3, , 1
WP-032500-00 CHASSIS ASSY, , PL325, 120V, 1
WP-032501-00 PCB ASSY, , PL325, 120V, 1
PL325 Power Amplifier (100V) (QSC part # FG-032500-01)
QSC Part # Description Qty. Reference Comments
CH-000579-00 COVER, BOTTOM, , PL3, 1
LB-000437-00 LABEL, WARNING, RAIN & MOISTURE, , 1
PM-000309-00 INSERT, SHIPPING, , PL380, , 2
PM-001401-00 BAG, SHIPPING, POLYPROPYLENE, SERIES 1, 20.5" X 12.5" X 24", 1
PM-001601-00 CARTON, SHIPPING, , PL380, 23.2" X 21.2" X 5.5", 1
PM-004001-00 PAD, SHIPPING, , EX/MXA, 21.2" X 23.2", 1
RD-000001-00 LABEL, SHIPG CRTN , THERMAL 0
SC-082051-PL PHILLIPS, PANHEAD SERATED BASE, #8, 0.313" L, BLACK, CR111 11
TD-000235-00 OWNERS MANUAL, PL3 SERIES, 1
TD-000254-00 MANUAL INSERT, CHINA ROHS, AMPLIFIERS, 1
TS-000004-WR 3 YEARS WARRANTY 3
WC-000341-00 AC CORD, POWERCON, 14 AWG, 3 CONDUCTOR, PL340, 72", 120V 1
WP-000952-00 SHIP KIT, , PL3, , 1
WP-032500-01 CHASSIS ASSY, , PL325, 100V, 1
WP-032501-01 PCB ASSY, , PL325, 100V, 1
PL325 Power Amplifier (230V) (QSC part # FG-032500-02)
QSC Part # Description Qty. Reference Comments
CH-000579-00 COVER, BOTTOM, , PL3, 1
LB-000437-00 LABEL, WARNING, RAIN & MOISTURE, , 1
PM-000309-00 INSERT, SHIPPING, , PL380, , 2
PM-001401-00 BAG, SHIPPING, POLYPROPYLENE, SERIES 1, 20.5" X 12.5" X 24", 1
PM-001601-00 CARTON, SHIPPING, , PL380, 23.2" X 21.2" X 5.5", 1
PM-004001-00 PAD, SHIPPING, , EX/MXA, 21.2" X 23.2", 1
RD-000001-00 LABEL, SHIPG CRTN , THERMAL 0
SC-082051-PL PHILLIPS, PANHEAD SERATED BASE, #8, 0.313" L, BLACK, CR111 11
TD-000235-00 OWNERS MANUAL, PL3 SERIES, 1
TD-000254-00 MANUAL INSERT, CHINA ROHS, AMPLIFIERS, 1
TS-000004-WR 3 YEARS WARRANTY 3
WC-000356-00 AC CORD ASSY, POWERCONN, 1.5MM2, 3 CONDUCTOR, PL3 SERIES, 100", 250VAC 1
WP-000952-00 SHIP KIT, , PL3, , 1
WP-032500-02 CHASSIS ASSY, , PL325, 230V, 1
WP-032501-02 PCB ASSY, , PL325, 230V, 1
Chassis Assembly PL325 (120V) (QSC part # WP-032500-00)
QSC Part # Description Qty. Reference Comments
CH-000118-00 RACK EAR, FRONT, , CX902, 2
CH-000693-00 SHIELD, LINE FILTER, , PL340, 1
CH-000694-00 CHASSIS, MAIN, , PL340, 1
CO-000320-00 AC INLET, FLANGE MOUNT, 3 POS, FEMALE, POWERCON A-TYPE 1
PL3 Series Service Manual 97
TD-000274-00 Rev. A
Page 100
Chassis Assembly PL325 (120V) continued
QSC Part # Description Qty. Reference Comments
HW-040001-00 BRACKET, ANGLE, #4-40, 0.375" L, ZINC PLATED STEEL, 2
HW-060090-SO STANDOFF, 0.250" HEX, #6-32, 0.563" L, ALUMINUM, FEMALE 2
LB-000053-00 FUSE, REPLACEMENT, FRENCH, , 1
LB-000077-00 WARNING, HIGH ENERGY, , PL 3RU, 1
LB-000409-00 AGENCY, 68FA LISTING, HORIZONTAL, , UL/CUL 1
LB-000439-00 LABEL, WIRE CLASS 2 AND 3, , , 1
LB-000652-00 PRODUCT, FACEPLATE, , PL3, 1
LB-000653-00 PRODUCT, FACEPLATE, , PL325, 1
LB-000673-50 LABEL, CHINA ROHS, 50 YEARS, , SMALL 1
LB-160226-05 FUSE, 25A, 125V, , UL 1
NW-060010-IT LOCK WASHER, #6, INTERNAL TOOTH, , CR111 4
PL-000054-00 KNOB, , PLX, POLYPROPYLENE/SANTOPRENE, BLACK, 2
PL-000066-00 CLIP, CHRISTMAS TREE, , NYLON, BLACK, 2
PL-000095-00 FAN INTERFACE, , PLX 2RU, POLYPROPYLENE, BLACK, 1
PL-000144-00 INSULATOR, FAN DUCT, PL2/2A, FISH PAPER, , 1
PL-000515-00 INSULATOR, FAN DUCT, PL3 SERIES, FISH PAPER, , 1
RD-000003-XX LABEL, SERIAL NUMBER, PLX/DCA/PL/PL2A/CM16A/PL/DCM 0
SC-025000-PP PHILLIPS, PANHEAD, M2.5, 6MM, BLACK, THREAD FORMING, CR111 8
SC-040041-PP PHILLIPS, PANHEAD, #4-40, 0.25" L, BLACK, CR111 4
SC-060042-PP PHILLIPS, PANHEAD SEMS INTERNAL TOOTH, #6-32, 0.25" L, BLK, ZINC, CR111 2
SC-060060-PS PHILLIPS, SEMS EXTERNAL TOOTH, #6-32, 0.375" L, ZINC, CR111 3
SC-063081-PO PHILLIPS, OVALHEAD, #6-20, 0.5 “L, BLACK, ZINC W/6107 LUBE, SELF TAPPING, CR111 2
SC-080122-HC HEX, CAP, #8-32, 0.75" L, BLACK, CR111 2
SC-080127-PS PHILLIPS, SEMS EXTERNAL TOOTH, #8-32, 0.75" L, ZINC, CR111 2
SC-082051-PL PHILLIPS, PANHEAD SERATED BASE, #8, 0.313" L, BLACK, CR111 14
SW-000037-00 AC SWITCH, DPST, 20A, 125VAC, 1
WC-000160-00 RIBBON CABLE ASSY, , 28 AWG, 20 CONDUCTOR, PL2A, 9" L, 2
WC-000312-00 RIBBON CABLE ASSY, , 28 AWG, 20 CONDUCTOR, DCM-3, 10" L, 1
WC-000343-00 CABLE ASSY, AC, 16 AWG, 3 CONDUCTOR, PL3, 5" L, 1
WC-000344-00 CABLE ASSY, INPUT POWER, 22 AWG, 3 CONDUCTOR, PL3, 14" L, 1
WP-000057-00 FAN ASSY, , PLC, , 1
WP-032502-00 PCB ASSY, INPUT, PL325, , 1
Chassis Assembly PL325 (100V) (QSC part # WP-032500-01)
QSC Part # Description Qty. Reference Comments
CH-000118-00 RACK EAR, FRONT, , CX902, 2
CH-000693-00 SHIELD, LINE FILTER, , PL340, 1
CH-000694-00 CHASSIS, MAIN, , PL340, 1
CO-000320-00 AC INLET, FLANGE MOUNT, 3 POS, FEMALE, POWERCON A-TYPE 1
HW-040001-00 BRACKET, ANGLE, #4-40, 0.375" L, ZINC PLATED STEEL, 2
HW-060090-SO STANDOFF, 0.250" HEX, #6-32, 0.563" L, ALUMINUM, FEMALE 2
LB-000077-00 WARNING, HIGH ENERGY, , PL 3RU, 1
LB-000439-00 LABEL, WIRE CLASS 2 AND 3, , , 1
LB-000652-00 PRODUCT, FACEPLATE, , PL3, 1
LB-000653-00 PRODUCT, FACEPLATE, , PL325, 1
LB-000673-50 LABEL, CHINA ROHS, 50 YEARS, , SMALL 1
LB-160226-00 FUSE, 30A, 125V, , UL 1
NW-060010-IT LOCK WASHER, #6, INTERNAL TOOTH, , CR111 4
PL-000054-00 KNOB, , PLX, POLYPROPYLENE/SANTOPRENE, BLACK, 2
PL-000066-00 CLIP, CHRISTMAS TREE, , NYLON, BLACK, 2
PL-000095-00 FAN INTERFACE, , PLX 2RU, POLYPROPYLENE, BLACK, 1
PL-000144-00 INSULATOR, FAN DUCT, PL2/2A, FISH PAPER, , 1
PL-000515-00 INSULATOR, FAN DUCT, PL3 SERIES, FISH PAPER, , 1
RD-000003-XX LABEL, SERIAL NUMBER, PLX/DCA/PL/PL2A/CM16A/PL/DCM 0
SC-025000-PP PHILLIPS, PANHEAD, M2.5, 6MM, BLACK, THREAD FORMING, CR111 8
SC-040041-PP PHILLIPS, PANHEAD, #4-40, 0.25" L, BLACK, CR111 4
SC-060042-PP PHILLIPS, PANHEAD SEMS INTERNAL TOOTH, #6-32, 0.25" L, BLK, ZINC, CR111 2
SC-060060-PS PHILLIPS, SEMS EXTERNAL TOOTH, #6-32, 0.375" L, ZINC, CR111 3
SC-063081-PO PHILLIPS, OVALHEAD, #6-20, 0.5 “L, BLACK, ZINC W/6107 LUBE, SELF TAPPING, CR111 2
SC-080122-HC HEX, CAP, #8-32, 0.75" L, BLACK, CR111 2
SC-080127-PS PHILLIPS, SEMS EXTERNAL TOOTH, #8-32, 0.75" L, ZINC, CR111 2
SC-082051-PL PHILLIPS, PANHEAD SERATED BASE, #8, 0.313" L, BLACK, CR111 14
SW-000037-00 AC SWITCH, DPST, 20A, 125VAC, 1
98 QSC Audio Products, LLC
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