BOSE FREESPACE Schematic

FREESPACE™ BUSINESS

MUSIC SYSTEM

1993 Bose Corporation Service Manual

TABLE OF CONTENTS

1. SAFETY INFORMATION.......................................................................................... 2

2. SPECIFICATIONS.................................................................................................... 3

3. BLOCK DIAGRAM.................................................................................................... 6

4. THEORY OF OPERATION....................................................................................... 7

5. DISASSEMBLY/ASSEMBLY PROCEDURES......................................................... 13

6. TROUBLESHOOTING TIP FOR HUM AND RFI REMOVAL................................... 18

7. TEST PROCEDURES.............................................................................................. 19

8. PARTS LISTS AND EXPLODED VIEWS................................................................. 23

9. SCHEMATICS AND PCB LAYOUTS....................................................................... 47

CAUTION: THE FREESPACE™ BUSINESS MUSIC SYSTEM CONTAINS NO USER-SERVICEABLE PARTS. TO PREVENT WARRANTY INFRACTIONS, REFER SERVICING TO WARRANTY SERVICE STATIONS OR FACTORY SERVICE.

PROPRIETARY INFORMATION

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 BOSE SERVICE CENTER OR OWNER OF THE BOSE PRODUCT, AND SHALL NOT BE REPRODUCED OR USED FOR ANY OTHER PURPOSE.

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.) 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) 1492 (71). 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 the exposed metallic part has a return path to the chassis, the reading should be between 1 and 5.2 Megohms. When there is no return path to the chassis, the reading must be "infinite". 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.

SPECIFICATIONS

Dimensions Module: 9.1"H x 12.6"W x 20.1"D (23.1x32x51.05 cm)

Satellite: 3.75"H x 4.5"W x 4.5" D (9.5x11.4x11.4 cm)

Weight Module: 37 lbs. (16.8 kg.)

Satellite: 1.24 lbs. (0.56 kg.)

Driver 1-8" Woofer Complement 4-2.24" Twiddlers™

Finish Module: Black or white,scratch-resistant,satin-finished

vinyl

Satellites: Black or white,painted polymer finish

PERFORMANCE SPECIFICATIONS

Amplifier Topology Class G (High efficiency) Amplifier Power Woofer channel (single): 100 watts min. into 4Ω @100Hz

Satellite channels (each): 50 watts min. into 8Ω @ 1 kHz

Output Short Circuit Current Threshold: 5 ± 0.2 amperes Protection Duration: Infinite

Input Impedance System: Balanced: 13.0 kΩ

Unbalanced: 26.5 kΩ Satellites: 4Ω Woofer: 4Ω

Distortion Bass Channel @ 100 W,100 Hz <0.1% THD typical

L/R Channels @ 50 W,1 kHz <0.1% THD typical

Output Noise Bass Channel (80 kHz LPF,inputs shorted) <1 mV

L/R Channels (A weighted,inputs shorted) <300µVrms

Output Hum Bass Channel <500µVrms (unweighted) Compressor Attack time: 10 ± 5 ms

Release time: 80 ± 20 ms Attenuation Range: 12 dB

Channel @1kHz >40 dB Separation @10 kHz >30 dB

Dynamic 0-16 dB variable boost @ 55 Hz (level dependent) Equalization

Crossover Transition 180 Hz Frequency

Input Sensitivity 0.4 Vrms @1kHz produces rated

Turn-On Delay 80ms ± 40 ms

(Auto)

Turn-on 300 µVrms ± 100 µVrms Threshold

Turn-Off Delay 90 sec. ± 30 sec. (Auto)

PERFORMANCE SPECIFICATIONS (Continued)

power in L/R channel outputs (at full volume)

Acoustic Output 95 dB SPL in IEC standard room @ 50W Power Consump- 350 W max.

tion Standby Power 8 W max.

Consumption DC Offset Bass Channel,L/R Channels <70mVDC

(with audio inputs shorted)

Frequency Response Curves

Bass channel frequency response curve with tone controls at min.,

max., and flat positions (for 30 and 200 mVrms input level)

Figure 1. Bass Channel Frequency Response Curve

Left/right channel frequency response with tone controls at min.,

max.,and flat positions.

Figure 2. Left/Right Channel Frequency Response Curve

BLOCK DIAGRAM

MAIN

Figure 3. Main PCB (Amplifier) Block Diagram

THEORY OF OPERATION

The following discussion references the Main PCB Schematic and Block Diagram. The schematic is located in the back of this manual and the block diagram is shown in Figure 3.

GENERAL

The FreeSpace™business music system electronics module powers the FreeSpace loudspeaker system,which is a derivative of the AM-5 system, FreeSpace is meant to be used in conjunction with the customer's audio source.

The FreeSpace system offers the following features:

- Automatic turn-on/ turn-off (audio sense or signal from the customer's audio source)

- Automatic (Bose

patented) dynamic equalization

- Bi-amplification for better power distribution to speakers

- High Efficiency Class G Power Amp stages (100W + 50W +50W)

- Active equalization for smoother frequency response

- Amplifier short-circuit protection

- Bass/treble room compensation controls

- Differential input stage (to reject hum)

- Dynamic compressor to prevent amplifier output overload distortion

- Electronic remote volume

powered loudspeaker system. As a powered

BLOCK DIAGRAM DESCRIPTION

Refer to the block diagram and schematic mentioned above. For discussion of L/R channels, only the right channel is discussed. Left channel operation is identical.

1. Power Supply

Two power supplies are used in the FreeSpace system. One is a low power regulated ±15VDC supply used to power the signal processing circuitry, the other is the high voltage main power supply used to power the three Class G power amplifier stages. Two voltages are provided for class G operation, ±20VDC and ±40VDC. The regulated ±15VDC supply always remains energized as long as the unit is wired to an active mains voltage source. The main power amplifier supply is turned on and off by the audio sensing circuit (discussed elsewhere).

2. Differential Amplifier

Audio input is at barrier strip P5, which is located on the I/O PCB Assembly (see the schematic located in the back of this manual). This connects directly to the left and right differential amplifier buffers on the Main PCB. This stage neatly accomplishes several functions: provides rejection of common-mode interference (like hum) that might be picked up by the long audio cable between the FreeSpace™ system and the customer's audio source; provides input overload protection through diodes (D101-104); strips off radio frequency interference through capacitors C203 and 204; provides DC blockage by capacitors C201 and 202; and provides a 10 dB gain through op-amp U201 .

3. Compressor

U201 (pins 5,6,and 7), under normal audio conditions, operates as a conventional inverting opamp stage with a gain of 6 dB. However, when overload is present at the speaker outputs, transistor U205 (pins 5,6 and 7) receives a turn-on signal from the compressor detect stage. When this happens the transistor acts as a variable resistor to ground, and with resistor R207 forms a voltage divider network that attenuates the audio signal. The amount of attenuation and timing action is carefully controlled by the compressor detect stage (discussed later).

4. Audio Sense

The FreeSpace system remains in standby mode until audio is detected at its inputs. Audio sense occurs after the compressor gain stage where the left and right signals are summed through resistors R222 and R224. The signal is preamplified by inverting op-amp summer stage U203 (pins 1,2,and 3), and is presented at the pin 1 input of turn-on chip U204 (NJM2072). Diodes D109 and D110 are necessary to prevent overload of the turn-on chip at high volume levels. With audio input ≥0.3 mVrms, U204 pin 7 goes high, and through diode D111 turns on transistor Q401. This triggers relay K1 to energize the main power transformer. Turn-on (settling) time of the power amplifier section is approximately 80 ms.

Without audio input,U204 pin 7 will go low after approximately 90 seconds, and will de-energize relay K1 and the main supply. This turn-off time is set by capacitor C217 .

5. Tone /Electronic Volume Control

A separate tone control PCB attaches to the Main PCB through a seven conductor ribbon cable connector. Power is derived from the main board. The tone control is a standard Baxandall circuit with +/- 6 dB bass and treble controls. Volume control function is accomplished through the use of current-controlled amplifier IC U502 (CA3280). The gain of U502 is directly proportional to the current flowing into pins 3 and 6 from transistor Q502. This current is proportional to the voltage at the base of Q502. This voltage is adjusted by the external 10K potentiometer that indirectly attaches across connector J502 through the I/O PCB barrier strip terminal P5,pins 7 and 8. At minimum volume (0Ω between J502,pins 1 and 2),the voltage at the base of Q502 should be .36 ± .1 VDC.

At maximum volume (10 KΩ between J502,pins 1

and 2),the voltage at the base of Q502 should be 4.6 ± .1VDC. A schematic is located at the back of this manual.

6. High Frequency Equalization

High frequency equalization is achieved through four cascaded active EQ stages consisting of U101, U102, U103 and U104. The purpose of equalization is two-fold. (1) It provides very sharp low frequency roll-off below 180 Hz (the crossover frequency of the acoustics) to prevent overload of the L/R speakers; and (2) It provides finely tailored correction in the L/R speaker pass band so that overall frequency response of the system is correct.

7. Automatic Dynamic Loudness

In order to compensate for the ear’s loss of bass response at low listening levels, a Bose patented automatic loudness circuit is employed. This circuit automatically senses the volume level of the incoming audio signal, and properly adjusts the amount of low frequency bass boost. When the volume level is high, the frequency response of the circuit is flat. When the volume level is decreased (at the customer's audio source or through the electronic volume control), the low frequency gain is increased in the region between 50 and 150 Hz.

Circuit operation is as follows: After passing through the Tone Control PCB, audio at U201 and U202 (pin 7) is summed by resistors R101 and R131. The AC signal at pin 5 of level detect opamp U203 is negative -peak detected, and is presented across emitter resistor R236. The peak detected signal has a 4 second hold time, and a 4 sec / 10 dB release time constant which is developed within the feedback loop of U203 (pins 5,6 and 7). The voltage across R236 is converted to a current source through transistor Q203, and flows into pin 5 of transconductance amp U206. U206 and U207 (pins 1,2 and 3) form a variable gain stage (delta-G). Action is such that increased current into pin 5 of U206 causes gain of the stage to decrease.

The variable gain stage is followed by a 55 Hz, Q=2.3 band pass filter (U207 pins 5,6 and 7). The output of this stage is summed through resistor R247 with the flat response signal present at resistor R248. This creates a composite frequency response curve with varying levels of 55 Hz boost. The overall desired loudness contour response is visible at U208 pin 1.

8. Low Frequency Equalization

A single stage Sallen-Key 2nd order high-pass / low pass network (U208 pins 5,6 and 7) provides bass channel equalization and low frequency roll-off to prevent subsonic low frequency overload.

9. Power Amplifier

All three power amplifiers are of identical topology and use high efficiency Class G power amp stages. The L/R amplifier channels are both rated at 50W. The bass channel is 100W, and because of its higher power, uses higher current output transistors and slightly different component values. Otherwise,operation is the same.

The basic concept behind Class G operation is simple. It takes advantage of the fact that music, even at loud volume settings, has only brief moments where peak voltage from the amplifier is needed. Unfortunately, with conventional Class B amplifiers, this peak voltage is always supplied to the output transistors and most of the time is wasted as heat. The result is that larger and more expensive power supplies and heatsinks are needed (or power output transistors are operated hotter with less reliability).

With Class G however, the amplifier operates off of two different power supply voltages. When the music is low the amp runs off of the low voltage supply, thus conserving power. When musical peaks occur, the amplifier “intelligently” switches to the higher voltage supply. On average, this voltage “conservation” translates to efficiency almost double that of class B.

Detail operation of the amp is as follows: First, it is important to understand that a Class G amp is really just a class B amp with special circuitry added to it to allow switching between two different power supply voltages. In the case of the bass amplifier, this switching action is accomplished on positive peaks by transistors Q335, Q334 and Q333, and on negative peaks by transistors Q342,343, and Q344.

A positive signal at op-amp input U303 pin 3 causes op-amp supply current to flow into pin 7 through transistor Q336. Q336 is used simply to buffer the +40V high voltage amp supply down to a safe +15V for the op-amp. This current also flows through resistor R353 and diode D325. R353 and D325, together with driver transistor Q332 and R355, form a current mirror which multiplies the current through Q336 by the ratio R353/R355. This increased current flows through driver transistor Q332, through diode D330, and into the base of output transistor Q337. The hFE current gain of Q337 (about 40) provides final current amplification before reaching speaker output.

Class G switching action occurs as follows: the voltage at the anode of D330 tracks one diode drop above the output voltage of the amp. When this voltage exceeds the nominal +20V power supply rail transistor Q335 conducts. This in turn causes transistors Q334 and Q333 to turn on. When Q333 turns on, it acts as a switch and connects the +40V supply present at its emitter to the collector of output transistor Q337. When this occurs, the +20V supply, normally fed through diode D329, is turned off because the +40V causes the diode to be reversed biased. When the music voltage drops below +20V, the reverse happens. The +40V supply is turned off, and the +20V supply is turned on again.

The power amplifier topology is complementary. Therefore, operation of the amplifier on negative half cycles is identical to positive cycle operation except for component reference designations.

10. Short Circuit Protection

The left and right power amp stages have short circuit protection. This is necessary to prevent damage to the amplifier from possible customer misuse. For example, the customer could accidentally short the speaker cable wires that connect to the cube speakers. If this happens, all three amplifiers will momentarily mute for about 3-4 seconds, and will continue to mute until the abnormal condition is removed.

This is achieved with the use of a .12 Ω resistor (R313 for right channel) in series with each amplifier output. Under normal conditions, peak current to the 8 Ω speakers should never exceed 3.5 amperes. If current flow exceeds 4.5 amperes, then Q309 will conduct and trigger monostable multivibrator circuit U401 (pins 1,2 and 3). Pin 1 will go high (for about three seconds) and will trigger mute transistors Q409 and Q408 through diode D406. Turn-on of Q408 causes all three amplifiers to mute. Referring to the bass power amplifier, actual muting occurs by turning on transistors Q331 and Q340, which cause both driver transistors (Q332 and Q341) to turn off. When the driver transistors are off ,there can be no current flow through the output transistors.

11. DC Offset Protection

If any one of the three amplifier channels should fail, it is likely that a large DC voltage will occur at the output of that channel. If this occurs, the DC offset detect circuit will cause the FreeSpace™ main power supply to shut down. It will remain shut-down until the unit is disconnected from the power source. This provides a safe indication to the customer that the unit requires service.

Detailed operation is as follows: Outputs of each amplifier are summed by resistors R407,R408 and R409, and low pass filtered by capacitor C401. For a sustained DC offset ,capacitor C401 will charge either negative or positive. A positive offset will trigger transistor Q404. A negative offset will trigger transistor Q405. This in turn causes transistor Q403 to conduct, and latches Q404 permanently on. Transistor Q402 then shorts the base of Q401 to ground. Q401 is the transistor that controls turn-on of relay K1 that is used to energize power transformer T2. Therefore, when Q401 is disabled, main power is shut down.

12. Compressor Detect

The output of each amplifier is half-wave detected through diodes D409,D410 and D411 respectively. This voltage is divided down by resistors R422 and R423, and is presented at the emitter of transistor Q410. Power supply voltage is similarly divided down by R425 and R426, and is presented at the base of Q410. Diodes D419 and D420 simulate the saturation voltage drop that occurs in the power amp stage. Therefore, when audio output of any amplifier comes within two volts of the instantaneous power supply voltage, Q410 will conduct, which signals the compressor (limiter) to act. Current from Q410 charges hold capacitor C405 and release capacitor C422. Voltage at C405 is buffered by transistor U205 (pins 1,2 and 3) and decreased down to a logarithmic voltage by resistor R438 and transistor U205 (pins 12,13 and 14). This voltage is then presented to the base of compressor transistors U205 (pins 5,6 and 7) and U205 (pins 8,9 and 10) which are described in the compressor section.

The DC control voltage present across capacitor C405 is derived through the use of an improved hold and release characteristic. This results in lower compressor distortion. In conventional limiters this DC control voltage is derived by a simple series diode and parallel RC network to ground. Unfortunately, for low frequency overload, this results in substantial ripple voltage present on the DC gain control voltage. This results in the all too common problem of mid-range voices or instruments being badly “modulated” by low frequency overload. This problem is solved by having two separate capacitors, one that “holds” the smooth DC voltage just long enough to prevent low frequency ripple (approximately 40ms) and the other (C422) that quickly releases the hold cap (through diode D412) at the proper time to maintain overall 100ms release time.

NOTES FOR FUTURE REFERENCE

DISASSEMBLY/ ASSEMBL Y PROCEDURES

NOTE: Figure numbers of exploded views will

be referred to throughout these procedures. Certain parts will be referenced. The item number in each figure which corresponds to the part will be enclosed in parentheses-i.e.-rod cover (4).

1. Mounting Hardware Removal

NOTES: Refer to Figure 4 for Procedures 1 and 2

unless otherwise specified.

Older versions of the unit contain different mounting hardware than newer versions of the unit.

If you have an older version of the unit,the mounting hardware will consist of 2 threaded tie rods (not shown) and 4 acorn nuts with washers (not shown). Refer to step A. for disassembly instructions.

If you have a newer version of the unit,the mounting hardware will consist of 2 long screws (3) which thread into 2 pem nuts (part of the mounting straps (5). These screws have a hex head with a built-in washer. Some newer units also have 1 additional washer located on the end of each screw. Refer to step B. for disassembly instructions.

A. For older units: Remove 1 7/16" acorn nut (with washer) from each threaded tie rod. Another tool may be needed to hold the acorn nut on the other end of each rod to prevent the rod from turning.

Slide each tie rod through the holes located in the mounting straps. (The rod covers (4) will also slide off). Remove the mounting straps by pulling them carefully away from the unit.

2. Mounting Hardware Replacement

A. For older units: Position the 2 mounting straps

(5) so that their screw holes line up with the 2 wide "channels" (between the fins) on each edge of the heatsink (Figure 6,Item 1). See Figure 4 for proper strap orientation. Slide the 2 threaded tie rods through the holes located in the mounting straps. Make sure that the rod covers (4) are in place before securing the rods to the straps.

NOTE: 2 of the acorn nuts (not shown) should already be secured on the end of each tie rod.

B. Replace 2 of the 7/16" acorn nuts (with washers). Another tool may be required to hold the other acorn nut on each rod.

C. For newer units: Position the 2 mounting straps so that their screw holes line up with the 2 wide "channels" (between the fins) on the edges of the heatsink. See Figure 4 for proper strap orientation. Some newer units have washers that are located on each screw. Make sure that the washers are in place before sliding the rod covers on. Make sure that the rod covers are in place before securing the screws to the straps. Thread the 2 long screws (3) into each pem nut (part of the mounting straps). Make sure that both long screws are securely fastened.

3. Junction Box Cover Removal

B. For newer units: Unscrew the 2 long screws

from the 2 pem nuts (part of the mounting straps). Pull the screws through the holes located in the mounting straps. (The rod covers will also slide off). On newer units there may also be 1 washer located on the end of each screw. It is not necessary to remove either washer. Remove the straps by pulling them carefully away from the unit.

NOTE: Refer to Figure 4 for Procedures 3 and 4 unless otherwise specified.

A. Loosen the 4 screws (9) which secure the junction box cover (8). Slide the cover so that the screws are released from the smaller section of the screw hole. Lift the cover away from the junction box (7).

4. Junction Box Cover Replacement

7. Tone Control PCB Assembly Removal

A. Position the junction box cover (8) over the 4

screws (9) still secured to the junction box (7). Lower the cover into place.

B. Slide the cover until all 4 screws are engaged with the narrow slot in each screw hole. Secure the cover to the junction box with the 4 screws.

5. Panel Assembly Removal

A. Remove the mounting hardware (Procedure

1) and the junction box cover (Procedure 3).

NOTE: Refer to Figure 4 for Procedures 5 and 6 unless otherwise specified.

B. Remove 14 screws (2) which secure the panel assembly (1) to the unit.

NOTE: If the panel assembly does not need to

be completely removed,then it may not be necessary to unhook the connectors as directed in step C.

NOTE: Refer to Figure 4 for Procedures 7 and 8 unless otherwise specified.

A. Remove the mounting hardware (Procedure

1), junction box cover (Procedure 3),and panel assembly (Procedure 5).

B. Remove the 2 control knobs (21) using either

your fingers or a flat blade screwdriver.

C. Remove the 2 3/8" hex nuts (22) and washers (23) which secure the PCB (24) to the unit.

D. Push the control knob shafts through the holes in the unit and carefully lift the PCB away from the unit.

8. Tone Control PCB Assembly Replacement

A. Slide the control knob shafts through the holes

in the unit. Make sure that the PCB is solder side up.

C. Unhook connectors J303 (woofer output) and J101 from the Main PCB Assembly (Figure 6, Item 13) and connector J502 from the Tone Control PCB Assembly (24).

D. Carefully lift the panel assembly away from

the unit.

6. Panel Assembly Replacement

A. Lower the panel assembly (1) into position.

B. Plug in connectors J303 (woofer output) and J101 to the Main PCB Assembly (Figure 6,Item

13) and connector J502 to the Tone Control PCB

Assembly (24) if they were previously disconnected.

C. Secure the panel assembly to the unit with 14 screws (2).

D. If no further troubleshooting is required,reinstall the junction box cover (Procedure 4) and the mounting hardware (Procedure 2).

B. Secure the PCB (24) to the unit with 2 3/8" hex nuts (22) and washers (23).

C. Replace the 2 control knobs (21) by pushing in gently towards the unit. The knobs are keyed and will only fit one way.

D. If no further troubleshooting is required,reinstall the panel assembly (Procedure 6) ,the junction box cover (Procedure 4) ,and the mounting hardware (Procedure 2).

9. Woofer Removal

NOTE: Refer to Figure 4 for Procedures 9 and 10 unless otherwise specified.

A. Remove the mounting hardware (Procedure

1),junction box cover (Procedure 3),and the panel assembly (Procedure 5).

B. Remove the 6 screws (2) which secure the

woofer (19) to the unit, and lift the woofer carefully away from the unit.

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