Bose Life CD5 Series I Service Manual page 14

THEORY OF OPERATION
There is one main serial data bus that controls source selection IC U101, volume control IC
U103, PLL frequency synthesizer U302, and EEPROM U401. The clock and data information
for all of these devices is sent out on U402, pins 5 and 7. However, U401, U302, U101 and
U103 each have their own chip select line. Data is sent to U302 whenever the tuner frequency
is changed. During the serial data transmission, U302, pin 3 is driven high. Data is sent to
U101 or U103 whenever a new source is selected or the volume is changed. At the completion
of this transmission, the STRB line (J9-5) is driven high briefly.
U401 is a nonvolatile EEPROM which is used for storing certain data such as tuner presets
and house codes. This data is protected from loss during a power outage. U401 communicates
with U402 over the main serial data bus. During communication to this chip, the chip select line
(U401, pin 1) is driven high.
There is another serial data bus between U402 and U505. These lines are labeled
CD_READY, CD_CLK, CD_ CMD, and CD_DATA on the schematic. The bus sends com-
mands (play, stop, etc.) to U505 and also sends track and time information to U402 so that it
may be displayed. This bus is constantly in use any time "CD" is selected as the source.
RR101 receives and demodulates commands from the RF remote control. R138, C130, C129,
R139, and D109 remove noise and shape the pulse. U106 squares up the pulse edges and
converts them to 5V logic levels. This signal is then fed to U402, pin 37. C401 prevents any
glitches at this pin.
In addition to the major functions mentioned above, U402 also performs several miscellaneous
tasks. The bipolar power supply for the audio circuits (+1OV/-12V) is turned on and off by
U402, pin 19 (power). Both supplies are turned on when this line is high (+5V), and off when it
is low. The unregulated supply is monitored by C414, R407, and R408. In the event of a power
failure, U402 will shut down the system gracefully. There are three independent muting cir-
cuits: Mute A, Mute B, and Power-on Mute. Mute A and Mute B are controllable from the
remote, and allows the A and B outputs to be controlled independently. The Power-on Mute is
used only during power-up (when the bipolar supply is turned on) to prevent pops and clicks.
When an external source is selected (AUX, VIDEO, or TAPE), the transport commands (FF,
FR, etc.) are passed through the serial data jack via Q401 and its associated circuitry.
Audio Circuits
There are two internal audio sources (CD and Tuner) and three external sources (AUX,
VIDEO, and TAPE). All of the sources are routed to U101. R101-106 and R201-206 provide
level matching for the different input sources. D101-106, D201-206, C101-103, and C201-203
provide static protection on the inputs. U101 selects 1 of the 5 input sources, and routes it to
its output on pins 5 and 9 (left), and pins 20 and 24 (right).
One half of U102 (pins 1-3 and 12-14) provides gain and buffering for the input signal. The
buffered output is routed to U103 and to the FIXED output on J103. U103 consists of two
sections. The first section attenuates the signal from 0 to 70dB in 10dB steps. The output of
the first section is buffered by the other half of U102 (pins 5-7 and 8-10) and is fed to the
second section. The second section attenuates the signal in 1dB steps. The two sections
together provide smooth attenuation from 0 to 80dB in 1dB steps.
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