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Page Title: Testing/Repair (cont)
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TM 5-4210-233-14&P-1 5-66.  APU GENERATOR-Continued (11) Check  the  field  winding  resistance  from  F2  in  the  control  box  to  the  F  +  connection  on  the generator (F + is connected to the positive brushes).  Resistance should be 1.46 ohms on standard ac models.  Other models will have the following resistances: 2.06 ohms for 24-volt cranking 3.80 ohms for 32-volt cranking 0.80 ohms for transistor flicker 5.14 ohms for 24-volt battery charger 8.8 ohms for 32-volt battery charger If the windings are warm from running, the resistance will be slightly higher. If the resistance is high, check for an open circuit in one of the parallel windings, step 12, otherwise go to step 13. (12)  Separate the parallel field windings (at F +) and check each for open circuit. (13) Check  for  open  circuit  in  the  series  winding with ohmmeter.  Touch probes to lead S1 and connection   F   +.   If   there   is   an   open   circuit, isolate each coil and check ft. (14) Test    for    short    circuit    between    the    starter windings   and   the   shunt   windings.      Before doing this, separate all windings at F +. (15) Commutator   Repair:   The   commutator   bars wear  down  with  use,  so  eventually  the  mica between  them  extends  over  the  tops  of  the bars  and  causes  sparking  and  noisy  brushes. When the mica on any part of the commutator is  touching  the  brushes,  it  must  be  undercut. A suitable undercutting tool can be made from a  hacksaw  blade.    Be  careful  not  to  injur  the bars.    After  undercutting,  remove  any  burrs formed  on  the  bars.    Cut  the  mica  to  about 1/32" under the bars. (16) If the commutator is grooved, out-of-round, or otherwise   damaged,   refinish   it.   Turn   it   in   a lathe and then undercut the mica as described above. Shield the ball bearing during refinishing.      Do   not   use   turning   centers   on shaft    because    they    probably    have    been damaged   and   are   no   longer   true   centers. Commutator  and  slip  ring  run  out  should  be less than .002 in. (17) Brush  Rig  Alignment:  The  brush  rig  must be  aligned  in  the  neutral  position.    If  it  isn't sparking    will    occur.    Normally    the    neutral position     is     identified     by     a     yellow     mark extending from the brush rig to the endbell.  If the  mark  is  lost,  or  a  new  brush  rig  installed, follow   these   instructions   to   find   the   neutral position. 5-368

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