THEORY OF OPERATION The HDC uses a unique double spool arrangement that serves to separate the null deadband from the feedback, giving  both  safety  against  null  drift  and  quick  dynamic response to command changes. The   command   signal   for   the   MCV106A   is   usually   a remote  hydraulic  control  handle,  as  shown  in  Figure  2, although  other  pressure  sources  could  be  used.    The handle  modulates  supply  pressure  from  the  pump  (or another source) so as to apply a proportional differential pressure across the C1 and C2 input ports. The HDC's null adjust is set with the modulating spring compressed  to  the  equivalent  of  12,  120  or  217  psi, depending on the output characteristics of the command source, representing the differential pressure required to move the actuator spool one direction or the other.  This is a factory setting that defines the width of the actuator INSTALLATION A highly reliable connection between the swashplate and the   drag   link   is   necessary   for   safe   operation.      An unreliable connection may result in loss of feedback with a  resulting  loss  of  control.    Series  3X  pumps  meet  this requirement,    but    all    Series    2X    units    not    already equipped  with  an  HDC  or  EDC  must  be  retrofitted  with the appropriate drag link, press fit pin and retaining ring, replacing  the  slip  fit  headless  pin  and  E-rings  used  to attach the original drag link. Series 3X pumps with serial numbers of 82-34-00000 or greater  will  accept  the  HDC  without  modification  of  the pump.  These units have a clearance notch cast into the swashplate    that    provides    additional    room    for    link movement.    Series  3X  pumps  with  serial  numbers  of less  than  82-33-99999  may  not  be  fitted  with  the  HDC without modification of the swashplate by Sundstrand. Prior  to  mounting  any  control  on  a  pump,  ensure  that both the control and the feedback link are correct for the pump  as  evidenced  by  the  series  number  stamped  on the link and the part number labeled on the control body. See Table A, below. spool deadband and cannot be changed in the field.  By tightening  or  loosening  the  null  adjust  screw,  the  fixed deadband is moved toward or away from the "A"' control port in order to null the valve. As differential control pressure (C1-C2) rises beyond the deadband, the actuator spool moves in one direction or the  other,  pivoting  the  crosslink  about  its  center.    The pivoting crosslink pushes or pulls the porting spool in the opposite   direction   of   the   actuator   spool.      When   the porting  spool  has  moved  far  enough  to  open  A  or  B  to supply pressure, oil is ported to the pump servocylinders to move the swashplate.  As the swashplate moves, the feedback  link  follows,  pivoting  the  crosslink  about  the stationary end of the actuator spool, driving the porting spool back to its neutral position.  Because the feedback signal is entered into the control loop after the command has    been    input,    response    time    and    accuracy    are enhanced. TABLE A Table A correlates the pump series number with the series number stamped on the side of the feedback link protruding from the control. SERIES LINK MARKING 20 20 No Spacer 21, 22 21, 22 23 23 24 24 25 25 26 26 27 27 33, 34, 36 33, 34, 36 Sundstrand Mobile Controls 3