Reset Ssc Pwb - SAC SS-800AVS Manual

2.2.12 Reset SSC PWB
The Reset SSC PWB supplies a reset pulse to the Autoshutdown PWB
of an SSC series transmitter at 1 minute, 1 hour, 2 hours, and 3
hours following a shutdown of the NDB by the Autoshutdown PWB. If
the transmitter is still transmitting at one minute following a
reset pulse, the Reset SSC PWB is cleared to its initial state,
ready to supply reset pulses at 1 minute, 1 hour, 2 hours, and 3
hours following any new shutdown
If the transmitter stops transmitting within 1 minute following a
reset pulse, the Reset SSC PWB waits until the next available reset
time to apply a new reset pulse. If the transmitter has not
successfully returned to proper operation and remained operating
for at least one minute following the fourth reset pulse (3 hours
after the original shutdown), the Reset SSC PWB is latched into a
permanent shutdown mode which will remain until the circuits are
manually reset by pressing the reset button on the Reset SSC PWB
or power is momentarily removed.from the transmitter.
Shutdown of the SSC transmitters is accomplished by SCR Q9 on the
Autoshutdbwn PWB being forced into a conducting condition by mon-
itoring circuits on the Autoshutdown PWB. As Q9 begins conducting,
the voltage on the anode of Q9 will change rapidly from approximately
55 volts to approximately one volt. This negative-going signal
passes through C1 on the Reset SSC PWB to UIA on the Reset SSC PWB.
UIA and UlB form a set-reset flip-flop which is normally in an
initial "reset" state, with the UlB output logic level = 0. This
initial level maintains U2 in a reset state, with all outputs of
U2 being a logic 0. When UIA detects a shutdown signal, the UlA/UIB
flip-flop is changed to a "set" state with the UIB output logic
level = i. This will cause U2 to begin a binary counting of the
RII, RI2, C3-4 oscillator clock cycles. At one minute following
the shutdown, U2 pin 5 will become logic i. If U2 pins 4, 6, 14,
13, and 15 are all logic 0, UID will then have a logic 1 on each
of its inputs, causing the UlD output to go to a logic 0 and causing
Q2 to stop conducting° QI, which was not initially conducting,
will be latched into conduction with the Q1 anode being pulled down
to approximately 1 volt. C2, following shutdown was charged with
+I4V on the end attached to Q1 and 1 volt on the end attached to
TB2 (6). When the Q1 anode end of C2 is forced to 1 volt as Q1
begins conducting, the end of C2 which is attached through TB2 (6)
to Q9 of the Autoshutdown PWB will force Q9 to be momentarily
reverse-biased, stopping the Q9 conduction as soon as C2 is dis-
charged. One minute following the reset of Q9, u2 pin 4 in the
Reset SSC PWB will go high. If the transmitter has not shut down,
TB2 (6) will be high (approximately +55V) and the UIA/UIB flip-flop
SS-800AVS Southern Avionics
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