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TM 5-4210-233-14&P-1 5-66.  APU GENERATOR-Continued e. Testing/Repair. NOTE Testing  of  Generator  consists  of  continuity  checks  using  wiring diagram (Appendix F). NOTE Most    of    the    following    tests    can    be    performed    without disassembling   the   generator.      Clearly   mark   all   leads   dis- connected, together with the point taken from. (1) Armature.  Remove all brushes from their holders. (2) Using ohmmeter, check for continuity in the ac winding for an open circuit between the slip rings.  If an open circuit is found, replace the armature. (3) Test both the slip rings and commutator for short or grounding to the shaft. (4) To test the armature for an open circuit in the dc windings 'O" reading indicates open circuit.  Check continuity between all adjacent bars of the commutator.  Visually inspect for bar burring, arcing and poor cranking. (5) Touch the probes to two adjacent bars and check for continuity.  Move each probe over one bar and check  again.    Continue  around  the  commutator.    Any  adjacent  bars  that  don't  show  continuity indicate an open armature winding. (6) This test can only be performed with the generator disassembled and requires a growler.  To test for shorts in the dc winding, place the armature in the growler.  Operate the growler and pass a steel strip back and forth over and above the armature windings.  If the strip is magnetically attracted to the  armature  at  any  point,  a  short  is  indicated.    After  testing  in  one  position,  rotate  the  armature slightly and repeat the test.  Do this for one complete revolution. (7) If the test indicates a short circuit in the dc  windings,  be  sure  the  commutator  is clean.        Carbon    dust,    dirt    or    grease between the bars or slip rings can cause a short. (8) If   the   tests   show   that   the   armature   is defective, replace it. (9) Field   Winding   Tests:      The   following tests can be performed without disassembling   the   generator,   but   the field coil leads must all be disconnected from their terminal points; brush   rig,   control   box,   and   external connections.      If   a   defective   coil   is found,  disassemble  the  generator  and replace the defective coil. (10) With an ohmmeter or continuity lamp, check for grounding to the generator frame.  Touch one prod to the coil terminals and the other to a clean, paint-free part of the frame.  If grounding is indicated, separate the windings and check each one. 5-367

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