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B.  On GJ B-2501 thru B-2503, the control panel has two indicator lights, a selector switch for Selcal-2, and a RESET button. Selcal-1 is connected to VHF-1 system and Selcal-2 can be switched between the HF and VHF-2 systems. The RESET button deactivates both decoders simultaneously.
C.  On GJ ALL EXCEPT B-2501 thru B-2503 and B-2509 thru B-2510, the control panel has two indicator lights, PRESS TO RESET buttons, selector switches, and a MARKER selector switch. Each selcal decoder can be switched between the HF-1, HF-2, VHF-1, and VHF-2 systems. The PRESS TO RESET buttons allow each decoder to be deactivated individuality.
3.  Dual Selcal Module
A.  The dual selcal module consists of two separate decoder units in a common case. One unit is connected to HF No. 1 and VHF No. 1 system receivers; the other is connected to HF No. 2 and VHF No. 2 system receivers. Each unit consists of an audio amplifier, dc amplifier, control relays and vibrasponder resonant reed assemblies (Fig. 2).
B.  The decoder unit audio amplifier consists primarily of two stages of amplification followed by push-pull power amplifier. The power amplifier is capacitively coupled to the coils of 12 vibrasponders (resonant reed assemblies). When a tone signal is received, the signal is amplified and fed to the coils of the vibrasponders. An AGC circuit is provided to maintain proper drive for the vibrasponders.
C.  The dc amplifier consists of the vibrasponder contactors, filter network and an amplifier. When the proper combination of tones is impressed on the vibrasponders, the vibrasponder reeds vibrate and complete the circuit through the reed contacts to the filter network and amplifier. When the amplifier is operating a circuit is completed through a decoder relay. When the decoder relay is closed, a voltage is applied to the coil of the relay causing it to effectively latch closed.
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D.  With the decoder relay energized, three sets of contacts are actuated. One set causes the relay to latch closed as described. A second set of contacts applies power to an oscillator relay circuit. The third set of contacts complete power to the selcal indicator light on the selcal control panel. The oscillator relay receives power through the decoder relay and thus becomes energized. When energized, two sets of contacts are operated. One set closes a circuit to the selcal system chime. The second set of contacts grounds the capacitor and transistor oscillator circuit connected to the relay coil. Since the relay is in a time delay circuit, it will remain closed momentarily and then open. After it opens, the capacitor will charge and the relay coil will again receive power. The close-open cycle will repeat until the decoder is reset.
E.  A reset circuit is completed whenever the toggle switch on the selcal panel is held to RESET. When the reset circuit is operated, power to the decoder relay is grounded causing the decoder relay to open. When the decoder relay opens, the oscillator relay is de-energized and the system is reset.
4.  Aural Warning Unit
A.  An electrical chime is in the aural warning unit with other aural warning components. The chime will emit a two-tone signal each time the oscillator relay of a decoder unit closes. The chime is connected in parallel with decoder unit No. 1 and decoder unit No. 2 oscillator relays; therefore, it will operate whenever either decoder unit receives the proper tone combination through the respective VHF communication receiver (Ref Chapter 31, Aural Warning and Call Devices).
5.  Operation
A.  Operation of the selcal systems is dependent upon the fact that each airplane is assigned a particular four-tone audio combination for identification purposes. A decoder is installed in the airplane to detect only the identification tones assigned to the airplane. Whenever a ground station attempts to communicate with a particular aircraft on VHF, they will transmit the proper identification tone combination for that airplane. Provided the proper frequency is selected on the HF or VHF system for the area in which the pilots expect to receive communications, the airplane selcal decoder will detect the tones and operate a chime and a green selcal alert light to indicate to the crewmembers that communication on VHF is desired from a ground station.
B.  The selcal system may be reset by pressing the RESET button located on the Selcal control panel. The selcal light will then go out and the chimes will stop.
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