Bose PS18T,PS28T,PS35 T Troubleshooting Manual
PS18/28/35 Troubleshooting Guide
Contents
Safety Information.............................................................................................................................2
Electrostatic Discharge Senstitive (ESDS) Device Handling ........................................................2
Specifications ....................................................................................................................................3
Theory of Operation .................................................................................................................... 4-21
Setting up a computer to issue TAP commands ..........................................................................22
Placing the Bass Module into TAP Mode ...................................................................................... 23
Figure 1. DIP switch Up/Down orientation ......................................................................................23
Equalizer Programming Method ....................................................................................................23
Scope Photos ............................................................................................................................ 24-25
Integrated Circuit Diagrams ..................................................................................................... 26-30
THIS DOCUMENT CONTAINS PROPRIETARY INFORMATION OF
BOSE® CORPORATION WHICH IS BEING FURNISHED ONLY FOR
THE PURPOSE OF SERVICING THE IDENTIFIED BOSE PRODUCT
BY AN AUTHORIZED SERVICE CENTER OR OWNER OF THE BOSE
PRODUCT, AND SHALL NOT BE REPRODUCED OR USED FOR ANY
OTHER PURPOSE.
PROPRIETARY INFORMATION
1
PS18/28/35 Troubleshooting Guide
SAFETY INFORMATION
1. Parts that have special safety characteristics are identified by the symbol on schematics
or by special notes on the parts list. Use only replacement parts that have critical characteristics
recommended by the manufacturer.
2. Make leakage current or resistance measurements to determine that exposed parts are acceptably insulated from the supply circuit before returning the unit to the customer. Use the following
checks to perform these measurements:
A. Leakage Current Hot Check-With the unit completely reassembled, plug the AC line cord
directly into a 120V AC outlet. (Do not use an isolation transformer during this test.) Refer to
UL6500 paragraph 9.1.1. Use a leakage current tester or a metering system that complies with
American National Standards Institute (ANSI) C101.1 “Leakage Current for Appliances” and
Underwriters Laboratories (UL) 6500, IEC 60065 paragraph 9.1.1. With the unit AC switch first in
the ON position and then in OFF position, measure from a known earth ground (metal waterpipe,
conduit, etc.) to all exposed metal parts of the unit (antennas, handle bracket, metal cabinet,
screwheads, metallic overlays, control shafts, etc.), especially any exposed metal parts that offer
an electrical return path to the chassis. Any current measured must not exceed 0.5 milliamp.
Reverse the unit power cord plug in the outlet and repeat test. ANY MEASUREMENTS NOT
WITHIN THE LIMITS SPECIFIED HEREIN INDICATE A POTENTIAL SHOCK HAZARD THAT
MUST BE ELIMINATED BEFORE RETURNING THE UNIT TO THE CUSTOMER.
B. Insulation Resistance Test Cold Check-(1) Unplug the power supply and connect a jumper
wire between the two prongs of the plug. (2) Turn on the power switch of the unit. (3) Measure the
resistance with an ohmmeter between the jumpered AC plug and each exposed metallic cabinet
part on the unit. When testing 3 wire products, the resistance measured to the product enclosure
should be between 2 and infinite Meg ohms. Also, the resistance measured to exposed output/
input connectors should be between 4 and infinite Meg ohms. When testing 2 wire products, the
resistance measured to exposed output/input connectors should be between 4 and infinite Meg
ohms. If it is not within the limits specified, there is the possibility of a shock hazard, and the unit
must be repaired and rechecked before it is RETURNED TO THE CUSTOMER.
ELECTROSTATIC DISCHARGE SENSITIVE (ESDS)
DEVICE HANDLING
This unit contains ESDS devices. We recommend the following precautions when repairing,
replacing or transporting ESDS devices:
• Perform work at an electrically grounded work station.
• Wear wrist straps that connect to the station or heel straps that connect to conductive floor mats.
• Avoid touching the leads or contacts of ESDS devices or PC boards even if properly grounded.
Handle boards by the edges only.
• Transport or store ESDS devices in ESD protective bags, bins, or totes. Do not insert unprotected devices into materials such as plastic, polystyrene foam, clear plastic bags, bubble wrap or
plastic trays.
CAUTION: THE BOSE
®
PS 28 AND PS 35 POWERED SPEAKER
VICEABLE PARTS. TO PREVENT WARRANTY INFRACTIONS, REFER SERVICING TO
WARRANTY SERVICE STATIONS OR FACTORY SERVICE.
CONTAINS NO USER-SER-
2
Specifications
Mechanical
PS18/28/35 Troubleshooting Guide
Dimensions:
Satellite: 3.1" W x 4.0" D x 6.02" H
Weight:
Module: 8.0" W x 23.0" D x 16.0" H
(20.32 x 58.42 x 40.64 cm)
Jewel Cube
Module: 35.9 lb (16.3 kg)
Satellite: 2.4 lb (1.1 kg)
Jewel Cube speaker: 1 lb (0.5 kg)
speaker:
(7.8 x 10.2 x 15.7 cm)
2.2" W x 8.0" D x 2.6" H
(39.4 x 20.3 x 6.6 cm)
Electrical
Drivers: Bass module: Two woofers, 5 1/4", 2.33 Ohms,
(wired in parallel)
Satellite speaker: Two Twiddler
(wired in series)
Jewel Cube speaker: Two Twiddler
(wired in series)
Amplifier power: Bass Channel: 125W, <0.2% THD, 40 Hz-200 Hz,
120 Vrms AC mains
L/R/C/LS/RS: 20W, <0.2% THD, 200 Hz-15 kHz,
120 Vrms AC mains
Input impedance: Module: 1.16 Ohms (two 2.33 Ohm woofers
wired in parallel)
Satellite: 8 Ohms (two 4 Ohm Twiddler speakers
wired in series)
Jewel Cube Speaker: 6.4 Ohms (two 3.2 Ohm Twiddler speakers
wired in series)
L/R/C/LS/RS output distortion: <0.1% THD at 0.5W
Bass distortion: <0.2% THD at 0.5W
L/R/C/LS/RS output noise: <500 uVrms, A weighted
Bass output noise: <2 mVrms, unweighted
L/R/C/LS/RS DC offset: <25 mVdc
L/R/C/LS/RS balance:
±2.0 dB
Channel separation: >40 dB at 1 kHz
(stereo mode)
>30 dB at 10 kHz
Turn-on delay: 1.5 seconds maximum
Turn-off delay: 200 ms maximum
TM
, speakers, 50 mm, 4 Ohms
speakers, 2 1/4", 3.2 Ohms
Main voltage: USA/Canada : 120 VAC, 60Hz
Inrush current: 20A peak for first 33.3 msec.
Europe, UK, AUS : 230/240VAC, 50Hz
Japan: 100VAC, 50/60Hz
Dual Voltage : 115/230VAC, 50/60Hz
3
Theory of Operation
1. Power Supply, Switch-Mode Audio Tracking
1.1 Introduction
PS18/28/35 Troubleshooting Guide
There is a growing demand for power in home theater systems. Traditionally a Bose
®
home theater
system uses an unregulated linear power supply to power multiple linear audio amplifiers. The
power supply is typically based on a line-frequency transformer and the audio amplifiers are either
Class-B or Class-G linear amplifiers. However, there exists a limit to power and size with such a
technology. High-frequency switching technology in the power supply and audio amplifier enables
us to achieve a higher power at a smaller package size. Nevertheless, such an approach is usually
not very cost effective because of the complexity of the circuit and the extra effort needed to
reduce EMI emissions associated with high frequency switching.
A new technology, switch-mode audio tracking power supply, has been developed for Lifestyle
®
home entertainment systems to achieve higher power from a small package at a reasonable cost.
A tracking power supply delivers power on demand to multiple amplifiers. When the audio signal is
low, the power supply output voltage is low. When the audio signal is high, the power supply output
voltage is high. A rail voltage that tracks audio level reduces the voltage drop at the power amplifier, resulting in lower power loss and hence less heat dissipation at the power amplifier. Consequently, traditional low-cost Class-B amplifiers can be used. While high frequency switching makes
it possible to keep the size small, Class-B amplification makes it possible to keep the cost down.
This document describes the theory of operation of a high-frequency switch-mode audio tracking
power supply, SD 254165, which is used as a power source for multiple audio power amplifiers in
Lifestyle® home entertainment systems. The purpose of this document is to assist in the troubleshooting of the switch-mode audio tracking power supply. Brief descriptions of major function
blocks of the power supply is presented first, followed by more detailed discussions on each
function block.
1.2 Functional Block Diagram
Figure 1 shows a block diagram of a switch-mode tracking power supply. Power flows from the AC
line input at left to the DC output at right. Control signal flows from right to left.
Figure 1. Block diagram of a switch-mode tracking power supply
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PS18/28/35 Troubleshooting Guide
Theory of Operation
AC power comes from line source to an EMI filter first, which prevents noise generated by high
frequency switching from flowing back to the line source. A bridge rectifier BR1 converts AC power
into DC power. There are three versions of the power supply; US, Euro, and Dual. For the US
version, the bridge rectifier BR1 is configured to a voltage doubler rectifier. For the Euro version,
the bridge rectifier BR1 is configured to a full wave rectifier. For the Dual version, an automatic
voltage selector IC turns on or turns off the triac Q401 depending on the line voltage, re-configuring the bridge rectifier into a full wave rectifier for 220V/240V line or a voltage doubler rectifier for
100V/120V line. Electrolytic capacitors C110 and C111 filter out line frequency and its harmonics.
A DC voltage at about 340V is obtained across the two capacitors regardless of the line voltage
variation.
The DC voltage is then inverted into a high-frequency quasi-square voltage by two power
MOSFETs Q101 and Q102 which are turned on and off at a high frequency (100kHz ~ 200kHz) by
the controller IC U101. The high-frequency quasi-square voltage is applied to the primary winding
of the center-tap transformer T102 via a capacitor C115. Capacitor C115 and leakage inductance
of the transformer form a resonant circuit, which shapes the resonant current waveform into a
quasi-sinusoidal waveform. The power to the transformer is controlled by adjusting the switching
frequency relative to the resonant frequency. A rectifier connected to the secondary of the transformer converts the high-frequency power into DC power and a low pass filter removes highfrequency contents. DC power is obtained at two output terminals. This circuit which converts DC
power to DC power is called a half-bridge high-frequency resonant DC-DC converter.
A negative feedback circuit made of an integrator, a nonlinear amplifier, an opto-coupler, and a
controller IC controls the above power converter in such a way that the output voltage of the power
converter follows or tracks the power supply control (PSC) signal, which is generated at the DSP
board to track the audio signal.
In addition to the above basic functions, overvoltage, amplifier fault, and over temperature protection circuits are designed to prevent the power supply and amplifiers from catastrophic failure. The
power-down circuit is also designed so that the power supply outputs decay slowly when the AC
input power is turned off. Two linear regulators are tapped off the two output rails to provide two
regulated low voltage rails.
The high-frequency transformer and the opto-coupler provide electrical isolation between the AC
line source and audio circuitry that is powered by the power supply.
Since the transformer is operated at a high frequency, it can be designed to be very small and yet
very efficient. The size of power supply is significantly reduced compared with a power supply
using a line-frequency transformer.
1.3 Resonant Controller IC
This section describes the high-voltage resonant controller IC that controls the resonant power
converter which is the brain of the power supply. Figure 2 shows the block diagram of the IC. Its
main function is to generate a high-frequency signal at a voltage-controlled-oscillator (VCO) and to
drive two power MOSFETs in a half-bridge circuit.
Figure 2. Block Diagram of high-voltage resonant controller IC
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PS18/28/35 Troubleshooting Guide
Theory of Operation
The IC has two enable pins, one of which (pin-8) disables the operation while the other (pin-9)
enables the operation. A voltage pulse higher than 0.6V at pin-8 shuts down the IC and a voltage
pulse higher than 1.2V at pin-9 wakes up the IC. An opamp, an under voltage lockout circuit, and a
soft start circuit are also built into the IC.
Figure 3 is a circuit diagram that shows how the IC is used in the tracking power supply. With a
12V voltage established at pin-12 (Vs), the VCO starts to oscillate, sweeping oscillation frequency
from its start frequency (350 kHz) downward. A gate-driving signal for a low-side MOSFET is
generated at pin-11 with respect to pin-10, and another gate driving signal for a high-side
MOSFET is obtained at pin-15 with respect to pin-14.
Figure 3. Resonant controller IC used in tracking power supply
Figure 4 shows the oscillograms of the two gate driving signals, when the IC alone is powered on
by a 12V DC supply. A dead time of about 280ns, during which both signals are at low level, exists
between the two gate signals. Such a dead time avoids shoot-through of the two power MOSFETs.
It also allows zero-voltage switching as will be explained later.
The frequency of the two signals is controlled by a voltage at pin-7, which is connected to the
output of an opto-coupler. The frequency is about 200 kHz with 0V at pin-7 and 100 kHz with 2V at
pin-7.
Figure 4. Scope photo of two gate driving signals
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PS18/28/35 Troubleshooting Guide
Theory of Operation
1.4 High-frequency Resonant Converter
This section describes the theory of operation for a high-frequency half-bridge resonant DC-DC
converter, the heart of the power supply. Figure 5 shows its detailed circuit, where the high-frequency transformer is represented by a leakage inductor Llk, a magnetizing inductor Lm, and an
ideal step-down transformer.
Figure 5. High-frequency half-bridge resonant DC-DC converter
With gate signals from the controller IC, the two power MOSFETs Q101 and Q102 are turned on
and off alternately to invert the DC input voltage to a high-frequency square wave voltage. The
high-frequency square wave voltage is applied to a series resonant circuit made of a resonant
capacitor C115 and leakage inductor Llk of the transformer. The resonant circuit shapes the
current waveform to a quasi sinusoidal waveform. Figure 6 shows oscillograms of the voltage
across MOSFET Q101 and current through capacitor C115. Such a smooth current waveform
lowers electromagnetic interference (EMI) emissions from the power supply.
Figure 6. MOSFET Q101 voltage and C115 current waveforms
The transformer transfers high-frequency power from the primary winding to two secondary
windings. The rectifier diodes D201A, D201B, D203, D204 rectify the high-frequency power and
the two capacitors C201 and C202 filter out high-frequency contents. A low pass L-C filter is
added to each rail to further reduce the high-frequency noise. Two DC voltages, one positive and
one negative, are obtained at the output.
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PS18/28/35 Troubleshooting Guide
Theory of Operation
The parameters of capacitor C115 and inductor Llk are selected in such a way that its resonant
frequency is lower than the range of switching frequencies. In other words, the switching frequency
is always above the resonant frequency. The resonant circuit is operated in inductive mode. Two
capacitors C113, C114 are added in parallel with the two MOSFET switches to slow down the
voltage transition across the two switches. Inductive mode operation and a dead time provided by
the IC, shown in Figure 4, allow the voltage across the two switches to transit smoothly from high
to zero and from zero to high. Figure 7 shows gate voltage, drain-source voltage, and drain current
for MOSEFT Q101. As can be seen from Figure 7, the power MOSFET is turned on and turned off
when its drain-source voltage is zero. Zero-voltage switching like this reduces switching loss and
switching noise.
Figure 7. Gate and drain-source voltage, and drain current waveforms for MOSEFT Q101
A characteristic of a resonant converter arrangement shown in Figure 5 is that the ratio of the
output DC voltage to the input DC voltage is determined by the ratio of the switching frequency to
the resonant frequency of capacitor C115 and inductor Llk. Figure 8 shows a plot of voltage
conversion ratio versus normalized switching frequency. Each of the curves represents a different
load resistance that is equivalent to the two loads at the two output rails. For example with load L3,
voltage conversion ratio decreases with the normalized switching frequency if operated above
resonance (inductive mode). Therefore, by moving the switching frequency away from resonance,
the output voltage is lowered.
Figure 8. Plot of voltage conversion ratio versus normalized switching frequency
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PS18/28/35 Troubleshooting Guide
Theory of Operation
1.5 Feedback Control Circuit
The feedback circuit controls the above power converter in such a way that its output voltage
follows or tracks the power supply control (PSC) signal. This section describes the feedback circuit
shown in Figure 9.
Figure 9. Feedback circuit
The feedback control circuit senses the negative output of the power converter and compares it
with the PSC signal. The error is amplified by an integrator. The amplified error is compensated by
a nonlinear amplifier for loop stability and is then applied to the resonant controller IC via an optocoupler.
If the magnitude of the negative voltage is lower than what the PSC signal demands, the output
voltage of the opto-coupler is higher, lowering the oscillation frequency of the resonant controller
IC. The IC moves the switching frequency closer to the resonant frequency of the power converter,
increasing the magnitude of output voltage. Such a negative feedback control minimizes the error
between the rail voltage and the PSC signal. The voltage at the power supply output terminals
tracks the audio signal. Figure 10 shows waveforms of a PSC voltage, two output voltages, and an
amplifier output voltage for a tracking power supply used as power source for one audio amplifier.
The PSC is derived from the audio signal and an offset is added for voltage drop at the amplifier
(amp saturation voltage). The two rails track PSC very well.
Figure 10. Power supply waveform showing tracking voltage
1.6 Protection Circuits and Others
This section describes protection circuits and some other circuits that are added to the power
supply. Figure 11 shows an over-temperature protection circuit and an AMP_FAULT protection
circuit.
9
PS18/28/35 Troubleshooting Guide
Theory of Operation
Figure 11. Over-temperature and AMP_FAULT protection circuit
A positive-temperature-coefficient (PTC) resistor RT100 is placed next to a heat sink for two power
MOSFETs. When the PTC RT100 is heated up to 125°C by the heat sink, the resistance of the
PTC increases dramatically, resulting in a logic-high at the PTC. Independently, when any audio
amplifier is in fault condition, the AMP-FAULT is pulled down, setting a logic-high at the output of a
second opto-coupler U103. Either over-temperature or amplifier fault sends a voltage pulse to pin8 of the resonant controller IC, shutting down the IC and hence the power supply.
Referring to the resonant controller IC circuit shown in Figure 3, the Vs-pin (pin-12) of the IC will
be slowly charged up beyond 12V in shutdown mode by a current through R156 since the IC
consumes very little power after it is shut down. Zener diode ZR101 starts conducting, charging
voltage at pin-9 up. Once the voltage at pin-9 reaches 1.2V, the IC is enabled. The power supply
resumes operation. The above shut down and recovery process repeats until both over-temperature and AMP-FAULT conditions are removed.
During normal operation, the resonant controller IC is powered by rectifying the dv/dt current
through capacitor C114 that is connected across MOSFET Q101 (Figure 5).
An overvoltage protection circuitry shown in Figure 12 is designed to protect the rail-to-rail voltage
from exceeding 52V. If the rail-to-rail voltage exceeds 52V, Zener diode ZR300 starts conduction,
activating transistor Q301 and hence transistor Q300. Current to opto-coupler U102 is pulled away.
The consequence of such is a lower rail-to-rail voltage.
A power-down circuit, shown in Figure 13, is designed for the output voltage to decay slowly after
the power switch on the AC line is turned off. Upon detection of low voltage at the AC line, transistor Q104 turns on, turning on transistor Q103. With the turn-on of transistor Q103, the power
converter operates in an open-loop manner. The output voltage of the power supply follows the
Figure 12. Overvoltage protection circuit
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