Harris DX 25U, Техническое руководство

Страница: 268 / 274

fied by U10. A DC offset voltage for U10 is provided by U9.
The reference for U9 is obtained from zener diode CR7 and is
adjusted for 2.73 Volts at TP3 with CALIBRATE potentiometer
R49.
Voltage divider R60, R61, R62, R63, and R64 provides the
reference voltage for the non-inverting inputs of comparators
U11 and U3. A voltage proportional to the temperature of RF33
is supplied from the output of U10-1 to the inverting input of
comparators U11-4, U11-10 and U3-4. The temperature of
RF33 can also be monitored externally.
A voltage proportional to the temperature of RF34 is supplied
from the output of U10-7 to the inverting input of comparators
U11-6, U11-8 and U3-6.
Since the operation of both circuits is identical, only RF AMP
TEMP SENSOR 1 operation will be discussed.
If the RF33 temperature increases, the voltage to the inverting
inputs of U11 and U3 will increase. Due to the reference voltage
divider action, the output of U3-2 will go LOW while the
outputs of U11-2 and U11-13 will remain HIGH. When the
output of U3-2 goes LOW, TEMP CAUTION LED DS3 will
illuminate.
If the RF33 temperature continues to increase, the output of
U11-2 will go LOW. This will enable multivibrator U12 and
cause U12-12 to toggle between HIGH and LOW. Each time
U12-5 goes high, U4 completes one clock cycle and TEMP
INDUCED LOWER LED DS2 will illuminate, indicating
power reduction is in process.
At this point NAND gate U8-9 is held HIGH by U11-13 while
U8-10 toggles from HIGH to LOW. Each time U12-12 goes
LOW, output U8-8 will go HIGH. This HIGH (pulse) to the
Controller will slowly lower the transmitter power until the
proper operating temperature is reached.
If RF33 temperature continues to rise while gradual power fold-
back is taking place, U11-13 will go LOW. This will cause the
output of U8-8 to go HIGH. The constant HIGH signal to the
Controller will cause the transmitter to reduce power rapidly. As
the temperature lowers, the signal at J3-9 will change from a
continuous HIGH, to a pulsing HIGH-LOW, to a continuous LOW.
When the temperature falls below the TEMP INDUCED LOWER
set point, the continuous foldback will stop.
The OVER TEMP LED DS2 will remain ON until the FAULT
IND RESET switch S3 is depressed.
S.2.3
Air Flow Detection
The air flow detection consists of an air flow monitor and a
two-stage fault detection circuit.
S.2.3.1
Air Flow Monitor
The Air Flow Monitor consists of an ambient temperature
sensor, a heated temperature sensor, and a differential amplifier.
Both sensors are mounted at the lower edge of the board in the
airstream from the PA Power Supply section of the Driver
Compartment.
The output of ambient temperature sensor U16-2 is connected
to the inverting input of differential amplifier U18-2. The heated
temperature sensor U17 is thermally connected to R91, a 30
Figure S-3. Driver Encoder Block Diagram
Driver Interlock Function
DX-25U
S-4 888-2297-002
WARNING: Disconnect primary power prior to servicing.