TM 55-1510-220-10 adjust both propeller controls at the same time.  This will keep the right governor setting within the limiting range of the left propeller.  If the synchrophaser is on but is unable to   adjust   to   the   right   propeller   to   match   the   left,   the actuator  has  reached  the  end  of  its  travel.    To  recenter, turn  the  switch  off,  synchronize  the  propellers  manually, and turn the switch back on. b. Control Box.  The control box converts any pulse rate   differences   into   correction   commands,   which   are transmitted to a stepping type actuator motor mounted on the  right  engine  cowl  forward  support  ring.    The  motor then  trims  the  right  propeller  governor  through  a  flexible shaft   and   trimmer   assembly   to   exactly   match   the   left propeller.    The  trimmer,  installed  between  the  governor control  arm  and  the  control  cable,  screws  in  or  out  to adjust the governor while leaving the control lever setting constant.      A   toggle   switch   installed   adjacent   to   the synchrophaser turns the system on.  With the switch off, the  actuator  automatically  runs  to  the  center  of  its  range of travel before stopping to assure normal function when used   again.      To   operate   the   system,   synchronize   the propeller in the normal manner and turn the synchrophaser  on.    The  system  is  designed  for  in-flight operations  and  is  placarded  to  be  off  for  take-off  and landing.    Therefore,  with  the  system  on  and  the  landing gear  extended,  the  master  caution  lights  will  illuminate and   a   yellow   light   on   the   caution/advisory   annunciator panel, PROP SYNC ON, will illuminate. c. Synchroscope.        The    propeller    synchroscope, provides an indication of synchronization of the propellers.    If  the  right  propeller  is  operating  at  a  higher RPM than the left, the face of the synchroscope, a black and   white   cross   pattern,   spins   in   a   clockwise   rotation. Left, or counterclockwise, rotation indicates a higher RPM of  the  left  propeller.    This  instrument  aids  the  pilot  in obtaining   complete   synchronization   of   propellers.      The system    is    protected    by    a    5-ampere    circuit    breaker placarded  PROP  SYNC,  located  on  the  overhead  circuit breaker panel (fig.  2-27). 2-48.  PROPELLER LEVERS. Two propeller levers on the control pedestal (fig.  2- 8),   placarded   PROP,   are   used   to   regulate   propeller speeds.    Each  lever  controls  a  primary  governor,  which acts    to    regulate    propeller    speeds    within    the    normal operation range.  The full forward position of the levers is placarded   TAKEOFF,   LANDING   AND   REVERSE   and also  HIGH  RPM.    The  full  aft  position  of  the  levers  is placarded  FEATHER.    When  a  lever  is  placed  at  HIGH RPM, the propeller may attain a static RPM of 2,000 depending upon power lever position.  As a lever    is    moved    aft,    passing    through    the    propeller governing  range,  but  stopping  at  the  feathering  detent, propeller RPM will correspondingly decrease to the lowest limit.    Moving  a  propeller  lever  aft  past  the  detent  into FEATHER will feather the propeller. 2-49.  PROPELLER REVERSING. CAUTION Do  not  move  the  power  levers  into reverse   range   without   the   engine running.      Damage   to   the   reverse linkage mechanisms will occur. CAUTION Propeller  reversing  on  unimproved surfaces   should   be   accomplished carefully to prevent propeller erosion  from  reversed  airflow  and, in    dusty    conditions,    to    prevent obscuring the operator's vision. CAUTION To  prevent  an  assymetrical  thrust condition,  propeller  levers  must  be in    HIGH    RPM    position    prior    to propeller reversing. The   propeller   blade   angle   may   be   reversed   to shorten landing roll.  To reverse, propeller levers must be positioned  at  HIGH  RPM  (full  forward),  and  the  power levers  are  lifted  up  to  pass  over  the  IDLE  detent,  then pulled  aft  into  REVERSE  setting.    One  yellow  caution light, placarded REV NOT READY, on the caution/advisory annunciator panel, alerts the pilot not to reverse  the  propellers.    This  light  illuminates  only  when the  landing  gear  handle  is  down,  and  if  propeller  levers are   not   at   HIGH   RPM   (full   forward).      This   circuit   is protected    by    a    5-ampere    circuit    breaker,    placarded LANDING GEAR RELAY, located on the overhead circuit breaker panel (fig.  2-27). 2-50.  PROPELLER TACHOMETERS. Two  tachometers  on  the  instrument  panel  register propeller  speed  in  hundreds  of  RPM  (fig.    2-30).    Each indicator    is    slaved    to    a    tachometer    generator    unit attached to the corresponding engine. 2-41