Bose CD-20 Service Manual page 10

THEORY OF OPERATION
3.0 Control Electronics (continued)
The micro monitors messages from the RF remote control with input port P30 (RF_DATA). RF
receiver module RR100 demodulates the signals received on the CD-20's remote antenna (the
digital ground conductor in the audio output cables) and converts it into a low-level AC signal.
This signal is filtered and converted to TTL levels by one-half of comparator U402 before it is
fed to the micro.
Key closures are detected by means of the micro's on-chip A/D converter. When a console key
is pressed, the voltage divider formed by R419 and R420 is changed by the switch being
pushed. Specifically, a new resistor will be shunted across either R419 or R420. This increases
or decreases the voltage at the micro sense lines, KEY_IN1, KEY_IN2 or KEY_IN3 (note that
the keys are divided into 3 groups of 4 keys each). The micro examines these sense lines and
determines which key is pressed based on the voltage it measures. If the micro measures a
voltage of about 2.5 volts, it knows no key is pressed within that particular group of 4.
The AC power status is monitored by the micro using port P10, pin 1 of U400 (PFAIL/). When
the AC input voltage to the wall transformer falls, its AC output voltage falls below the nominal
12V. When the voltage falls below a certain point (about 88 VAC in FM mode, about 93 VAC in
CD mode), the DETECT line from the power supply (which monitors the raw DC input voltage
to the +5V regulator, U1) causes comparator U402's output to go low. This warns the micro
that the power is failing, and it responds by shutting the system down (muting the speaker
outputs, blanking the display, etc.). When AC power is OK, U402's output (PFAIL/) is high.
4.0 Audio Electronics (Schematic Diagram Sheet 2)
The CD-20 has 2 internal audio sources (CD and tuner) and four external sources (Tape, Aux,
Video 1 and Video 2). All of the sources get routed through audio matrix chip U101. This IC
controls which zone(s) the sources will be connected to. The chip has three left and right
outputs. One is used for Zone 1, two for Zone 2, and three for the tape outputs. The micro
instructs the chip to connect each of the outputs to the correct input by using the clock and
data lines as described above. If a zone is not active (or muted), the audio matrix chip may
connect that output to an internal reference rather than one of the inputs for extra muting. The
matrix has internal buffer amps, allowing its outputs to be used directly as the Zone 1 and
Zone 2 fixed audio outputs.
Output #3 from U101 (pins 16 and 17) is used for the CD-20's left and right Tape output. The
micro sees to it that this output nominally is connected to the same input source as the Zone 1
output. This means that any tape deck connected to the Tape outputs would record the same
source that is being played on Zone 1. However, if the Tape input is selected for Zone 1, the
Tape outputs are muted by the micro to prevent a feedback loop.
The first two matrix chip outputs (pins 12 through 15), as described, are then fed to Zone 1 and
Zone 2 volume control ICs (U100 and U102 respectively). Each IC contains the volume-control
cell, output-buffer cell and output mute cell for that particular zone. Volume settings, mute
settings and output-buffer switching are all controlled by the micro using the ACLOCK,
ADATA_1 (for Zone1) and ADATA_2 (for Zone2) lines. The final LR audio outputs (called the
"adjustable outputs," after volume control and muting) are pins 24 and 25. From here, the
audio passes through DC blocking capacitors C114, C214, C124 and C224 (the volume control
ICs run on a single +10V supply, so the audio has a 5 Vdc offset component) before running to
output circular DIN connectors J101A and J101B.
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