TM 11-5820-921-40-1
been completed without faults. Initiation of the Automatic Tune Cycle locks the receiver-transmitter into
the Tune mode of operation and starts fine tuning the power amplifier. The antenna coupler cannot start
its fine tune cycle yet, because it has no rf input. The power amplifier servo system is initially forced to
rotate the variable coil 1A8A5L1 contact toward the correct tune setting, until sufficient output is sensed
at the tube plates. When this occurs, the servo control is automatically transferred to the phase discrimina-
tor (e.g., Tune Detector Module Assembly 1Al0) for final tuning adjustments of the variable coil.
2-74. When the power amplifier rf output (input to the antenna coupler) reaches about 10 to 20 watts,
the antenna coupler starts to tune automatically. For a short period of time, the power amplifier and the
antenna coupler tune simultaneously. When the antenna coupler has tuned properly (within a set time
period and with a VSWR less than 2:1), an ANT CPLR READY signal (ground) is sent to the power
amplifier. This signal is AND-gated with a PA READY signal (ground), when the power amplifier fine
tuning cycle is completed, to unlatch the Tune Mode and return the system to the normal receive-transmit
operating condition. The system is then ready to operate at the selected frequency, in either receive or
transmit, as controlled by the operator.
2-76. The tube output stage Tank Circuit Module Assembly 1A8 is tuned in two independent operations:
(1) the preprogrammed bandswitching of the shunt elements and (2) the servo controlled adjustment of
2-77. PREPROGRAMMED BANDSWITCHING-DETAILED DESCRIPTION
2-78. As shown in
figure 3-72, the
proper pi-network input and output shunt reactance for tuning the
desired frequency are selected in eight bands by motor driven ganged rotary switches 1A8A1S1-B and
1A8A2S1-C. Switch wafer 1A8A4S1-A, ganged to 1A8A1S1-B and 1A8A2S1-C, is used to translate the
eight BAND CODE signals, from Band Control PWB Assembly 1A6, to an angular position of the switch
shaft. This is accomplished by using the "ground-seeking" method of position control, i.e., the motor
1A8A4B1) control circuit on Band Control PWB Assembly 1A6 is designed to stop the motor when the
1A8A4S1-A switch wiper hits the contact with a "grounded" BAND CODE signal. The "ground" BAND
CODE signal is present on only one of the eight lines from Band Control PWB Assembly 1A6 and the
grounded signal line corresponds to the selected operating frequency band. All of the other BAND CODE
signal lines, tied to the 1A8A4S1 -A switch contacts, are open circuited. The SWITCH WIPER signal from
1A8A4S1-A is fed back to Band Control PWB Assembly 1A6 as described in the following paragraphs.
2-80. Bandswitch wafer 1A8A4S1-A, as shown in figure 3-72, is in the band B position (3-4 MHz).
Therefore, the input BAND CODE signal on line 10 of connector 1A8P1 is "grounded" and all other BAND
CODE signal input lines are open. The ground signal, SWITCH WIPER, is returned to the Band Control
circuit through pin 6 of connector 1A8P1. As shown in figure 3-72, the grounded SWITCH WIPER input
at pin 10, of connector 1A6P1, biases transistor 1A6Q11 ON and 1A6Q9 OFF. This in turn cuts OFF
transistor 1A6Q10 and biases transistor 1A6Q12 into saturation. With transistor 1A6Q12 saturated, no
operating dc voltage (BAND MOTOR +) is applied to bandswitch motor 1A8A4B1, stopping the drive.
Since all switch wiper contacts, for 1A8A4S1-A, 1A8A1S1-B and 1A8A2S1-C are aligned, all three switches
stop on position 10. This selects capacitor 1A8A1C8 and C9 for the input capacitance circuit and inductor
1A8A2L2 and capacitors 1A8A2C17 and C18 for the output reactance circuit, thus tuning the pi-network
for the 3-4 MHz frequency range,
2-81. When the system operating frequency is changed, for example, to 15.5000 MHz, a new band is
selected, in this case band F, the 12 to 16 MHz band. The BAND CODE signal line B (switch position 10)
is open circuited and BAND CODE signal line F (switch position 2) is grounded. With the ground removed
from the SWITCH WIPER signal input to Band Control PWB Assembly 1A6, transistor 1A6Q 11 is cut OFF
while transistor 1A6Q9 is biased ON. This biases transistor 1A6Q10 ON and 1A6Q12 OFF, applying +19
Vdc (the BAND MOTOR (+) signal) through saturated transistor 1A6Q10 to Bandswitch Drive motor