Bose FREESPACE 4400, Service Manual

Page: 83 / 107

83
THEORY OF OPERATION
Each mic sense path consists of input protection circuitry and a resistor to the 8V supply fol-
lowed by a 330uF DC blocking capacitor, input protection diodes to the plus and minus 15V rails
and an op-amp serving as an input gain stage of 37dB. The output of the op-amp passes
through another DC blocking cap to ½ of a volume control IC. This output passes through
another capacitor into an op-amp gain stage followed by a low-pass filter, a high pass filter and
finally a precision rectifier/average circuit and off the board to one of the PIC A/D inputs. There
are two attenuators (U203, U211), two dual op amps (U206, U213) and four quad op-amps
(U207, U208, U214, U215) on the board.
5. Wall Plate Sense Board
The wall plate sense board contains the circuitry to scan up to four wall plates that are con-
nected through four RJ-45 jacks and to drive the LEDs on the wall plates. In addition there are
connectors and passive protection components for the music on hold output, the line 4 expan-
sion output and its associated control line, and the control input to allow an external device to
wake up the system.
Power is applied to the board even when the E-4 Series II is in standby and is regulated for the
board and the external wall plates by a voltage regulator (U401). The wall plate input lines are
scanned by three parallel in, serial out shift registers (U404, U405, U406) Resistors on the wall
plate sense board pull the input lines up, switch closures on the wall plates pull the inputs down.
Two serial in, parallel out shift registers (U402, U403) apply multiplexed drive to the LEDs on the
wall plates in the interim when the switches are not being scanned.
6. Flash Memory Board
The flash memory card is powered from the main DSP board. The board contains a 2 Mb, 5V
flash ROM (U701) and power supply bypass capacitor. This memory stores the system configu-
ration data as well as the host microprocessor operating program.