Pioneer PDP-503CMX Product Training Manual page 40

Fan Drive & Audio Block
The fan drive circuit is located on the Audio MX assembly. Physical
location between the RGB and the X-Drive assembly's. In normal
operation the two fans are off. The Main Microcomputer on the RGB
board (IC5505) monitors temperature sensor three and when the
temperature reaches 35° Celsius the fan drive signal is sent to the
regulator (IC8701) and the fans start at low speed. As the
temperature increases the fan speed also increases at a linear rate
until full speed has been reached. The panel will go into shut down
if the temperature reaches 78° Celsius (green flashing LED). Fan
rotation detectors are located in each fan motor, if no rotation is
detected IC8703 will instruct the CPU to enter shut down mode.
A DC detector monitors the output from the 2 watt audio amplifier
IC8601. If DC is detected on the right or left channel outputs the
detector sends an Audio_NG signal to the RGB CPU and the unit
enters the shut down state. (green flashing LED)
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