TM 10-3930-671-24 3. Remove the three black leads from the heat sink screws and gently push the heat sink out of  the housing.  Observe loose leads as plate is being removed, so as not to damage leads (Figure No. 28). NOTE Three mica insulators are set against the   inside   of   the   rear   housing   and insulate  positive  heat  sink  from  rear housing  (Figure  No.    28).    The  mica insulators are retained by a recess in the   rear   housing,   plus   the   use   of silicon grease. FIGURE NO.  28. OUT   OF   CIRCUIT   RECTIFIER   DIODE   TEST   -   If   a commercial  alternator  rectifier  diode  tester  is  available, follow  manufacturer’s  instructions  to  test  all  diodes.    Do Not Use 120 Volt AC test lamp. A  12  Volt  battery  operated  test  lamp  may  be  used  if  a commercial  tester  is  not  available.    Connect  one  test lead  to  diode  heat  sink,  the  other  to  each  diode  wire terminal, Figure No.  29, then repeat test with test leads reversed.    Lamp  should  light  with  leads  in  one  position, but should not light with  test  leads  reversed.    All  diodes in heat sink (Figure No.  29) or rear housing (Figure No. 30) should show the same results. FIGURE NO.  29. FIGURE NO.  30. If lamp lights, regardless of how test leads are switched, the  diode  is  shorted.    If  lamp  fails  to  light  in  either  test, the  diode  is  open.    Replace  defective  diodes,  observe correct   polarity   by   color   of   stamping   used   to   list   part number on diode. REPLACING DEFECTIVE RECTIFIER DIODES- Note, head of output stud, is a metal to metal contact. Figure No.  31 shows three positive, rectifier diodes, and terminal studs assemblied in the positive heat sink. FIGURE NO.  31. F-288