Bose AM-30P Manual de servicio - Página 8

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2.3 Microcontroller (Refer to Figure 8 PCB Block Diagram)

Upon request (via the remote control or console) the CD-20 sends out serial data commands

to the AM-30P microcontroller to select mode of operation (2, 3, 5 channel), adjust volume

level (master, center, or surround), and control on/off mute. These customer settings are stored

in the EEPROM.

The microcontroller also acts as a compressor to automatically regulate the master volume

control IC during amplifier clipping (from CLIP DETECT).

The microcontroller also handles mute and unmute commands to the amplifier during power-

up and down of the system and monitors the status of several protection circuits in order to

shutdown the power supply in the event of a system malfunction such as DC offset at the

speaker outputs or abnormal power supply voltage as received by DC FAULT DETECT.

AC power is applied to the Main PCB and is controlled by TURN-ON/OFF and then exits to the

primary winding of the power transformer. Secondary AC voltage of the transformer is rectified

and filtered to DC on the Amplifier PCB and distributed to the power amplifier stages and the

Main PCB.

3. DETAILED THEORY OF OPERATION ( Refer to schematic sheets 1 and 3)

3.1 Power Supply

The 10 Volt DC turn-on signal from the CD-20 is received at pin 1 of DIN connector J5. It is

direct coupled to the base of Q601. When the AM-30P receives the "turn-on pulse" from the

CD-20, Q604 and Q601 conduct and provide a current path through opto-coupler U604. U604

is a opto-coupler triac driver that requires a minimum of 5mA to trigger. The opto-coupler

conducts through pin 4 and 6 to trigger the AC Mains triac Q605. The triac acts as a switch to

energize the primary winding of the power transformer.

The universal transformer (dual voltage) has dual primary windings that are wired in series for

240V operation, and in parallel for 120V operation. U.S. and European transformers have

single primary windings. Resistor R626 and capacitor C613 form a snubber network to sup-

press the high voltage transients that can occur across the triac.

Secondary voltages are fully developed within 80 milliseconds of turn-on. Three main power

supplies are energized: ±17VDC for the low voltage power amplifier rails; ±34VDC for the high

voltage (Class-G) power amplifier rails; and ±12VDC regulated for the low power signal pro-

cessing circuitry. +5.6VDC for the microcontroller is derived from the +12VDC supply through

resistor R620 and zener diode D600. When the 12V supply crosses the 10V threshold at

power-up (at about 40 milliseconds), diode D606 in the turn-on circuit conducts, and takes over

the task of supplying current to opto-coupler U604. At this point, the current draw requirement

from the CD-20 drops a factor of 100 to less than 100uA, and Q601 acts as an emitter follower.

THEORY OF OPERATION