Bose Lifestyle PS 28 Troubleshooting Manual page 20

Assuming that the POST is successful, the Sharc sets up the registers for the S/PDIF receiver
(U2), codec (U401), and Compressed Audio Decoder (U103) to function as needed. The
PSC_PWM control is set to its minimum value, the amplifier Mute control lines LRCB_MUTE and
LSRS_MUTE are asserted (low).
After the POST has been successful and the program code is running, the DSP is initially in the
SmartSpeaker "Off" state and polls the serial communications input for valid SmartSpeaker com-
mands. It flashes the green LED briefly once every 5 seconds to signal that it is on and in the "Off"
state.
2.19 Power Down Sequence
Since the bass module never really "shuts off" (it only puts the audio power amplifiers in Standby
mode when sent the SmartSpeaker "Off" command), the only thing that happens when an "Off"
command is received is to start sending all zero data to the D-to-A converters and mute the power
amplifiers.
When power is lost, the DSP is eventually put into Reset by the master reset IC U602 when the
3.3V supply drops below 3.08V. When power comes back, the process described in "Power On
and Boot Up Sequence of Events" will occur.
3. Amplifier PCB
The amplifier board consists of three integrated dual amplifier ICs that provide the six channels of
required amplification. This IC, the TDA7265 manufactured by ST Microelectronics, has thermal
and short-circuit protection built-in. The five satellite channels directly use the output of these
amplifier chips, whereas the bass channel, due to its need to deliver high current, is buffered (in
complimentary emitter-follower configuration) by power transistors Q601 and Q602. These transis-
tors provide the necessary current gain required to drive the low impedance bass speaker.
All six amplifier channels have a voltage gain factor of 3.5 (11.0 dB) with flat frequency response.
3.1 Mute Circuit
The amplifier chips operate in standby mode (no output and low power consumption) until the
mute pin (pin 5) is pulled 6 volts below the +V power supply (pin 3). This is accomplished by
current source transistor Q401 for the surround amp IC (U1) and Q603 for all other channels (U2
and U3). These current sources are controlled by the DSP board mute lines LSRS_mute and
LRCB_mute. These signals are either 0 VDC (muted) or 3.3VDC (unmuted).
If the positive supply rail falls below 9 volts the amplifier chips will mute. This feature is primarily
used to prevent turn-on or turnoff pop. Under this condition, diode ZR2 and transistor Q604 turnoff
and this in-turn causes unmute transistors Q401 and Q603 to turnoff.
3.2 DC Offset Circuit
All six amplifier channels are summed together through 100K resistors (R105, R506, R605,
R305,R405,R609) and low pass filtered by capacitor C1 to create an average DC voltage that is
normally 0 VDC. If a fault condition occurs, where any one or more amps experience a sustained
DC shift of more than 5 volts, transistor Q1 (for a positive fault) or Q6/Q7 (for a negative fault) will
turn-on and pull-down J2 pin 24 voltage from 5 Volts to 0 Volts. This signals the power supply
board that there is a fault condition.
Theory of Operation
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PS18/28/35 Troubleshooting Guide