Am Auto Shutdown Pwb - SAC SS-800AVS Manual

2.2.9 AM Auto Shutdown PWB
The Auto Shutdown PWB incorporates detectors and logic circuits to
monitor output current and modulation.
The output RF current level is coupled by a sense coil to the PWB
input where it is detected by diode CR6. The demodulated signal
has both a DC component proportional to RF current level, and an
AC component proportional to the modulation level. The level of
these components is adjusted by R6. The two components are then
separated with the DC component going to U1 pin 3 and U2 pin i.
The DC signal applied to Ul pin 3 is compared with a reference level
established by R8 and R9. When the transmitter RF output current
drops below the reference at Ul pin 2, a low power level condition
will be signaled as Ul pin 1 goes to approximately 0.5 VDC. This
will force TP2 low, resulting in eventual transmitter shutdown.
The AC component of the detected signal, which may contain both
voice and tone identification signals, is fed through R6 to syn-
chronous detector U2 where the tone is separated out and detected.
U2, a transmission gate, turns on and off between pins 1 and 2
synchronously with a reference tone obtained through U1 B from the
AM Recycle PWB. The output, U2 pin 2, of the synchronous detector
is a DC voltage proportional to the modulating level of the iden-
tifying tone. The DC level from U2 pin 2 is compared at UlC pin
i0 with a carrier reference level at UIC pin 9. The presence of
a tone modulation percentage higher than that established by R22
is indicated by a +14 V signal on UlC pin 8. A low-tone level will
be indicated by a .5 V signal at Ul pin 8. With the transmitter
operating properly, Ul pin 1 will be approximately +14 V and U1 pin
8 will alternate with the code tone pulses between approximately
+14 V and 0.5 V. TP2 (Yellow) will alternate between +14 V and +6
V. The transitions in the TP2 voltages are coupled through C5 to
Q2. This turns on Q2 and prevents C6 from charging through RI2 and
RI3. If the RF current or tone modulation percentage drops below
the levels set by R6 and R22 respectively, TP2 will be held at +1.5
V or 6 V respectively, causing C6 to charge through resistors RI2
and RI3 firing unijunction transistor Q7 after a nominal 25 second
delay. If the tone is on continuously, C5 will charge, and Q6 will
not turn on, with the same end result. Q7 fires the SCR which then
applies a 0 volt signal to the base of Q4, turning Q4 and relay K1
off, and removing the 14 V supply from the preamplifier. This
completely turns off the RF output until the SCR is again reset by
opening switch Slo
Also included on the Auto Shutdown PWB is a 14 volt regulated power
supply.
SS-800AVS Southern Avionics
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