TM 55-4920-429-13
1-18. Threshold-Detector Circuit.
(See FO-2, FO-3, and FO-8.) The threshold-detector circuits contain two threshold-detector amplifiers
and two lamp-driver stages. These control the on/off condition of the STATUS A and STATUS B indi-
cators.
a. The STATUS A threshold detector amplifier U4-9 is biased to +4.6 volt dc at its inverting input
by voltage divider R81/R82. When the input signal is below 4.6-volt dc, the output at U4-8 is-12-volt
dc. When the input signal is above 4.6-volt dc, the output is +12-volt dc. When the output is negative,
transistor Q6 is cut off, and relay K1 is deenergized. The ground return of STATUS A indicator thru
normally-open contacts 3, 4 is open, and the indicator is out. When the output is positive, transistor Q6
is turned on, and relay K1 is energized to light the indicator. Diode CR11 protects transistor Q6 from
the voltage self-induced when relay K1 is released.
b. The STATUS B threshold circuit operates the same as the STATUS A circuit, except that a +0.76
volt dc bias voltage is applied to the non-inverting input of the amplifier at U4-12. Its output voltage at
U4-14 switches positive when input voltage drops below +0.76 volt dc.
c. The ground return path to the indicators thru relays K1 and K2 is open with the SEL 1/OFF/SEL
2 switch at OFF. This prevents spurious lighting of the indicators when the discrete signal monitor is
not in use. Buffer diodes CR13 and CR14 provide an alternate ground-return during self-test.
1-19. Discrete Signal Monitor Programs.
(See FO-3 and FO-8. Refer to table 1-2.)
a. Program No. 1. Selected for monitoring CYCLIC MAG BRAKE ON, CLTV MAG BRAKE ON,
SWIVEL LOCKED, and AFCS ENGAGE ON signal functions for +28 volt dc or open circuit. Program
1 uses the ac/dc amplifier, programmable-gain amplifier (gain –1.0), and threshold-detector.
(1) An active-low signal on this input line is applied to matrix inputs U7-11 and U7-1. This
causes electronic-switch sections U11-6, 8, 9 and U14-13, 1, 2 to turn on. It allows electronic-switch sec-
tions U11-5, 4, 3 and 14-6, 8, 9 to be activated.
(2) U11-8,9 contacts connect resistor R62 to logic ground. This provides a O volt dc input level to
the ac/dc amplifier when the +28 volt dc signal is inactive (open circuit). U1 1-473 contacts bypass the
ac to dc converter.
(3) U14-1, 2 contacts connect the output of the ac/dc amplifier to the programmable-gain am-
plifier thru resistor R78.
(4) U14-8, 9 contacts bypass the offset/inverter amplifier.
b. Program No. 2. Selected for monitoring L ROLL BEEP TRIM ON and R ROLL BEEP TRIM
ON signal functions for 13 volts ac or open circuit. Program 2 provides an ac mode signal for the dmm.
It uses the ac/dc amplifier, ac to dc converter, programmable-gain amplifier (gain -1.92), and
threshold-detector.
(1) An active-low signal is applied to matrix inputs U7-13, U8-12, and U10-1. This causes
electronic switch sections U11-6, 8, 9, U11-U13, 1, 2 and U12-6, 8, 9 to turn on. It also causes section
U11-5,4, 3 to turn off, and allows electronic-switch section U14-6, 8,9 to be activated. Transistor Q5 is
turned on to energize relay K3. Relay K3 connects an ac mode level to the METER SOURCE switch.
When the METER SOURCE switch is set to DISCRETE MON, the dmm displays the ac signal.
(2) U11-8,9 contacts connect resistor R62 to logic ground. This provides a O-volt dc input level to
the ac/dc amplifier when the 13 volt ac signal is inactive (open circuit).
(3) U11-4,3 contacts open. This opens the bypass around the ac to dc converter.
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