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1-18.  Threshold-Detector  Circuit

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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. 1-7

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