18
The thermal detect line feeds into pin 9 of the microcontroller. This is the output of a compara-
tor that incorporates a thermistor to monitor the temperature of the Amplifier PCB near the
heat sink. At room temperature (25
°
C) the thermistor resistance is 22k
Ω
. It forms a voltage
divider with resistor array R608 (pins 3-4), and a mirrored voltage divider exists R608 (pins 1-
2) with R607 on the inverting input of the comparator (U600 pins 1, 6 and 7). At 80
°
C (which is
the maximum U.L. temperature of the outside cover material), the thermistor drops in value to
1.0k
Ω
and triggers the comparator low. In response to this, the microcontroller issues a mute
command to all the amplifiers. With all amplifiers muted, power dissipation of the system is
less than 12 watts (including the transformer) and the temperature should cool enough within a
couple minutes to trip the comparator high, at which time the microcontroller unmutes the
amplifiers and normal operation resumes.
The offset line feeds into pin 8 of the microcontroller. This is the output of a comparator that
monitors the DC offset voltage of all five of the power amplifiers. Each amplifier output is
connected to the inverting input of comparator U600, pin 8 through a 1M
Ω
resistor. This node
has a normal bias of 2.0VDC. U600 is configured as a window comparator that will trigger
below 1V or above 3V. If any one of the amplifiers experiences a DC offset voltage exceeding
6 volts DC, the comparator will trip low. In response to this, the microcontroller will power-off
the system. At power-up there is a one second delay in acknowledging the status of the offset
line, due to the time required to charge up capacitor C605 to its bias voltage.
The power fail line feeds into pin 6 of the microcontroller. This is the output of a comparator
that monitors the DC power supply rails. Several power supplies tie into this comparator. The
±
12VDC lines connect to the inverting input of U600, pin 4 through R634 and R635. The
voltage sum at this point is normally 5.6V. The
±
17VDC supplies are summed through resistors
R16 and R17. Under normal conditions this voltage is zero on the Amplifier PCB. This voltage
sum is brought over to the Main PCB through connector J2, pin 13 (protect). There it is
summed through resistor R602 to create a bias of 6VDC at the middle of D601. The balancing
of this circuit is such that if any one or more of the power supply voltages fail or become
heavily loaded and suffer an abnormal voltage drop, the comparator will trip low. In response
to this, the microcontroller will power off the system.
THEORY OF OPERATION
