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 TM 55-1905-219-14-9
4-64.1. TANK LEVEL INDICATION-RECEIVER-MAINTENANCE INSTRUCTIONS
(Continued).
(e) Blocking diode CR1 in the high alarm circuit applies the fluid level signal to gate bias
network R5-CRZ1. Filter C2-R4 shunts high frequency amplitude changes in the signal to ground in order to
prevent the alarm circuit from responding to changes caused by transient signals. Under normal (no alarm)
conditions, gate bias network R5-CRZ1 combines with threshold bias network RP2-R6-R7 to maintain threshold
detector Q1 in a cutoff state. Threshold select potentiometer RP2 in threshold bias network RP2-R6-R7 is
adjusted to cause threshold detector Q1 to conduct only when the input to gate bias network R5-CRZ1 exceeds
a value that corresponds to an alarm condition.
(f) When threshold detector Q1 does conduct (there is an excessively high fluid level in the
tank), switch Q2 is turned on. With switch Q2 turned on, a current path is completed for relay driver Q3-Q4 that
energized relay K1. Protective diode CR9, connected across the coil or relay K1, provides reverse EMF
suppression and protects relay driver Q3-Q4 transistors from reverse current flow.
(g) When relay K1 is energized, one set of transfer contacts connect the +17 volt dc supply
to changing network Rll-R12-R13-C3. Changing network Rll-R12-R13-C3 develops a positive going high alarm
pulse when the +17 volt dc level rises at its input. Blocking diode CR6 steers the high alarm pulse to relay
driver Q1-CR5 on the power supply and master alarm module PS( ). The PS1 module is associated with the
operation of control modules CM1 to CM7. Receipt of the high alarm pulse by relay driver S1-CR5 on the
power supply and master alarm module causes the relay driver to energize relays K1 and K2 on that module.
(h) When relay K1 on the power supply and master alarm module is energized, the A contact
set closes. Closing of these contacts completes a holding path that maintains relays K1 and K2 in the
energized state after the high alarm pulse delays. The B contacts of relay K1 connect the flashing voltage
developed by the power distribution circuits containing the flashing option to the normally open B contact of
relay K1 in the control module. With relay K1 in the control module energized, the flashing voltage is
connected to HIGH LEVEL lamp L( )-A on the receiver panel. The HIGH ALARM lamp on the receiver panel
flash on and off in response to this input.
(i) At the same time that the contacts of relay K1 in the power supply and master alarm
module enable the flashing of the alarm lamps, one set of contacts of energized relay K2 on that module con-
nect a ground that sounds an audible alarm on the receiver panel.
4-1331
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