Specifications

returns, the counter will reset to 00000000 respectively, and the
servo will return to the lower limit. The output of the Digital
Comparator will generate a Servo Fault that applies an RF Mute
to the transmitter when manually turned on again. The Servo
Fault in conjunction with the A/C Mains Fault will alert station
personnel to go the Frequency Change mode to reset the refer-
ence voltage for the selected frequency.
R.5.20.3 Servo Fault
If the Tune Fault input from the detector or the Digital Compa-
rator output is logic low, a Servo fault will be generated.
R.5.21
Load Servo Circuits
TheLoadServomotorB1inthe2A7ServoDriveAssemblyis
connected via a gear assembly to variable inductor L2. As the
servo is varied, the total inductance changes, and the output
matching is tuned to the proper value.
The Load Servo circuits are the same as the Tune circuits.
R.5.22
SERVO PAL
The servo logic circuitry is responsible for turning on the servo
circuits when a need to tune the transmitter occurs. To prevent
the servos from operating full time, the logic detects when a
request for servo action is required and powers the servo drive
circuits long enough to allow the tuning to occur. The servo
control circuits do not allow the reference to be changed unless
the transmitter is in the proper Frequency Change mode.
Refer to Figure R-4 SERVO Encoder for the following example.
R.5.22.1 Clock Shifter
When the Frequency Change input is logic high, the Clock output
will be 12.5Hz, and the active high Tune Raise/Lower and Load
Raise/Lower are enabled.
When the Low Power Tune of High Power Tune input is logic
high, the Clock output will be 3.1Hz, and the active high Tune
Raise/Lower and Load Raise/Lower are enabled.
When in the Normal Mode, the reference voltage can not be
changed.
R.5.22.2 Servo Supply Enable
When any of the raise/lower outputs is active high, the Timer
Trigger output will be logic high. This will activate retriggerable
one-shot timer U23. The TimeInput on U18-13 will be logic high
and if the Door Interlock is logic high, the Servo Enable will turn
on the servo supplies.
When all raise/lower outputs go to a logic low, 60 seconds later
the Servo Enable will go low and turn off the supplies.
Anytime an interlocked door is opened, the supplies will be shut
off.
R.5.23
Servo Supply Regulators
Regulators are used for the servo driver amplifier supply voltage
to allow steady state voltage to be available to the amplifier when
required. One minute after the last servo raise/lower movement,
the regulators are turned off.
R.5.23.1 +17/-15 Volt Servo Supplies
The + and - 20VDC supplies are connected to zener regulated
pass transistors Q3 and Q5 that provide the respective voltages.
An indicator will be illuminated GREEN when the supply is
activated and the voltage may be measured at the test point.
R.5.23.2 Servo supplies Relay Enable
When K1 is de-energized, the base voltage is shorted to ground
and the supplies are turned off. When Servo the Enable output
from U18-14 is logic high, Q4 is turned on, K1 is energized, and
the supplies are turned on.
R.5.24
Tune Servo Fault Detector
This circuitry will also compare the reference voltage with the
position voltage and produce a fault output if the two voltages
are not the same. This condition would occur if there was a
problem with the servo circuit such that the servo motor could
not produce the same position voltage as the reference. Examples
Figure R-3. Reference Counter PAL
888-2339-002 R-7
WARNING: Disconnect primary power prior to servicing.