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CIRCUIT DESCRIPTION
6.8.6 BBM FAN CONTROL
The voltage supply to the thermal measurement
circuit is taken from the 26.5V DC line into the BBM.
A precision temperature sensor U106 is mounted on
the PC board near a screw into the BBM bracket
which transfers heat to the sensor. The output of this
sensor is 10 mV/°C with a ±1% accuracy. This volt-
age is amplified by U105 with resistors R153/R154
setting the gain.
The output of this gain stage (pin 1) is fed to
another gain stage that performs as a comparator. The
output (pin 7) will go high when the heatsink tempera-
ture reaches 45°C and will go low when the tempera-
ture goes below 35°C. This output is sent to the power
supply through Q106 to turn the fan on and off.
March 1999
Part No. 001-2009-600
Figure 6-13 BACKPLANE CONNECTORS
6-32
6.9 CARD RACK
The card rack provides slots for up to eight logic
cards; including Main Processor Card (MPC), Main
Audio Card (MAC) and the Interface Alarm Card
(IAC). The IAC has a notch in the card to accommo-
date a pin in Slot-8 so that no other card can be
plugged into this slot.
On the back of the card rack is the Backplane
with plug-in connectors to the cards and cables to the
RF modules, Power Supply and External Connector
Board.
Refer to the component layout and schematic dia-
gram in Section 10 for more information on the
repeater backplane.
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