TM  55-4920-402-13&P 2-7. Strobex Modes of Operation. The mode of operation depends on the rotor being balanced (main or tail), the number of Interrupters, number of rotor blades, and speed of  rotors.  The  following  paragraphs  discuss  the  criteria for  mode  selection. a. Mode A. Mode A is used for main rotor track, where the helicopter is fitted with one interrupter-per- blade, and for tail rotor balance. (1) In this mode, the Strobex is a slave only flash- ing once in response to each external command. The elec- trical  commands  come  from  the  Magnetic  Pickup  secured to the fixed swashplate, One interrupter-per-blade of mag- netic  material  is  attached  to  the  rot  sting  swashplate,  and each  passage  of  an  Interrupter,  in  proximity  to  the  Mag- netic  Pickup,  generates  an  electrical  pulse  to  trigger  the Strobex. When the Strobex is directed at the tip path, both  Tip  Targets  will  be  seen,  superimposed,  at  two  points in azimuth. The targets will be seen front and back on a two-blade  helicopter. (2) This mode is also used for tail rotor balancing where  the  Balancer,  in  response  to  the  Accelerometer signal, provides the trigger pulse to the Strobex. The clock angle  observed,  together  with  vibration  amplitude  from  the Balancer  meter,  defines  a  point  on  the  Balance  Chart  for the rotor, which in turn, indicates corrective action. When in Mode A, the Strobex operates at its lower intensity. b. Mode B. Mode B is used for main rotor track when the helicopter is fitted with one interrupter-per-revolution, or when a brighter light is required than provided by Mode A. (1) In Mode B, the Strobex operates in a locking oscillator mode. The rotating swashplate is fitted with only one  Interrupter,  thus  delivering  a  one-per-revolution  pulse (not one-per-blade) to the Strobex. (2) The oscillator is set by the ten-turn dial to flash once-per-blade. (The ten-turn dial is set to equal, or less  than,  rotor  RPM  times  number  of  blades  times  0.4). The  one-per-revolution  signal  from  the  Magnetic  Pickup locks (or resets) the oscillator each revolution. The Tip Targets  will  be  seen  superimposed,  at  as  many  azimuth positions  as  there  are  blades.  However,  if  the  RPM  dial is set to a slightly lower rate, the blades will still be seen at the same position, but will be spread uniformly for each  resolution.  The  locking  pulse  always  keeps  the  tar- get in the same azimuth position. This is typically used on five, six, or seven blade helicopters where superimposed targets  are  difficult  to  distinguish. (3) If the brighter light is desired when working the  smaller  helicopters,  which  are  generally  fitted  with  one interrupter-per-blade,  Mode  B  is  used.  Simply  set  the flash rate to the blade rate, times 0.4, or slightly less, and the  remaining  operation  is  identical  to  Mode  A  except the light is about four times greater. The tip pattern can- not be spread in Mode A because the Interrupters restart the  oscillator  each  time  a  blade  passes,  and  the  oscillator never fires the Strobex. By setting the ten-turn dial to double the blade rate (use a multiplier of 0.8 instead of 0.4), the tip pattern can be seen at twice as many points in azimuth. When in Mode B, the Strobex operates at its  higher  intensity. c. Mode C. Mode  C  is  used  for  speed  RPM  measure- ment.  activates  the  free-running  oscillator.  and  discon- nects  any  external  signals.  Flash  rate  is  controlled  only by the ten-turn dial, which reads directly in RPM, to an accuracy of about ±2%. Its maximum flash rate is 1000 per minute for measuring RPM in the range of 100 to 1000 RPM. In Mode C, the Strobex operates at its higher intensity. d. Mode D. Mode D is used for tail rotor tracking and speed  (RPM)  measurement. (1) This is a free-running oscillator as in Mode C. However, the dial reading is multiplied by ten generating a flash  rate  to  10,000  flashes  per  minute. (2)  Since  no  Magnetic  Pickup  is  attached  to  the tail  rotor,  tracking  is  accomplished  by  the  free-running oscillator. For two and four blade tail rotors, the RPM is set to four times rotor rate and fine adjusted to cause the single Tip Target to appear as a stopped image of four. The rotor disc is viewed edge-on and the reflective Tip Targets are viewed for track. When in Mode D, the Strobex oper- ates  at  its  lower  intensity. e. Mode E. Mode E is used for tracking propellers. Mode E activates a locking oscillator exactly as Mode B, except  it  operates  over  the  higher  flash  rate  range  to 10,000  flashes  per  minute.  It  is  used  primarily  t  o  track airplane  propellers  where  the  flash  rate  is  set  to  three times  propeller  rate  (for  three  blades),  and  the  Magnetic Pickup, pulsed by a single Interrupter, locks the oscil- lator  at  one-per-revolution.  When  in  Mode  E,  the  Stro- bex  operates  at  its  lower  intensity. 2-8. Balance and Tracking Charts. The Balance Charts are computers that are used to plot the measurement of the vibration  amplitude  and  clock  angle  and  determine  the weights required to balance the rotor. Figure 2-6 illus- trates a typical Balance Chart. Balance Charts consists of the  following. a.  A  clock  face  (12  radial  lines)  representive  clock  angle. b. A set of ten concentric circles, representing IPS, drawn over the clock face, with zero at the center and 1.0 at the outside. c. A graph over the clock face and IPS circles to indi- cate amount, direction, and location of change required to weight, sweep, pitch link, tab, etc., to-correct the problem. The intersection of IPS circles and clock angle lines define a point on the chart. From this point, lines to the axes of a  graph  show  the  amount  and  location  of  weights  (or  main rotor  sweep)  required  to  accomplish  balance,  or  pitch  link change  to  accomplish  track. 2-9. Reading the Charts. Reading the charts is about the same, whatever the geometry of the rotor represented. 2-8