QSC ISA 280 Operating manual

Technical Service Manual
ISA™ Series
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▲ISA280
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▲ISA300T
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▲ISA300Ti
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▲ISA450
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▲ISA500T
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▲ISA500Ti
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▲ISA750
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▲ISA800T
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▲ISA800Ti
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▲ISA1350
*TD-000163-00*
TD-000163-00
Rev. B
QSC Audio Products, LLC
Costa Mesa, CA 92626 USA
www.qscaudio.com
RoHS

Technical Service Manual 1
ISA Series Power Amplifiers
QSC Audio Products, LLC
Technical Services Group
Phone: 1-800 QSC AUDIO (1-800-772-2834) USA only
[phone]
Fax: [phone]
Skype ID: qscaudio
Postal: 1665 MacArthur Blvd.
Costa Mesa, California 92626 USA
E-mail: tec[email protected]
Web: http://www.qscaudio.com (product information and support)
http://www.qscstore.com (parts and accessory sales)
ISA280
ISA450
ISA750
ISA1350
ISA300T & ISA300Ti
ISA500T & ISA500Ti
ISA800T & ISA800Ti
ISA Series
Technical Service Manual
Copyright 2007 and 2009 QSC Audio Products, LLC All rights reserved.

2QSC Audio Products, LLC
TD-000163-00 Rev. B
ISA amplifier, rear view

Technical Service Manual 3
ISA Series Power Amplifiers
Table of Contents
ISA Series Performance Specifications ................................................................................................................................ 5
1. Introduction............................................................................................................................................................................... 7
1.1 Restriction of Hazardous Substances Directive (RoHS) .......................................................................................................................... 7
1.2 Service bulletins ................................................................................................................................................................................................... 7
1.3 The well-equipped service bench ................................................................................................................................................................. 7
1.4 Working with surface-mount components .................................................................................................................................................. 8
Two-terminal components (resistors, capacitors, diodes, etc.) ........................................................................................................................................................................ 8
Three-terminal components (transistors, etc.) ................................................................................................................................................................................................. 8
Multi-pin components (ICs, etc.) ...................................................................................................................................................................................................................... 9
1.5 Series description .............................................................................................................................................................................................. 10
Outputs for distributed lines .......................................................................................................................................................................................................................... 10
1.6 Technical descriptions and theory of operation ..................................................................................................................................... 10
Power supplies .............................................................................................................................................................................................................................................. 10
Audio circuitry ................................................................................................................................................................................................................................................10
Clip detection ................................................................................................................................................................................................................................................ 11
DC protection ................................................................................................................................................................................................................................................ 11
Class H ........................................................................................................................................................................................................................................................... 12
Bridged mono operation and protection ....................................................................................................................................................................................................... 12
Output transformers and autoformers .......................................................................................................................................................................................................... 12
1.7 ISA1350 circuitry ................................................................................................................................................................................................. 13
Class H step drivers ....................................................................................................................................................................................................................................... 13
Bias circuitry .................................................................................................................................................................................................................................................. 13
Muting via the bias network ......................................................................................................................................................................................................................... 13
DC fault protection ........................................................................................................................................................................................................................................ 14
1.8 Revisions made during production ............................................................................................................................................................... 14
ISA750 and ISA800T ..................................................................................................................................................................................................................................... 14
2. Component identification and pinout ..............................................................................................................................15
NE5532AN Dual operational amplifier ......................................................................................................................................................................................................... 15
LM311 Voltage comparator .......................................................................................................................................................................................................................... 15
4N29 Opto-isolator ........................................................................................................................................................................................................................................ 15
LM13600 Dual operational transconductance amplifier .............................................................................................................................................................................. 15
2N5064 Sensitive gate thyristor ................................................................................................................................................................................................................... 15
IRFZ44 TMOS power field effect transistor .................................................................................................................................................................................................. 16
2N3904 (NPN) and 2N3906 (PNP) Small-signal transistors ......................................................................................................................................................................... 16
MJE15032 (NPN) and MJE15033 (PNP) Driver transistors .......................................................................................................................................................................... 16
MAC224 Triac ............................................................................................................................................................................................................................................... 16
2SC5200 (PNP) and 2SA1943 (NPN) Power transistors ............................................................................................................................................................................... 16
3. Troubleshooting: Symptoms, causes, & remedies .......................................................................................................17
3.1 Excessive current draw .................................................................................................................................................................................... 17
Symptoms covered: ....................................................................................................................................................................................................................................... 17
Possible situations: ........................................................................................................................................................................................................................................ 17
3.2 Protection, muting, and turn-on/turn-off delay problems ...................................................................................................................... 17
Symptoms covered: ....................................................................................................................................................................................................................................... 17
Possible situations: ........................................................................................................................................................................................................................................ 17
3.3 Faults with signal present ............................................................................................................................................................................... 18
Symptoms covered: ....................................................................................................................................................................................................................................... 18
Possible situations: ........................................................................................................................................................................................................................................ 18
3.4 Instability .............................................................................................................................................................................................................. 19
Symptoms covered: ....................................................................................................................................................................................................................................... 19
Possible situations: ........................................................................................................................................................................................................................................ 19
3.5 Power supply & rail balancing problems ................................................................................................................................................... 19
Symptoms covered: ....................................................................................................................................................................................................................................... 19
Possible situations: ........................................................................................................................................................................................................................................ 19
Table 1: Bias and current limit adjustments .................................................................................................................................................................................................. 20
4. ISA calibration procedures ................................................................................................................................................21
4.1 Setting bias ........................................................................................................................................................................................................... 21
Tools and resources you will need: ................................................................................................................................................................................................................ 21
Procedure ...................................................................................................................................................................................................................................................... 21
4.2 Setting positive and negative current limits............................................................................................................................................. 21

4QSC Audio Products, LLC
TD-000163-00 Rev. B
Tools and resources you will need ................................................................................................................................................................................................................. 21
Procedure ...................................................................................................................................................................................................................................................... 21
5. Servicing ISA amplifiers ......................................................................................................................................................23
5.1 Mechanical disassembly and re-assembly ............................................................................................................................................... 23
Removing the channel modules .................................................................................................................................................................................................................... 23
Removing the AC board ................................................................................................................................................................................................................................ 23
6. Replacement parts ................................................................................................................................................................24
6.1 ISA280 parts and assemblies .......................................................................................................................................................................... 24
ISA280 Power Amplifier ................................................................................................................................................................................................................................ 24
Chassis Assembly ISA280 (beginning June 2006) ........................................................................................................................................................................................ 24
Chassis Assembly ISA280 (before June 2006) ............................................................................................................................................................................................. 25
Main PCB Assembly ISA280 ......................................................................................................................................................................................................................... 26
PCB Assembly, AC/Input ISA280, 300Ti, 300T, 450, 500Ti, & 500T .............................................................................................................................................................. 28
6.2 ISA300T parts and assemblies ........................................................................................................................................................................ 30
ISA 300T Power Amplifier ............................................................................................................................................................................................................................. 30
Chassis Assembly ISA300T ........................................................................................................................................................................................................................... 30
Main PCB Assembly ISA300T ....................................................................................................................................................................................................................... 31
PCB Assembly, AC/Input ISA280 300Ti, 300T, 450, 500Ti, & 500T ............................................................................................................................................................... 33
6.3 ISA300Ti parts and assemblies ....................................................................................................................................................................... 34
ISA300Ti Power Amplifier .............................................................................................................................................................................................................................. 34
Chassis Assembly ISA300Ti (beginning June 2006) ...................................................................................................................................................................................... 34
Chassis Assembly ISA300Ti (before June 2006) ........................................................................................................................................................................................... 35
Main PCB Assembly ISA300Ti ...................................................................................................................................................................................................................... 36
PCB Assembly, AC/Input ISA280, 300Ti, 300T, 450, 500Ti, & 500T .............................................................................................................................................................. 38
6.4 ISA450 parts and assemblies .......................................................................................................................................................................... 39
ISA450 Power Amplifier ................................................................................................................................................................................................................................ 39
Chassis Assembly ISA450 (beginning June 2006) ........................................................................................................................................................................................ 39
Chassis Assembly ISA450 (before May 2006) .............................................................................................................................................................................................. 40
Main PCB Assembly ISA450 ......................................................................................................................................................................................................................... 41
PCB Assembly, AC/Input ISA280, 300Ti, 300T, 450, 500Ti, & 500T .............................................................................................................................................................. 43
6.5 ISA500T parts and assemblies ........................................................................................................................................................................ 44
ISA 500T Power Amplifier ............................................................................................................................................................................................................................. 44
Chassis Assembly ISA 500T .......................................................................................................................................................................................................................... 44
Main PCB Assembly ISA 500T ...................................................................................................................................................................................................................... 45
PCB Assembly, AC/Input ISA280, 300Ti, 300T, 450, 500Ti, & 500T .............................................................................................................................................................. 47
6.6 ISA500Ti parts and assemblies ....................................................................................................................................................................... 48
ISA500Ti Power Amplifier .............................................................................................................................................................................................................................. 48
Chassis Assembly ISA500Ti ........................................................................................................................................................................................................................... 48
Main PCB Assembly ISA500Ti ...................................................................................................................................................................................................................... 49
PCB Assembly, AC/Input ISA280, 300Ti, 300T, 450, 500Ti, & 500T .............................................................................................................................................................. 51
6.7 ISA750 parts and assemblies .......................................................................................................................................................................... 52
ISA750 Power Amplifier ................................................................................................................................................................................................................................ 52
Chassis Assembly ISA750 (beginning May 2006) ......................................................................................................................................................................................... 52
Chassis Assembly ISA750 (before May 2006) .............................................................................................................................................................................................. 53
Main PCB Assembly ISA750 ......................................................................................................................................................................................................................... 54
PCB Assembly, AC/Input ISA750, 800Ti, & 800T .......................................................................................................................................................................................... 56
6.8 ISA800T parts and assemblies ........................................................................................................................................................................ 58
ISA800T Power Amplifier .............................................................................................................................................................................................................................. 58
Chassis Assembly ISA800T ........................................................................................................................................................................................................................... 58
Main PCB Assembly ISA800T ....................................................................................................................................................................................................................... 59
PCB Assembly, AC/Input ISA750, 800Ti, & 800T .......................................................................................................................................................................................... 61
6.9 ISA800Ti parts and assemblies ....................................................................................................................................................................... 62
ISA800Ti Power Amplifier .............................................................................................................................................................................................................................. 62
Chassis Assembly ISA800Ti ........................................................................................................................................................................................................................... 62
Main PCB Assembly ISA800Ti ...................................................................................................................................................................................................................... 63
PCB Assembly, AC/Input ISA750, 800Ti, & 800T .......................................................................................................................................................................................... 65
6.10 ISA1350 parts and assemblies ...................................................................................................................................................................... 66
ISA1350 Power Amplifier .............................................................................................................................................................................................................................. 66
Chassis Assembly ISA1350 (beginning May 2006) ....................................................................................................................................................................................... 66
Chassis Assembly ISA1350 (before May 2006) ............................................................................................................................................................................................ 66
Main PCB Assembly ISA1350 ....................................................................................................................................................................................................................... 66
Table of Contents continued

Technical Service Manual 5
ISA Series Power Amplifiers
ISA Series Performance Specifications
ISA280 & ISA300Ti ISA450 & ISA500Ti ISA750 & ISA800Ti ISA1350
OUTPUT POWER in watts
FTC: 20 Hz–20 kHz @ 0.1% THD, both channels driven, direct outputs
8W per channel 185 260 450 800
4W per channel 280 400 650 1300
FTC: 50 Hz–16 kHz @ 0.5% THD, both channels driven, transformer outputs
70V or 100V (per channel) 300 (ISA 300Ti) 500 (ISA 500Ti) 800 (ISA 800Ti)
25V (per channel) 240 (ISA 300Ti) 400 (ISA 500Ti) 640 (ISA 800Ti)
FTC: 20 Hz–20 kHz @ 0.5% THD, both channels driven, direct outputs
70V (per channel) 1500
EIA: 1 kHz @ 0.1% THD, both channels driven
8Ωper channel 200 280 500 850
4Ωper channel 300 450 750 1400
1 kHz @ 1% THD, typical, both channels driven
2Ωper channel 430 700 1200 2000
Bridge Mono:
8Ω, 20 Hz–20 kHz, 0.1% THD 530 800 1300 2600
8Ω, 1 kHz, 0.1% THD 600 900 1500 2800
4Ω, 1 kHz, 1% THD, typical 830 1400 2400 4000
140V or 200V, 50 Hz–16 kHz, 0.5% THD 600 (ISA 300Ti) 1000 (ISA 500Ti) 1600 (ISA 800Ti)
140V, 20 Hz–20 kHz, 0.5% THD (direct outputs) 3000
50V, 50 Hz–16 kHz, 0.5% THD 480 (ISA 300Ti) 800 (ISA 500Ti) 1280 (ISA 800Ti)
DYNAMIC HEADROOM 2 dB @ 4Ω2 dB @ 4Ω2 dB @ 4Ω2 dB @ 4Ω
DISTORTION
SMPTE-IM < 0.01% < 0.01% < 0.02% < 0.02%
FREQUENCY RESPONSE 20 Hz–20 kHz, +0/-1 dB
(at 10 dB below rated output power) -3 dB points: 5 Hz and 50 kHz
DAMPING FACTOR > 300 @ 8Ω> 300 @ 8Ω> 300 @ 8Ω> 250 @ 8Ω
NOISE (unweighted 20 Hz to 20 kHz, below rated output) 100 dB 100 dB 100 dB 100 dB
VOLTAGE GAIN 31.6× (30 dB) 40× (32 dB) 50× (34 dB) 64× (36 dB)
INPUT SENSITIVITY, V RMS
full rated power @ 8Ω1.15v (+3.4 dBu) 1.15v (+3.4 dBu) 1.23v (+4.0 dBu) 1.24v (+4.1 dBu)
7. Schematics and diagrams ..................................................................................................................................................71
7.1 ISA Series Chassis Assemblies ...................................................................................................................................................................... 71
7.2 ISA280 (1st generation) Schematics............................................................................................................................................................. 94
7.3 ISA280 (revised) Schematics........................................................................................................................................................................... 97
7.4 ISA300T Schematics......................................................................................................................................................................................... 100
7.5 ISA300Ti Schematics........................................................................................................................................................................................ 103
7.6 ISA450 (1st generation) Schematics........................................................................................................................................................... 106
7.7 ISA450 (revised) Schematics......................................................................................................................................................................... 109
7.8 ISA500T Schematics......................................................................................................................................................................................... 112
7.9 ISA500Ti Schematics........................................................................................................................................................................................ 115
7.10 ISA750 (1st generation) Schematics ........................................................................................................................................................ 118
7.11 ISA750 (revised) Schematics ...................................................................................................................................................................... 121
7.12 ISA800T Schematics ...................................................................................................................................................................................... 124
7.13 ISA800Ti Schematics ..................................................................................................................................................................................... 127
7.14 ISA750, ISA800T, ISA800Ti Component locations ..................................................................................................................................130
7.15 ISA1350 Schematics ....................................................................................................................................................................................... 132
7.16 ISA1350 Component locations .................................................................................................................................................................... 136
Table of Contents continued
continued

6QSC Audio Products, LLC
TD-000163-00 Rev. B
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Idle
1.2 A
8 Ω
6.4 A 12.5 A 25.5 A
4 Ω
10 A 20 A 42 A
2 Ω
14.5 A 31 A* 66 A*
ISA Series Performance Specifications (continued)
ISA 280 & ISA 300Ti ISA 450 & ISA 500Ti ISA 750 & ISA 800Ti ISA1350
INPUT IMPEDANCE 10 kΩunbalanced
20 kΩbalanced
CONTROLS Front: AC switch, Ch. 1 and Ch. 2 gain
Rear: 10-position DIP switch
INDICATORS POWER: Green LED CLIP: Red LED, 1 per channel
SIGNAL: Yellow LED, 1 per channel
CONNECTORS Input: Active balanced; XLR (pin 2 +) and detachable Euro-style terminal blocks
Output: Screw terminals with safety shroud
DataPort V2: HD-15 female
COOLING Continuously variable speed fan, back-to-front air flow
AMPLIFIER PROTECTION Full short circuit, open circuit, thermal, ultrasonic, and RF protection
Stable into reactive or mismatched loads
LOAD PROTECTION Turn-on/turn-off muting, AC coupling, triac crowbar (on each channel)
OUTPUT CIRCUIT TYPE AB AB H (2-tier) H (3-tier)
AB: Class AB+B complementary linear output
H: Class AB+B complementary linear output with Class H 2- or 3-tier high efficiency circuit
DIMENSIONS 19.0" (48.3 cm) wide, 5.25" (13.3 cm) tall (3 rack spaces)
15.9" (40 cm) deep (rack mounting to rear support ears)
WEIGHT Shipping: ISA 280: 42 lb (19.1 kg) ISA 450: 46 lb (20.9 kg) ISA 750: 50.5 lb (23 kg) 68 lb (30.8 kg)
ISA 300Ti: 50 lb (22.7 kg) ISA 500Ti: 55 lb (25 kg) ISA 800Ti: 63 lb (28.6 kg)
Net: ISA 280: 35 lb (15.9 kg) ISA 450: 40 lb (18.2 kg) ISA 750: 44.5 lb (20.2 kg) 77 lb (34.4 kg)
ISA 300Ti: 44 lb (20 kg) ISA 500Ti: 49 lb (22.3 kg) ISA 800Ti: 57 lb (26 kg)
POWER REQUIREMENTS Available for 120 or 220–240 VAC, 50/60 Hz
POWER CONSUMPTION @ 120 VAC
(both channels driven)
Multiply currents by 0.5 for 230V units
US patents pending
SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE

Technical Service Manual 7
ISA Series Power Amplifiers
1. Introduction
1.2 Service bulletins
Contact QSC Technical Services to make sure you have the most up-to-date service bulletins for ISA Series amplifiers. Service bulletins
may be distributed in hard copy, via fax, and electronically (Adobe Acrobat PDF) via CD-ROMs, FTP from the QSC web site
(www.qscaudio.com), and e-mail.
One service bulletin has been issued for the ISA Series at the time this manual was printed: ISA0001, “ISA Turn-on Hangup” (ISA450,
ISA500T, ISA750 and ISA800T)
1.1 Restriction of Hazardous Substances Directive (RoHS)
Beginning in June 2006 (serial number date code 0606xxxxx), all ISA Series amplifiers are manufactured to conform to the European
Union’s RoHS Directive, which reduces the amount of hazardous substances allowed in products for sale within its member nations. In
electronic equipment such as audio power amplifiers, this applies primarily to certain toxic heavy metals, such as lead, which may be
present in electronic components, solder, and other parts.
RoHS-compliant materials
When servicing RoHS-compliant amplifiers, it is important for the service technician to use only RoHS-compliant components and solder
(lead-free). All replacement parts provided by QSC for RoHS-compliant products are certified as RoHS compliant.
RoHS-compliant tools
Soldering irons and desoldering apparatus used on RoHS-compliant products must also not be contaminated by hazardous substances,
such as lead. Therefore, you cannot use the same soldering and desoldering tools for RoHS-compliant products and solder as you do for
non-compliant products and solder. You must either use separate soldering irons, desoldering tools and braid, etc., or at the very least
designate separate tips and braids and use only the appropriate ones. If you contaminate a tip or braid, even inadvertently, by using it on
a non-compliant product or solder, you can no longer use it with RoHS-compliant products or solder.
1.3 The well-equipped service bench
To properly service ISA amplifiers, a technician needs the right tools. The technician’s service bench should have the following equipment:
• Digital multimeter with RMS AC voltage and current
• Digital clamp-on ammeter
• Dual-trace oscilloscope
• Audio distortion analyzer
• Non-inductive load resistors, configurable as 8 ohms (min. 500 watts capacity), as 4 ohms (min. 750 watts capacity), and 2 ohms (min.
1200 watts capacity)
• Variable AC voltage source, such as a Variac or Powerstat variable transformer, with a rated current capacity of up to 25A (for 120V
models) or 12A (for 230V models)
• Low-distortion audio sine wave generator
• Philips and flat screwdrivers
• Soldering iron with a fine tip (25–60W recommended)
• Rosin-core solder (RoHS-compliant solder, or 60/40 or 63/37 for non-RoHS product only)
• Long-nose pliers
• Diagonal cutters
• Wire strippers
Automated test equipment, such as an Audio Precision workstation, is very useful for servicing ISA amplifiers. Contact QSC Technical
Services to obtain applicable AP test files.

8QSC Audio Products, LLC
TD-000163-00 Rev. B
Solder braid
Solder
Solder
Tweezers
Solder
1.4 Working with surface-mount components
ISA amplifiers, like many modern electronic products, use surface-mount technology (SMT) components where appropriate in order to
make high-density circuitry that is reliable and economical to manufacture.
SMT components in the ISA amps are used in the small-signal and control circuits, so they do not handle significant amounts of power;
therefore, they are subject to very little stress and should seldom fail. Sometimes they do fail, or they require replacement for a
performance upgrade or modification. Thus, it is important to know how to work with SMT components.
Specialized tools and equipment exist for soldering, unsoldering, and removing SMT components quickly and efficiently, but they are often
expensive. Most SMT repairs, though, can be handled reasonably well with common tools and equipment, such as tweezers, solder braid, and
fine-tip soldering irons. The original factory components are tacked to the board with a spot of glue, so you might have to apply some force
to break the adhesive.
Two-terminal components (resistors, capacitors, diodes, etc.)
Removal
1Use two soldering irons, preferably about 25 to 40 watts, with fine tips.
2With a soldering iron in each hand, hold one tip on the solder at one end of the component and the other
tip on the other end (Figure 1.1).
3Once the solder melts on both ends, grip the component between the two tips and lift it from the circuit
board.
4Use solder braid and a soldering iron to remove the solder from the two pads (Figure 1.2).
Insertion
1With a soldering iron and appropriate solder, melt just enough solder onto one pad to create a small
mound (Figure 1.3).
2Grasp the component in the middle with tweezers. Melt the small mound of solder with the iron and
place the component across the two pads (in the correct orientation, if the component is sensitive to
direction) and press it flat against the circuit board, with one end of the component immersed in the
melted solder (Figure 1.4).
3Hold the component in place and take the soldering iron away. Let the solder harden to tack the component in place.
4Fully solder the other end of the component to its pad. Let the solder harden (Figure 1.5).
5Fully solder the tacked end of the component to its pad (Figure 1.6).
Three-terminal components (transistors, etc.)
Removal
1With a soldering iron and solder braid, remove as much solder as possible from the middle terminal of
the component.
2With a soldering iron in each hand, hold one tip on the solder at the terminal at one end of the compo-
nent and the other tip on the terminal at the other end.
3When the solder on both ends melts, grip the component between the two tips and lift it from the circuit
board. You might need to quickly touch the pad on the middle terminal with a soldering iron to melt any
remaining solder that might be holding the component down.
4Use solder braid and a soldering iron to remove the solder from the three pads.
Insertion
1With a soldering iron and appropriate solder, melt just enough solder onto one pad to create a small
mound of solder.
2Grasp the component with tweezers. Melt the small mound of solder with the iron and place the
component in the correct orientation across the three pads and press it flat against the circuit board,
with one terminal of the component pressed into the melted solder.
3Hold the component in place and take the soldering iron away. Let the solder harden to tack the
component in place.
4Fully solder the other terminals of the component to their pads. Let the solder harden.
5Fully solder the tacked terminal of the component to its pad.
Figure 1.1.
Figure 1.2.
Figure 1.3.
Figure 1.4.
Figure 1.5.
Figure 1.6.

Technical Service Manual 9
ISA Series Power Amplifiers
Multi-pin components (ICs, etc.)
Removal
Removing a multi-pin SMT component is a delicate procedure. Ideally, you should use a soldering iron with an attachment that allows you
to heat all the pins simultaneously.
If such a soldering device is not available, use this procedure:
1Use a soldering iron and solder braid to remove as much solder as possible from the pins of the component.
2With fine tweezers, carefully try to lift each pin to see if it’s free. If it’s not, touch it with the tip of the soldering iron and if necessary,
use the solder braid to remove the remaining solder.
3Repeat the process until all the pins are free and you can remove the component.
Insertion
1With a soldering iron and appropriate solder, melt just enough solder onto one pad to create a small mound of solder. It is usually
easiest to use a pad that corresponds to one of the end or corner pins of the component.
2Grasp the component with tweezers. Melt the small mound of solder with the iron and place the component in the correct orientation
upon its pads and gently press it flat against the circuit board, with the appropriate terminal of the component pressed into the
melted solder.
3Hold the component in place and take the soldering iron away. Let the solder harden to tack the component in place.
4Fully solder the other terminals of the component to their pads. Let the solder harden.
5Fully solder the tacked terminal of the component to its pad.

10 QSC Audio Products, LLC
TD-000163-00 Rev. B
1.5 Series description
QSC’s ISA Series amplifiers are entry-level professional audio
products for installed sound systems, designed for good, basic
performance and reliability at low price. The series comprises
seven current models—the ISA280, ISA300Ti, ISA450, ISA500Ti,
ISA750, ISA800Ti, and ISA1350—and three discontinued ones—
the ISA300T, ISA500T, and ISA800T.
Outputs for distributed lines
While all ISA models can drive low-impedance speaker loads as
low as 2 ohms per channel, the three ending in “Ti” also have
isolated transformer-coupled outputs for driving 25-, 70-, or 100-
volt lines. The “T” models preceded the “Ti” ones and had non-
isolated autoformer-coupled outputs; other than this distinction,
they are the same as the “Ti” models. Note: For the sake of
brevity, information that pertains to both versions will be
combined in descriptions of the “Ti” models, except where
otherwise noted.
Each model has two audio channels and is three rack spaces tall.
See page 2 for complete specifications.
The ISA280, ISA300Ti, ISA450, and ISA500Ti have single-sided
printed circuit boards. The ISA750, ISA800Ti, and ISA1350 use
double-sided boards.
1.6 Technical descriptions and
theory of operation
Note: Some of these descriptions concern circuitry that is
duplicated in the amplifier’s two channels. For the sake of
simplicity, the descriptions are of Channel 1 only. Components in
Channel 1 have a 3-digit designation with “1” as the first digit;
their equivalents in Channel 2 have a “2” as the first digit,
followed by the same two numerals. For example, R122 and R222
have identical functions in their respective channels.
Power supplies
Unlike other recent QSC amplifiers, the ISA line uses strictly
conventional power supplies, with large transformers that operate
at the 50 or 60 Hz frequency of the AC line. The electrical current
in the secondary circuitry is converted to DC through a full-wave
bridge rectifier. The resulting 100 or 120 Hz ripple is filtered out by
large capacitors that also serve as current reservoirs for short-
term, transient demands.
The supply provides a bipolar set
of supply rails for each channel,
with equal quiescent positive and
negative voltages, as shown in
Figure 1.7. Note that unlike many
bipolar supplies for complemen-
tary transistor arrangements, the
secondary windings are not
connected to ground at the center.
+Vcc
-Vcc
+Vcc
-Vcc
Channel 1
Channel 2
+110V
-110V
+55V
-55V
Channel 1
Ch. 1 Center Tap
To Channel 2 Center Tap
12 5W
×2
Ω
0.047 µF
This is because the output
transistors are directly
mounted to the heat sink,
metal-to-metal, to maximize
heat transfer; this grounds
the collectors, requiring
somewhat different output
and power supply arrange-
ments. The grounded-
collector concept is
described later in this
chapter.
In the ISA 750 and ISA 800T, the secondaries are tapped to provide
an intermediate set of bipolar rails for the Class H output circuitry.
Figure 1.8 shows one channel. Class H operation is described later
in this chapter.
The 24-volt cooling fan is driven by a separate DC supply that is
powered by a 20-volt tap on the transformer primary. To minimize
fan noise, the fan speed is controlled by varying its actual DC voltage
in response to the amplifier’s heat sink temperatures. An
optocoupler isolates the fan control circuitry from the thermal
sensors.
Audio circuitry
The audio inputs are balanced to offer a reasonably high amount
of common-mode noise rejection. The input balancing is done
using a single op amp (one half of an NE5532 dual op amp)
arranged as a differential amplifier. The degree of common-mode
rejection is dependent on a close match between the input
resistors (R100 and R101 in Figure 1.9) and between the feedback
resistor and the shunt resistor (R105 and R106). The circuitry uses
1% precision resistors to ensure at least 40 dB of common-mode
rejection.
The feedback and shunt capacitors, C101 and C103, add a first-
order high-frequency roll-off, down 3 dB at 88.4 kHz (over two
2.5K LINEAR
RIGHT ANGLE POT
R106
10.0K
^R_0805
R101
10.0K
^R_0805
R100
10.0K
^R_0805
R105
10.0K
^R_0805
CW
CCW
W
R112
2.5K
A1
R102
1K
^R_0805
+IN_A
-IN_A
C101
180p-5%
^C_0805
C103
180p-5%
C106
47µF NP
3+
2-
1
NE5532
U101:1
A1
R113
274
^R_0805
To LM13600 operational
transconductance amp
Figure 1.7
Figure 1.8
Figure 1.9

Technical Service Manual 11
ISA Series Power Amplifiers
octaves above the high end of the audio spectrum). This makes the
amplifier less susceptible to RF interference, high-frequency
oscillations, etc.
Also in this stage, the feedback loop contains one half of a 13600
dual operational transconductance amplifier (Figure 1.10). The OTA is
part of the clip limiter circuitry; when the clip limiter is activated, a
control voltage increases the transconductance of the OTA, which
essentially decreases the impedance of the feedback loop and
reduces the gain of the stage in order to reduce the amount of
clipping.
The gain control uses a linear potentiometer, but the impedances
loading the wiper to ground make the pot approximate an audio
taper over most of its rotation. After the wiper, RC networks roll
off the low end, if the LF filter is set for that channel, at either 30
or 50 Hz, depending on the DIP switch setting on the rear panel.
The next active device is another 5532 op amp, U101:2. Its output
drives the driver transistors, which in turn drive the output transis-
tors. The output section has a Class AB+B configuration; the
drivers (a complementary pair, Q105 and Q106, comprising an NPN
MJE15032 and a PNP MJE 15033) are class AB. A series network
of two diodes and a 100-ohm trimpot provide the small amount of
forward bias on the transistor pair to keep crossover distortion
minimal. In parallel with the trimpot is a 50-ohm thermistor with a
negative temperature coefficient; as the circuitry warms, its
resistance decreases. This reduces VBE on both Q105 and Q106,
decreasing the bias current to reduce the threat of thermal runaway.
The base of each driver transistor is tied to ground through a diode
and a 2.2K trimpot in series; these set the current limiting threshold
for their respective signal polarities.
The collector of each driver transistor directly drives the bases of
its output transistors, which are the main power-handling signal
devices. If you’re not familiar with the grounded-collector scheme,
the arrangement of the output transistors might look somewhat
R111
100
^R_0805
R109
100
^R_0805
10.0K
^R_0805
R103
3
+
8
75
1
4
-
2
U10:1
LM13600M
R108
7.50K
^R_0805
+14V
A1
R104
150K
^R_1206
-14V
R110
270
^R_0805
C105
100-25V R107
^R_0805
39K
Q100
3906
R115
820
^R_1206
To input op amp U101:1
11
CLIP LIMIT
SWITCH
(Open to defeat
clip limiter)
From clip
detection
strange: the positive voltage swings are handled by PNP transis-
tors, while the negative swings are handled by NPN devices. The
collectors all connect to ground, which allows them to be mounted
directly to the heat sink—metal-to-metal, without insulators in
between—for the best possible transfer of heat away from the
transistors. The emitters of the PNP and NPN transistors are
coupled through resistors to the positive and negative supply rails,
respectively, forming banks of common-emitter circuits driving the
supply rails. Consequently, the devices drive the rails with the
audio signal, which rides atop the DC. The output to the speaker
load is taken from the point between the positive and negative
reservoir capacitors; this is also where the negative feedback is
taken from. The nature of this arrangement, with audio signal
riding on the supply rails, is why the power supply has no ground
reference.
Another unusual characteristic of the grounded-collector output
section is that the signal at the output to the speaker is actually
opposite in polarity to the signal at the op amp output. This is why
the negative feedback resistor, R122, connects to the op amp’s
non-inverting input instead of the inverting input.
The output point of the circuit couples to the output connector
through an RLC network (R160, R161, R162, L100, and C124) that
serves as a high-frequency snubber and also helps keep the amp
circuitry stable when driving capacitive loads.
Clip detection
The output of the op amp also drives a group of four diodes (D102,
D103, D105, and D106) arranged as a full-wave rectifier. Normally,
the op amp’s output signal level is about 1 volt or less, which is all
it takes to drive the driver transistors.
But because this point is within the overall feedback loop, when
clipping occurs, the op amp puts out a much higher signal voltage
to try to make the output signal track the input. The four diodes
rectify the voltage to drive the clip indicator LED, LD100. The
current exiting the full-wave rectifier passes to ground through
R127 and also drives the base of transistor Q100 through R115. If
the clip limiter is switched on, Q100’s emitter is grounded, and
when the voltage across R127 goes sufficiently negative to
forward-bias Q100, which sends current through R111 and R103
into the amplifier bias input of the operational transconductance
amplifier (OTA), U10:1. The OTA is in the negative feedback loop of
U101:1, and increasing its transconductance essentially reduces
the impedance of the feedback loop, which reduces the gain of the
op amp stage. This reduces the signal level until the amount of
clipping is minimal. When the clipping stops, Q100 is no longer
forward-biased, and the gain returns to normal.
DC protection
The ISA750 and ISA800T have a crowbar circuit, based on a triac
and two silicon controlled rectifiers, on each output to protect
against DC faults. If an amp channel puts out a DC voltage, which
could be the result of a component or circuit failure, it will first
trigger either D119 or D120, depending on the polarity of the
Figure 1.10

12 QSC Audio Products, LLC
TD-000163-00 Rev. B
voltage. The triggered SCR will in turn trigger triac Q113, shorting
the output to ground through fuse F100. The fuse will blow,
safeguarding the speaker load from the DC fault.
The output sections of the ISA280, ISA300T, ISA450, and ISA500T
are AC coupled.
Class H
The ISA750 and ISA800Ti utilize a two-tier Class H output section.
It is essentially a Class AB+B circuit but with two sets of bipolar
supply rails. On both the positive and the negative sets of rails, a
comparator circuit, called a “step driver,” compares the audio
signal to the lower rail voltage. When necessary to fully reproduce
the signal’s voltage swing—just before the signal voltage reaches
the lower rail voltage—the step driver turns on a TMOS power FET
to pull the output transistors’ supply rail up from the lower voltage
to the higher one, and then back down again when the signal
allows. By keeping the transistors’ supply rails low whenever
possible, the devices dissipate less unused power and generate
less waste heat, making the amplifier more efficient than a
straight class AB amplifier with the same power points.
The comparators are 311-type ICs: U170 on the positive step and
U171 on the negative. Each one drives a high-gain complementary
transistor pair (2N3904 + 2N3906), which drive the gate of their
respective MOSFET.
The ISA 1350 has a three-tier Class H output section. It works in
much the same way as the two-tier arrangement, but the
additional step further increases the amplifier’s electrical
efficiency.
Bridged mono operation and protection
When the amplifier is operated in bridged mono, its two channels
work in tandem to produce up to twice the voltage swing that a
single channel is capable of. To do this, Channel 2 produces a
signal identical to Channel 1’s, but opposite in polarity—in other
words, a mirror image.
Channel 2’s signal feed (bus BR_MONO_FEED) is an attenuated
version of the signal on Channel 1’s speaker bus. Closing DIP
switch #6 (set to “BRIDGE MONO ON”), connects the
BR_MONO_FEED bus on Channel 1 to the BR_RET bus on Channel
2. The BR_RET bus drives the non-inverting input of op amp
U201:2 directly.
With two channels operating as one, but each having its own
feedback and protection circuitry, it is vital to keep both running as
mirror images. A protection circuit monitors the balance between
Channel 1’s and Channel 2’s signals. Resistors R22 and R23 (R22A,
R22B, R23A, and R23B in the ISA 750 and ISA 800T) are equal in
value and form a voltage divider between the two channel outputs.
If the output signals are mirror images, the voltage at the junction
of the resistors (bus BR_BAL) will be zero. If the signals are not
mirror images—for example, one channel is defunct, distorting, or
reduced in gain—a voltage will appear on BR_BAL. Through DIP
switch 7, the BR_BAL bus becomes bus BR_CUT and feeds the
From amplifier
output circuitry
~34V:
8, 4, or 2Ω
70V
100V
25V
AUTOFORMER
bases of transistors Q8 and Q6, which are part of a 4-transistor
circuit across the +15V and -15V rails that supply the op amps and
the input circuitry. If the voltage on BR_CUT goes positive enough
to forward-bias Q8, the transistor’s collector will collapse the +15V
rail. At the same time, the emitter current from Q8 will flow
through R25 and into the emitter of Q7, forward-biasing it, too.
The collector of Q7 will then collapse the -15V rail.
Similarly, if BR_BAL goes sufficiently negative, it will forward-bias
Q6, in turn forward-biasing Q9, and these will collapse the ±15V
rails.
With the rails collapsed, the op amp and the input circuitry will not
function, which will mute the audio.
Output transformers and autoformers
The amplifiers whose model numbers end in “T” have a tapped
autoformer on each channel output. The autoformer allows a
channel to drive distributed speaker lines by converting the output
voltage to an appropriate level. For example, either channel of the
ISA300T (Figure 1.11) can put out a maximum signal level of about
34 volts rms at its low-impedance output terminals, but the
autoformer steps it up to about 70 volts (between the 0 and 70
terminals) or 100 volts (between the 0 and 100 terminals) for
driving lines of those respective voltages. Also, the channel can
drive a 25-volt line connected across the 70 and 100 terminals.
Note that one side of the output is at chassis ground potential.
The “T” models were replaced by the “Ti” ones, which use
transformers to develop the 25-, 70-, and 100-volt outputs (Figure
1.12). The main benefit is the full electrical isolation between the
primary (the amplifier’s internal circuitry) and the secondary (the
distributed loudspeaker line), which some electrical and safety
codes may require.
Figure 1.11

Technical Service Manual 13
ISA Series Power Amplifiers
From amplifier
output circuitry
25V
70V
100V
8, 4, or 2
1.7 ISA1350 circuitry
Compared to the utter simplicity of the lower power ISA models
like the ISA280, the ISA1350’s circuitry might seem so complex
that it should belong to another family of power amps. This added
complexity primarily concerns two particular areas of the ampli-
fier—the class H power supply rail steps and the bias circuitry—
and provides these benefits:
•Three-tier class H output section—the additional tier
further exploits the dynamic nature of audio to improve the
amplifier’s electrical efficiency to as much as 55% better than a
class AB amplifier of a comparable power rating. Despite the
added circuitry and complexity, this allows substantial savings
in components and materials costs by scaling down power
transformers, using fewer output devices, and removing less
waste heat.
•Improved thermal bias tracking and better fault protec-
tion muting—instead of using a simple bias network based on
a series diode pair, the ISA1350 features an “active” bias
network that uses current sources to set bias. An NTC ther-
mistor mounted to the heat sink tracks the operating tempera-
ture of the output devices and directs the current sources to
compensate for optimal bias; the thermal tracking circuitry is
thus isolated from the audio signal, so it can perform with
great precision. The protection circuits for faults, overcurrent,
etc., are also tied into the bias network, so they can shut off
the bias if a serious problem occurs. Shutting off the bias then
shuts off current in the output transistors. This is a highly
effective and inherently balanced approach to fault protection,
which is especially important in a very high power amplifier.
This is also how the turn-on and turn-off muting is handled.
Class H step drivers
The step driver circuitry in the ISA1350’s three-tier class H system
is very similar to the one used in the two-tier system of the ISA750
and ISA800Ti. Since the normal voltage limit for 1206-type
surface-mount resistors is 200 volts and the voltages that the step
driver’s voltage sensing resistors see may reach 300 volts, the
ISA1350 uses 100 kΩ1206 resistors in series pairs instead of
single 200 kΩcomponents.
Bias circuitry
The new circuitry (see Figure 1.13) for the bias network features
transistors Q108 and Q109, with surrounding resistors, forming 8
mA current sources, with less loading of the op amp output than
the resistors used on older RMX and ISA models. These currents
forward bias diodes D106 and D107, which are in contact with the
heat sink. Trimmer R171 adjusts the bias voltage.
The bias currents are controlled by signals +BIAS and –BIAS,
which come from a current source Q142, Q143, and R250. The
single voltage across R250 determines the voltage across both
R161 and R166, which sets the bias currents equally for the PNP
and NPN drivers.
The voltage on R250 is controlled by the voltage at the collector of
Q132, which forms a Vbe multiplier controlled by R226 and R227.
The collector voltage is a multiple of the base voltage, with a
nominal value of 5.45 times the base voltage. A typical 25°C base
voltage is 0.68 V, resulting in a collector voltage of 4.4 V. The
current thru R233 subtracts about 1 V from this, and stabilizes the
voltage against changes in supply voltage. As the amplifier heats
up, the base voltage declines somewhat, and therefore, the BIAS
signals are reduced, which adds more thermal compensation.
The thermal sensor R239 is a 10 kΩNTC resistor (not a PTC as
used in the other models); its resistance decreases with rising
temperature. This increases voltage across R240, and thus reduces
voltage between Q142 and Q143, which reduces bias currents. The
voltage on R240 ranges from almost zero when cold, to about 0.6 V
at maximum temperature.
Muting via the bias network
Muting occurs when the emitter current for Q141 is removed, thus
shutting down Q142 and Q143 and removing all bias from the
output section. The emitter current for Q141, through R238, is
controlled by the voltage on C131.
• When the amp is turned on, C131 ramps up from zero, using a
small current from R220. When the voltage on C131 exceeds
the voltage on the collector of Q132 (the bias reference
voltage), Q141 is turned on, and the bias circuit is enabled.
• When the amp is turned off, circuitry on the AC board pulls
down on D119 and quickly discharges C131, thus muting the
amplifier.
• When the amplifier reaches full temperature, circuitry on the
AC board senses the voltage on the FAN-SP line, from D122,
and triggers the discharge circuit for D119, thus muting the
amplifier.
Figure 1.12

14 QSC Audio Products, LLC
TD-000163-00 Rev. B
1.8 Revisions made during
production
ISA750 and ISA800T
April 2002—Zener diodes D107, D110, D207, and D210 changed
from 5.6V to 4.7V. This is to eliminate the tendency of either
channel to hang up when the amplifier is turned on. Channel 1 was
most susceptible, and various circumstances tended to aggravate
the problem, such as having the low-frequency filters engaged,
having an accessory powered by the DataPort, etc. For amps made
before the change went into effect, use service bulletin ISA0001 to
correct related hang-up problems.
DC fault protection
The ISA1350 uses a DC crowbar circuit, similar to that used on the
ISA750 and ISA800Ti, on each channel output.
Figure 1.13

Technical Service Manual 15
ISA Series Power Amplifiers
2. Component identification and pinout
1
2
3
4
8
7
6
5
AB
OUTPUT A
INVERTING
INPUT A
NON-INVERTING
INPUT A
V-
OUTPUT B
INVERTING
INPUT B
NON-INVERTING
INPUT B
V+
1
2
3
4
8
7
6
5
GROUND 1
NON-INVERTING
INPUT
INVERTING
INPUT
V-
OUTPUT
BALANCE/
STROBE
BALANCE
V+
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
AB
NON-INVERTING
INPUT A
V-
AMP BIAS INPUT B
NON-INVERTING
INPUT B
INVERTING INPUT B
OUTPUT B
BUFFER INPUT B
BUFFER OUTPUT B
V+
DIODE BIAS B
AMP BIAS INPUT A
DIODE BIAS A
INVERTING INPUT A
OUTPUT A
BUFFER INPUT A
BUFFER OUTPUT A
NE5532AN Dual operational amplifier
LM311 Voltage comparator
LM13600 Dual operational transconductance
amplifier
4N29 Opto-isolator
1
2
3
6
5
4
2N5064 Sensitive gate thyristor
KAG

16 QSC Audio Products, LLC
TD-000163-00 Rev. B
BCE
12
3
BE
C
B C E
TOSHIBA
GDS
BCE
2N3904 (NPN) and 2N3906 (PNP) Small-signal
transistors
MJE15032 (NPN) and MJE15033 (PNP) Driver
transistors
IRFZ44 TMOS power field effect transistor
2SC5200 (PNP) and 2SA1943 (NPN) Power
transistors
MAC224 Triac
MT1
MT2
G

Technical Service Manual 17
ISA Series Power Amplifiers
When first checking the operation of an amplifier on the bench,
always turn your variable transformer down to zero before
plugging the amplifier in. After you turn the amplifier on, gradually
turn up the AC voltage as you observe the amplifier’s behavior and
its current draw; this will help you determine what, if anything, is
wrong with it. If you see or smell smoke, flames, or any other signs
of short circuits or excessive current draw, quickly turn the AC back
down to zero. If no such problems occur, it is usually safe to turn
the AC up to the amplifier’s full operating voltage for further
testing.
Moderate increase in current draw (current increases
slowly, doesn’t become excessive until about ¼ of the
amplifier’s full AC operating voltage)
• One polarity’s drivers and/or power transistors (NPN or PNP) is
shorted, on one or both channels.
• Individual supply clamp diodes D117, D118, D217, or D218 is
reversed or shorted.
• Bias diodes D108, D109, D208, or D209 or bias trimpots R131 or
R231 is open.
Slow increase in current draw (current doesn’t become
excessive until about half of the amplifier’s full AC
operating voltage; amplifier may pass signal)
• The bias is severely misadjusted, or bias diodes D108, D109,
D208, or D209 is defective.
• An oscillation is causing excessive current demand.
Runaway current draw (current increases sharply at about
25 to 33% of the amplifier’s full AC operating voltage)
One or more reservoir capacitors is reversed. CAUTION: the gas
buildup in a reversed electrolytic capacitor can cause it
to vent explosively. Immediately turn off power and let the
capacitor cool down before replacing it.
3. Troubleshooting: Symptoms, causes, & remedies
3.1 Excessive current draw
The customer complains of blowing circuit breakers or fuses, or
burning smell or smoke.
Symptoms covered:
• Fuses blow immediately
• The amplifier quickly gets very hot
• Line circuit breakers trip at turn-on
• The amplifier hums loudly and the chassis vibrates
• The amplifier emits smoke
• The amplifier gives off a burning smell
If the symptoms indicate a possible problem in the channel circuits
or output sections, you can isolate either channel module from the
power supply by pulling its fuses from the AC board.
Possible situations:
Excessive current with no signal present
If the amplifier seems to run hot and draws higher-than-normal
current when idling at full AC voltage, the cause could be bias
misadjustments in the output circuitry of one or both channels. See
the calibration procedures in the next section.
Fast increase in current draw (current increases rapidly at
only a few volts AC)
• The main bridge rectifiers BR100 and/or BR200 (all models) and
BR101 and/or BR201 (ISA750 and ISA800Ti) is reversed or
shorted.
• Supply clamp diode pairs D117 and D118 and/or D217 and D218
is reversed or shorted.
• The drivers and/or power transistors is shorted on both
polarities (NPN and PNP) on one or both channels.
3.2 Protection, muting, and turn-
on/turn-off delay problems
The customer complains of amplifier locking up, or not turning on
and off correctly.
Symptoms covered:
• Both channels do not come out of protect
• Amplifier will not thermally shut down when it should
• Power LED doesn’t light
• Too little or too much muting delay
• No clip limiting
• Fan doesn’t run, or runs always at high speed
Possible situations:
Both channels stay in protect after turn-on
• Q4 or Q5 is shorted base-to-emitter.
• Voltage across D8 should be 14 to 15 volts DC. If it is low, check
D8 and R10; also, C7 and C8 is leaky.
• D9, R4, and/or R7 is open.
The amplifier will not thermally shut down when it over-
heats
Check for shorted D9, R4, or R7; check also for open LD1 (“POWER”
LED), R2, or R5.

18 QSC Audio Products, LLC
TD-000163-00 Rev. B
Too much or too little muting delay
• Excessive delay at turn-on: check for open LD1, R2, or R5; check
for incorrect R10 or R15; check for shorted D9; also see service
bulletin RMX0002.
• Too short at turn-on (amplifier unmutes before the circuits stabilize,
causing a thump): check for incorrect or bad C7, C8, or R10.
• Amplifier doesn’t stay muted at turn-off: see service bulletin
RMX0003
Power LED doesn’t light
Check for open or shorted LD1; check R2.
No clip limiting
• U10 is defective.
• DIP switches SW1:1 or SW1:10 is defective.
Fan doesn’t run
• The fan is defective.
• Check the fan connection to the AC board.
• Check R1 and BR1 on the AC board.
Fan runs always on high speed
• Q3 and/or U2 are shorted.
• Check PTC thermistors R4 and R7.
The supply rails are OK with no signal but collapse with a
signal
• C112, C113, C212, and/or C213 is leaky.
• Check C114, C124, C214, and C224.
The amplifier gets too hot with no load
• Bias trimpots R131 or R231 are misadjusted, burned, or open.
• Bias diodes D108, D109, D208 and/or D209 are incorrect (should
be 1N4934).
• If the amplifier is producing high-frequency oscillations, check
C114, C124, C214, and C224.
• Check resistors R136, R137, R236, and R237.
• Op amp U101 or U201 is unstable.
One channel clips prematurely
• R146 or R147 (Channel 1) or R246 or R247 (Channel 2) are open.
• R139 or R140 (Channel 1) or R239 or R240 (Channel 2) are
misadjusted.
• Check R157, R158, R198 (ISA750 and ISA800Ti only), D115, and
D116 (Channel 1) or R157, R258, R298 (ISA750 and ISA800Ti
only), D215, and D216 (Channel 2).
• (ISA280, ISA300Ti, ISA450, and ISA500Ti) Check R118 and R119
(Channel 1) or R218 and R219 (Channel 2).
• R130 or R132 (Channel 1) or R230 or R232 (Channel 2) is open.
Excessive hum in loudspeaker when no signal is present
(ISA280, ISA300Ti, ISA450, and ISA500Ti)
Check R118, R119, R218, and R219.
Excessive current draw with signal present (ISA750 and
ISA800Ti)
• Check triacs Q113 and Q213.
• Check D119, D120, D219 and D220.
3.3 Faults with signal present
The customer compains that the amplifier passes a signal but
doesn’t run correctly.
Symptoms covered:
• The output signal breaks up or is distorted
• “Ringing” sound in loudspeaker when no audio signal is present
• The output signal collapses when driving a normal speaker load
• Supply rails OK with no signal, but collapse when a signal passes
• The amplifier gets too hot
• One channel clips prematurely
• Excessive hum in loudspeaker when no audio signal is present
Possible situations:
The output signal breaks up or is distorted
• (ISA280, ISA300Ti, ISA450, and ISA500Ti) Check the hum-null
resistors R118, R119, R218, and R219.
• Check the ground traces for continuity among speaker ground,
input ground, and AC ground.
“Ringing” sound in loudspeaker when no audio signal is
present
• Check C114, C124, C214 and C224.
• Check or replace dual op amp U101 or U201.
The output signal collapses when driving a normal speaker
load
• R139, R140, R239, and/or R240 is misadjusted.
• Check R118, R119, R218, and R219.