TM 11-5820-921-40-1
2-66. When any of the receive-to-transmit switching inhibit conditions exists, fault switching transistor
lA3Q4 is deactivated. Under normal conditions, with no faults present at pins 9, 11, and 20 of System
Control PWB Assembly 1A3, transistor 1A3Q4 is biased ON by the +26.5 Vdc line through resistor
1A3R18. This shorts pin 7 and (through diode 1A3CR9) pin 8 to ground, accomplishing two functions:
Grounding pin 7 of 1A3 turns on the ready (RDY) lamp 1A14A4DS2 on the front panel.
a.
Grounding pin 8 of 1A3, the RT FAULT line, disables the receive-transmit inhibit circuitry in the
b.
receiver-transmitter. Under this condition, normal receive-transmit switching is possible using the
PTT key on the handset or the CW key.
turning OFF the RDY lamp on the PA front panel. At the same time the RT FAULT line is also open
circuited, with +26.5 Vdc on pin 8 of 1A3 blocked by diode 1A3CR9. In the receiver-transmitter, this
open circuit is sensed as a FAULT, inhibiting the transmit keying so that the system cannot be switched
into the Transmit Mode until the malfunction has been cleared.
VARIABLE COIL WIPER TRAVEL LIMITING - (see figure 3-72). Cam operated microswitches
a.
1A8A3S1 and 1A8A3S2 stop the servo drive motor 1A8A3B 1 when the 1A8A5L1 coil wiper
approaches the extreme low or high inductance ends of coil 1A8A5 L1, respectively. This prevents
the wiper from running off the coil windings and damaging the assembly.
WARM-UP DELAY - (See the Dc-to-Dc Converter Description paragraph 2-129. ) The application
b.
of the +1600 Vdc (i.e., the start of the Dc-to-Dc Converter) to the plates of vacuum tubes 1A14V1
and 1A4V2 is dealyed 60 seconds 10 seconds after primary dc power (+26.5 Vdc) has been
switched ON. This is necessary to allow the tube filaments and cathodes to heat up and reach their
normal electron emission capability.
2-69. This section describes the operation of power amplifier circuits as they are used during tune cycle
operation, The rf signal amplifier circuits used in the power amplifier, with the exception of Tank Circuit
Module Assembly 1A8, are all of the wideband type and therefore require no tuning when operated over
the 2 to 30 MHz frequency range. However, π network tube Tank Circuit Module Assembly 1A8 must be
tuned at the desired operating frequency to match the tube plate resistance (RP), of about 1600 ohms, to
attenuate unwanted signals harmonically related to the carrier frequency. A Tune Cycle is initiated automat-
ically when the dc power is initially switched on or when the receiver-transmitter frequency setting is
changed, with the exception of the 100 Hz setting. The general sequence of operations during the Tune
Cycle is shown in figure 2-6 for the power amplifier and the antenna coupler. The Tune Cycle is divided
into the Preset and Wait Cycle and the Automatic Tune Cycle.
2-71. The Preset and Wait Cycle is automatically initiated by a >30 millisecond ground pulse (Tune
Start) from the receiver-transmitter. After Tune Start initiation, the power amplifier and the antenna
coupler tuning circuits are latched into the Tuning Mode of operation, thus inhibiting the normal Receiver-
Transmit (Rx-Tx) mode of operation. As shown in figure 2-6, during the T 1 and T6 time periods, power
amplifier and antenna coupler circuits and relays are switched and variable tuning elements returned to
their home (or starting) positions (e.g., variable coil 1A8A5L1 is driven to the maximum inductance
setting) in preparation for initiation of the Automatic Tune Cycle.
2-73. The Automatic Tune Cycle is initiated by momentarily depressing the PTT key or CW key on the
handset. This applies a ground to the KEYLINE signal line which goes to System Control PWB Assembly
1A3 in the PA, provided the warm-up time delay has timed out and all preprogrammed band switching has
