(2) Inspect    the    armature    for    short    circuits using a grower fixture.  Place the armature in the growler as shown in figure 4-6.  Hold a thin strip of steel, such as a  hacksaw  blade,  about  1/32  inch  from  the  armature. While  holding  the  steel  strip  in  this  position,  rotate  the armature slowly in the growler.  A short circuit will pull the steel  strip  tightly  against  the  armature  core  and  cause the   strip   to   vibrate.      If   a   short   circuit   is   found,   the armature must be replaced. Figure 4-6.  Testing the armature for short circuits. (3) Inspect the armature shaft and commutator for runout using a lathe or "V" blocks and a dial indicator (fig.  4-7).  If runout exists, the commutator must be resurfaced until true. Figure 4-7.  Checking commutator runout. (4) Inspect  the  commutator   contact   surface. A  satisfactory  condition  is  indicated  by  an  even,    highly burnished,  dark-copper  color.    If  the  contact  surface  is rough, pitted, scored, burned, or coated with hard carbon or  oil,  the  commutator  must  be  resurfaced.    If  the  mica insulation  between  the  commutator  bars  is  not  0.025  to 0.032  inch  below  the  surface  of  the  bars,  it   must   be undercut to the correct depth. c. Inspect the field coils for grounds by placing one probe of a test lamp (fig.  4-8) on the field yoke and the other probe on the field terminal.    If  the  lamp  lights,  the field   coils   are   grounded   and   must   be   replaced   if   the ground cannot be located and required. Figure 4-8.  Testing field coil for grounds. 4-5