JBL LSR28P Operating manual

1
Technical Service Manual
Rev. A
3/14/2003

2
TABLE OF CONTENTS
PRODUCT OVERVIEW 3
OVERALL FREQUENCY RESPONSE 4
SERVICING NOTES AND DISCLAIMERS 5-6
SPECIFICATIONS 7
THE JBL LIMITED WARRANTY 8
TOOLS NECESSARY FOR SERVICE 9
SERVICE PROTOCOLS 10-11
A WORD ABOUT BALANCED AND UNBALANCED INPUTS 12
CONNECTOR WIRING/TYPICAL BENCH SETUP 13-14
SELECTIBLE INPUT SENSITIVITY 15
BLOCK DIAGRAM 16
THEORY AND CIRCUIT DESCRIPTIONS 17-25
POWER SUPPLY
INPUT BUFFER AMPLIFIER
LOW FREQUENCY PROCESSING
HIGH FREQUENCY PROCESSING
FINAL LOW FREQUENCY FILTER
LOW FREQUENCY OUTPUT AMPLIFIER
HIGH FREQUENCY OUTPUT AMPLIFIER
FAULT INDICATION CIRCUITRY
TROUBLESHOOTING GUIDE 26-45
FINAL TEST PROCEDURES 48
EXPLODED VIEWS AND GRAPHICS 49-51
WIRING DIAGRAM 52
SEMICONDUCTOR PINOUTS 53
PCB PICTORIALS 54-57
MASTER PARTS LIST 58
ELECTRONIC FAILURE QA CODES
SCHEMATICS

3
PRODUCT OVERVIEW
The Linear Spatial Reference, LSR, series of professional active loudspeakers is
especially designed for professional applications. Its two-way architecture employing a 1-
inch damped titanium-composite tweeter and an 8-inch, Differential Drive with Dynamic
Braking Voice Coil woofer reproduces the audible spectrum effortlessly.
The woofer's patented, Differential Drive Mechanism provides a greater voice coil surface
area allowing the LSR28P to better dissipate excessive voice coil heat. Since voice coil
heat directly affects the compression of the cone, the sound from traditional speakers will
deteriorate when used during long sessions. JBL has effectively devised a method of
releasing this heat. More aptly, by spreading the heat over a larger surface area so that it
is absorbed by the surrounding magnetic structure, this reduction has caused two
noticeable advantages--longer useful transducer life and a flatter impedance curve
throughout the frequency spectrum. What this means to the user is that the LSR28P will
exhibit consistent performance at low, medium and high levels of volume during the
entire musical session.
The Dynamic Braking Coil prevents excessive speaker cone excursion during those high
levels. Although, the LSR28P does require a pre-amp to provide a nominal -10db level to
be used for proper operation, the LSR28P provides the final signal frequency processing
and power amplifier modules.
The composite signal is efficiently separated into the low and high frequency components
by utilizing BI-amplified topology for signal dispensation. Each frequency-specific
element is sent to the respective power amplifier. For the low frequency section, a
discrete, push-pull Darlington-configured amplifier is employed with a measured gain of
15 or an increase of 23dB. For the high frequency section, an integrated circuit capable of
being driven to 100 watts of continuous output with a gain of 14 is used.which is
comparable to a 21db gain. Both amplified signals are sent to the matched low and high
frequency proprietary transducers for audible reproduction. Employing this method of
frequency separation eliminates the total harmonic and phase distortion that commonly is
associated with passive networks.
The final result is a time-independent, smooth, and distortion-free sound at all levels of
output! And, since the entire LSR Series has been designed with the philosophy
incorporating the best components available to attain the highest performance possible
for a particular targeted area, they are able to faithfully reproduce their respective
frequency range. For the LSR28P, the entire audio range from a low of 50Hz extending
to 20KHz.and beyond with a crossover frequency of 1.7KHz is reproduced. (The
frequency response for the LSR28P amplifier is shown graphically on the following
page.) Needless to say, they have become the standard speaker monitors used in
professional studios and by audiophiles alike.
.

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OVERALLFREQUENCY RESPONSE
The frequency response of the output from the crossover system is shown in the figure below.
A
UDIO PRECISION &vs Crossover Response
-30.00
-25.00
-20.00
-15.00
-10.00
-5.000
0.0
5.0000
10.000
LEVEL(dBu) 2-CHAN(dBu)
20 100 1k 10k 50k
FREQ(Hz)
When a system under test shows a different result in the frequency response test, as shown
above, using a +4dBu input signal to the XLR connector with no DIP switches selected, the
source of the error can be investigated by referring to the following sections located within the
troubleshooting guide.

5
SAFETY PRECAUTION STATEMENT
There are no user servicible parts inside! Attempting to repair this
product or opening the cabinet will expose the user to hazardous high
voltage. Servicing and repair should be referred to authorized
technical personnel only.

6
LIABILITY WAIVER
JBL PROFESSIONAL IS NOT LIABLE FOR ANY DIRECT OR INDIRECT DAMAGE TO
THIS OR ANY ASSOCIATED PROPERTY ARISING FROM THE USAGE OR THE
INABILITY TO USE THIS PRODUCT.

7
SPECIFICATIONS
ACOUSTIC & ELECTRICAL SPECIFICATIONS
Power Consumption: 220W, (IEC 265) P
Power Capacity: High F 100W, Low F 250 W
Power Requirements: 115/230VAC, 50/60 Hz (user selectable)
Frequency Response: 50 Hz - 20 kHz (+1, -1.5db)
Sensitivity (XLR Input): +4dB, 96dB, @1 Meter
(1/4” Input): -10dB, 96dB, @ 1 Meter
(All DIP Switches Set Off) +4dBu Signal
Crossover Frequency: 1.7 kHz
Signal Input: XLR Balanced with pin 2 Hot @ 120k Ohm impedance
SYSTEM COMPONENTS
Cabinet Resonance Frequency: 38 Hz
Low Frequency Transducer
JBL #(218F) 203mm (8 in.) Cone
DC Resistance: 1.8-ohm ± 10%
Differential Drive, Dynamic Braking
High Frequency Transducer:
JBL# (053Ti)
DC Resistance: 3.5 ohm - 3.8 ohm
Amplifier:
Low Frequency: Class A-B Discrete
High Frequency: Class A-B, Monolithic
THD@ ½ Power: <0.05%
AURAL SWEEP TEST SPECIFICATIONS
(XLR Input All DIP Switches set default off)
System Aural Sweep Test: 1.5V Input, 20 Hz to 30 kHz
L.F. Aural Sweep Test: 7.0V Input, 20 Hz to 5 kHz
H.F. Aural Sweep Test: 1.5V Input, 200 Hz to 20 kHz
PHYSICAL SPECIFICATIONS
Enclosure Dimensions: 406mm x 330mm x 324mm D
(16.0 x 13.0 x 12.75 in. D)
Net Weight: 45 lbs. (20.5 kg.)

8
WARRANTY INFORMATION
All JBL LSR products carry the transferable, JBL Professional Limited
Warranty covering all defects in material and workmanship for a full
five years from the original date of purchase. Electronic components
and circuitry is warranted for three years. For more specific
information, consult the warranty card that is packed with the product.
Click here to view the JBL Limited Warranty Statement
http://www.jblpro.com/pub/technote/warranty.pdf

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ESSENTIAL TOOLS
In order to successfully service and maintain this speaker system, the technician
should have, at a minimum, the following test instruments.
Oscilloscope, at least 20 MHz bandwidth
Digital Multimeter, with a minimum of 50kohm Impedance for various
troubleshooting
Line Voltage Variac with Ammeter for measuring proper voltage/ current
consumption.
Hand tools (i.e. screwdrivers, pliers’ etc.)
4 ohm resistive load capable of handling 500 watts continuous.
Serial Number specific schematic diagram

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SERVICE PROTOCOLS
The maintenance of an electronic system can be divided into many sequential processes that
have to be explicitly followed in order to achieve a final result. Partial repair or incomplete
repair will not suffice for the professional technician intent on complete customer satisfaction
and will not adequately restore proper operation to a malfuntioning unit.
Today’s audio requirements necessitate that the electronics engineer design amplifier
circuitry to operate within very tight standards or tolerances. Extended high and low frequency
circuit attributes demand that the components that are used within the circuitry be of excellent
quality and are able operate within the designated parameters to produce the desired results. The
Q Point or operating point defines the class of operation of the specific amplifier. Both the low
and high frequency amplifiers operate in the range of class AB operation. This not only is
efficient, but also avails the designer to achieve those desired results. In addition, electronic
equipment will only properly operate if the circuit is operating at 100% efficiency. In order to
achieve the efficiency the service technician needs to apply a sort of fault analysis, approaching
each breakdown from the standpoint of cause and effect analysis. Since electronic
troubleshooting is synonymous with fault analysis, as applied to the parameters of the circuit
under observation, it must be methodically continued until successful completion or repair.
The skill that is necessary to successfully troubleshooting electronics to the specific faulty
component requires that the technician understand how the equipment correctly operates.
Obtaining an accurate, serial number-coordinated schematic diagram is also an essential tool.
Then, by determining what is operating correctly and what is not, the technician can eliminate
the electronic circuitry not associated with the fault and concentrate on the problem area. Testing
for typical voltages and oscilloscope waveforms at various test points and, as a last resort,
initiating methodical signal tracing can usually isolate the fault to a particular stage. Further
diagnoses within that stage will usually reveal the root cause of the failure. Most often, the root
cause will reduce to a defective part, faulty trace or intermittent connection. Sometimes there
are several root causes that interact with one another causing erroneous measurements and,
subsequently, inaccurate diagnoses of the malfunction. In these instances, the technician needs
patience and persistence to accurately diagnose and then repair the problem to restore proper
operation.
Many times a faulty or intermittent connection at the leg of a component that is supposedly
attached to a specific trace will appear to be okay when in reality it is not. These "cold solder
joints" occur because of several different reasons. Either the joint did not get hot enough for the
solder to bind the trace and the component leg together. Sometimes, metal fatigue can break the
connection. Oxidation or contamination at the solder joints can occur especially if the unit is
operated within a very humid atmosphere. In any case, the technician needs to be aware that
physics is constantly at work and can cause variety of acute symptoms to occur.
Unfortunately, several scenarios are not as straightforward. If the observed behavior is an
oscillation, a very high total harmonic distortion or simply a DC voltage seen on the outputs,
these abnormalities will be seen throughout the amplifier. Additionally, a single part can cause
other parts to fail and, consequently, will require all defective components be replaced
concurrently to restore proper operation. This is especially true of modern amplifier design that
includes the LSR Series. In order to attain the phase and distortion-less sound, the discreet
topology of the main amplifier is capacitor-less. This design was intended and does enable the

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listener to hear the input media more accurately albeit at the expense of a more difficult and time
consuming troubleshooting analysis.
Most of the repairs that are necessary to the LSR monitor system can be traced either to
the PCB connections at the component legs to an actual failure of the electromechanical
components . . . intermittent DIP switches, noisy potentiometers, faulty connectors. The printed
circuit boards and their respective Surface Mount Technology components are reliable. The
problems that do arise, and all electronic equipment will exhibit problems eventually, can
usually be attributed to the failure of the larger through-hole components.

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A WORD ON BALANCED AND UNBALANCED INPUTS
In order to achieve an undistorted sound, the installation technician has to connect the
LSR28P properly. The introduction of noise, hums, pops and whistles in the sound media
detracts from the audience’s understanding, and ultimately, their comprehension whether that
media is of a vocal or musical nature. So, it is of critical importance that the Audio System be
correctly installed.
There are three methods of connecting the source output to the source input of the LSR28P
System. Each method has its own attributes and the NUETRIK Combo connector can
accommodate all methods of hookup, however, the quietist and most universally accepted
method of hookup for the professional environment is the first method or “balanced input.”
The first method of hookup, called a “balanced input”, of which the LSR28P was designed, uses
XLR connectors throughout the system. The actual reason why this is a more accepted method
of hookup is the fact that XLR connectors utilize three conductors. Each conductor has a
purpose—one conductor for the hot lead, one conductor for the audio ground and, lastly, one
conductor for the chassis ground, which is connected to the shield of the input cable. This third
conductor, the shield, which is terminated at the LSR28P, eliminates extraneous noise that would
ordinarily be induced into the input leads or ground plane and carried onward to the
amplification process.
The second method of hookup, called an “unbalanced input”, is by using ¼” phone plug which
is predominantly used within the semiprofessional environment.
The phone plug can be plugged into the center of the Nuetrik connector. Again, each method
will work but the ¼” phone plug method is more susceptible to inducing the annoying hum that
is commonly referred to as a ground loop due to the fact that the chassis ground and audio
ground are hard wired together. This condition will occur especially if a large distance separates
the monitor and preamplifier allowing differing levels of voltage potential to reside upon the
ground plane. The unwanted signal is admitted into the system via the input circuitry and, or the
power supply.
If the unwanted signal enters the input circuitry, it is usually through input cable induction and,
most of the time, the hum that will be induced is a 60cycle hum. If the unwanted signal enters
through the power supply the entire amplifier will be affected. In order to avoid this scenario,
the technician should use a 3-conductor cable, preferably shielded R58 coaxial cable. Again,
two of the conductors carry the signal and the other conductor is connected to the chassis
ground.
If the “unbalanced input” method must be used, termination of the ground must be done at
the input to the LSR28P. This will avert the above possible problems by preventing a complete
circuit forming and possible internal electronic circuit damage resulting from a malfunction or
internal a.c. short of either equipment.

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BALANCED AND UNBALANCED CONNECTIONS
SUGGESTED INSTRUMENT SETUP
INPUT
METER
OUTPT
METER
CHNL CHNL
1 2
DISTORTION
ANALYZER
UNIT UNDER
TEST
V
OLT/OHM
METER
VARIAC
WITH
CURRENT
METER
ISOLATION
XSFRM
R
O’SCOPE
4 Ω
LOAD
SIGNAL
G
ENERATO
R
A

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The third method of connecting signal to the LSR28P is still an unbalanced hookup and is a
variation of the above XLR method by using a two-conductor cable or RCA cable with two XLR
adapters at each end or two Tip/Ring/Sleeve connectors at each end.
The Tip/Ring/Sleeve connectors are also commonly referred to as 1/4" connectors and can be
used with the LSR28P's Neutrik connectors. The popularity of TRS connectors is related to their
early use and acceptance within the consumer audio industry. The tip of the TRS connector or
the center conductor of the phonograph cable is the “hot” lead and should be connected to Pin 2
of the XLR connector for signal transfer. The use of the RCA connector is, by far, the least
expensive method of hookup, the noisiest and most problematic, too. They are simply two
conductors one hot and one cold, however, they do connect both grounds together and have
caused many problems including damaging equipment. They are not recommended for
professional sound systems.
THEREFORE, IT IS HIGHLY RECOMMENDED THAT THE TECHNICIAN USE A
THREE-CONDUCTOR CABLE WITH XLR CONNECTORS TO CONNECT THE
PREAMPLIFIER OR SOURCE TO
THE LSR 28P MONITOR

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INPUT SENSITIVITY
The LSR28P is very versatile in that the input audio sensitivity can be adjusted. Monitor inputs
are normally at nominal levels of +4 dBu or –10dbv. These input levels are called professional and
semiprofessional, respectively, and reflect the type of connectors used for hookup. The nominal level for
the XLR input is +4 dBu and -10 dBv for the 1/4" TRS input. If less sensitivity is needed to match other
brands of professional or semiprofessional equipment, 4,8, or 12 dB of signal attenuation can be inserted
in the input line by using the DIP switches on the back of the LSR28P. See the illustration below:
Located on the right of the Neutrik connector but before the power switch, is a bank of switches that are
accessible with the use of a pen or a very small screwdriver. These are the DIPswitches.
•Switch 1-enables the input trim potentiometer when switched to the up position. This
potentiometer is located to the left of the bank of DIPswitches and can be accessed
with a small screwdriver. This allows the user to attenuate the input level from the
fixed; factory set default of +4 dBu to a variable input level from +4 dBu to –12 dBu.
•Switch 2—attenuates the input level by a fixed 4 dBu
•Switch 3—attenuates the input level by a fixed 8 dBu.
•Switch 4—aligns the bass response to roll off at 24 dB per
Octave
•Switch 5—aligns the bass response to roll off at 36 dB per
Octave corresponding to a –2 dB bass response
•Switch 6—keeps the bass response at 36 dB per Octave but
Increases the emphasis of bass frequencies +2 dB
•Switch 7—attenuates only the High Frequency amplifier
Input level by -2 dB.
•Switch 8—enhances the High Frequency amplifier input
Level by +2 dB

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HFP
LFP
3
BLOCK DIAGRAM
LSR28P
Signal
Gain
Trim
DIP Switch
PreAmp
PreAmp
PWR. AMP
PWR. AMP
HF
LF
Speaker
Speaker
TRIM
TRIM

17
THEORY AND CIRCUIT DESCRIPTIONS

18
POWER SUPPLY
Initially, raw alternating current enters the LSR28P from the IEC connector on the signal input
processing board and is directly connected to the power switch S3 through the main fuse F1. .
From there, it travels to the voltage selector switch that determines which primary on the
transformer connected. The voltage that has been selected is then connected to the toroidal
transformer and stepped down to a more useful voltage. This voltage then enters the main PCB,
where it is rectified or transformed to positive and negative Direct Current by bridge rectifier
D13. It is filtered by capacitors C16 and C17. . . the end result is a stable power supply of
nominally positive and negative 36 volts. Mainly, this voltage is used for the rails of the power
amplifiers, which provide a full voltage swing from peak to peak of 78 volts. The rectified
voltage is also passed through a resistive ladder network which steps down the magnitude further
and is regulated at positive and negative 15 volts to drive the housekeeping/fault detection
circuitry. Voltage doubling circuitry supplies the positive and negative 70 volts necessary to
supply the drivers and predrivers. By utilizing this bootstrap configuration, the adjacent stages
are prevented from scavenging the voltage from the rails during demanding informational or
musical passages.
The main power transformer that is employed in this product has its core in the form of a
toroid. This physical shape provides better magnetic flux permeability between the primary and
secondary windings, and, thus, has a better instantaneous power delivering ability than
conventional bundled, stamped-steel. cores. A lower hysteresis loss also is beneficial since less
heat is generated. It has long been known that the toroid allows better flux lockup between the
primary and secondary windings and, therefore, a better instantaneous voltage delivery than
traditional transformers. The combination of using this type of transformer and the bootstrap
circuitry almost guarantee that sufficient voltage will be available on demand for full bass
response. . . without the “bottoming out’ of the low frequency driver at crucial moments in the
music.

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INPUT BUFFER AMPLIFIER
The integrated circuit, U3, provides several functions. U3A is used as an input buffer amplifier
which isolates the source signal from deterioration caused by the user selectable DIP switches,
SW1A.,B,AND C. The resultant gain of U3B will vary depending upon which DIP switches are
enabled. The final composite signal on pin 7, U3B, is sent to coupling capacitors C4 and C26
which provide access to the low and high frequency processing circuits, respectfully.
Input = 0 dB/2.2 Vpp
@ 100 Hz
Out
p
ut= 1.1 V
pp

20
HIGH FREQUENCY PROCESSING
The circuit, shown above, is used for the high frequency processing of the input signal.
Physically, it is located on the input circuit board. Its purpose is to actively filter out or attenuate
the low frequency component from the composite signal leaving only the high frequencies to be
contoured by the user and, eventually, sent to power amplifier for final amplification.
Specifically, signal from the previous stage enters the non-inverting input pin 3 of
integrated circuit U1A through coupling capacitor C26. U1A, acting as a second order high pass
filter, attenuates signals below the critical frequency, according to the equation:
f(c) = 1/[2π(R1R2C1C2)1/2] = where R1 = 9kΩ, R2 = 10kΩ, C1 = C2 = 10nF
= 1.677kHz
Because of this relationship, the frequency response will “roll-off” or be attenuated at a
rate of –40 dB per decade assuring a gain of .707 volts at this critical frequency.
Integrated circuit U1B is configured as a buffer amplifier to isolate the effects of the user
selectable high frequency “trim” potentiometer, R4, and includes some additional second-order
filtering. Partial equalization occurs from components R13 and C5. The signal enters at pin 5
and pin 6 of this IC whereby unwanted noise is eliminated through CMRR, and it is amplified
from a signal voltage of .38V to a usable voltage level of .45 volts. The signal exits at pin 7.
Enabling of SW1G and SW1H cause signal frequency equalization boosts or cuts to occur.
These can be attributed to the effects of U4A and B along with U8.
Out
p
ut= . 8 V
pp
In
p
ut= 1 V
pp