TM 11-5820-921-40-1
output from the receiver-transmitter to the input of the first driver stage in the power amplifier, and con-
necting the output of the tube tank circuit to the input of the antenna coupler.
2-11. The fault switching function inhibits the transmit operation whenever one of the following mal-
functions occurs:
a.
The antenna coupler does not tune properly (to a preset VSWR within a preset time).
b.
The power amplifier tube tank circuit does not tune correctly.
c.
The system (power amplifier and antenna coupler) do not tune in less than 20 seconds.
d.
The high voltage overload is tripped.
The bandswitch motor is running.
e.
2-12. The Automatic Level Control (ALC) Detector (1A7A2) samples the RF power output level and
provides a proportional positive-going signal to the receiver-transmitter for controlling the Tx output of
the exciter. The ALC loop gain is preset to maintain a 400 watt 1 dB output with variations of gain in
the signal path.
2-14. The tuning control circuit provides the sensing, switching and sequencing of operation for proper
tuning of Tube Tank Circuit 1A8. The tuning control circuits consists of Band Control Assembly 1A6,
Bandswitch Motor 1A8A4B1, Tune Detector 1Al0, Servo Amplifier 1AS, Servo Motor 1A8A3B1, and
Tune Control 1A4.
2-15. The band control assembly changes the binary MHz frequency code (from the receiver-transmitter)
to eight signals that are used to control the position of the bandswitch (S 1 ) wiper through a position De-
coding Switch (S1-A) and Bandswitch Motor 1A8A4B 1. Bandswitching takes place automatically when
the frequency is changed on the receiver-transmitter.
2-16. Servo tuning takes place in two steps. The first step occurs when the frequency is changed (except
the 100 Hz digit) at the receiver-transmitter or when the system is switched "ON" and the power amplifier
has to be retuned. During the first step, the system control circuit (1) latches the power amplifier into
the TUNE mode, (2) energizes Tune Relay 1A7A1K2, which switches the dummy load in series with the
power amplifier output and modifies the ALC voltage to prevent overdriving the amplifier during the
second step of the TUNE cycle, and (3) enables the servo amplifier, allowing variable inductor 1A8A5L1
to drive to its starting position for the start of the servo-controlled fine tuning cycle. After the above
operations are completed, the second step of the system fine-tune cycle is initiated by momentarily de-
pressing the PTT (press-to-talk) switch on the system handset.
2-17. During the second step, the servo amplifier is forced to tune toward the proper variable coil
setting by signals from Tune Control Assembly 1A4, until the RF signal at the plate (PLATE SAMPLE) is
sufficiently high to permit servo tuning with phase difference signals (0 ERROR "A" and 0 ERROR "B")
from the phase detector. The servo loop rotates the coil sliding contact until an in-phase signal condition
of the CATHODE SAMPLE and PLATE SAMPLE signals indicates the correct setting of the coil. Servo
motor 1A8A3B1 then stops. When the PA is nearly tuned, the antenna coupler tune cycle begins. When
the antenna coupler is tuned, a COUPLER READY signal is sent to the power amplifier and AND-gated with
the PA READY signal to switch the radio set (AN/GRC-193 or AN/MRC-138) into OPERATE-( ready for
normal RX-TX operation) at the newly selected frequency.
Note
The TUNE cycle is interrupted if a fault occurs or the frequency is changed
(except the 100 Hz digit). It may be repeated by going through the frequency
change and the PTT (press-to-talk) keying sequence previously described.
