TESTING POWER RECTIFIERS AND TESTING AND ADJUSTING CONTROLLED RECTIFIERS TESTING POWER RECTIFIERS AND CONTROLLED RECTIFIERS The power rectifiers and controlled rectifiers, used in the SR 4 Generator Sets, are of the stud mounted type.  You must  be  extra  careful  during  the  installation,  testing  and replacement of these components. Failure of these units can be caused by: 1. Too much current. 2. Too much voltage. 3. Too much heat. 4. Wrong torque during installation. The type of failure normally found in a rectifier or controlled   rectifier   is   a   short   circuit   from   anode   to cathode.  An open circuit failure will not be seen with any frequency.    Controlled  rectifiers  can  have  a  failure  with either  a  short  or  an  open  from  gate  to  cathode.    The performance of the excitation and regulation circuits can be   less   than   desired   as   a   result   of   a   change   in   the characteristics of these components.  This type of failure can not be found with an ohmmeter or continuity tester. Rotating rectifiers (CR1, CR2 and CR3) and field rectifier (CR10) are of the normal polarity type: that is the terminal   end   is   the   anode   and   the   stud   end   is   the cathode.  Rotating rectifiers (CR4, CR5 and CR6) are of the  reverse  polarity  type:  that  is  the  stud  end  is  the  - anode and the terminal end is the cathode. Controlled  rectifier  (CR9)  is  made  with  the  stud end   as   the   anode,   the   longest   terminal   end   is   the cathode and the shorter terminal end is the gate. Ohmmeter  or  continuity  tester  (8S4627)  checks can be made on these rectifiers but only to find short or open  circuit  conditions.    To  do  these  tests,  the  positive cable  is  connected  to  the  anode  and  the  negative  cable to the cathode, then reverse the cables. A shorted rectifier will have an indication of zero or very low resistance with an ohmmeter, or the light will be  on  with  the  circuit  tester  in  both  directions.    An  open rectifier  will  have  an  indication  of  infinity  (maximum)  on an  ohmmeter  or  no  light  indication  with  the  continuity tester  in  both  directions.    A  "good"  rectifier  will  have  a much    greater    reverse    resistance    (cathode    positive; anode     negative)     than     forward     resistance     (cathode negative;  anode  positive).    Typical  reverse  resistance  is 30,000    ohms    to    300,000    ohms.        Typical    forward resistance is less than 10 ohms. The   values   you   get   can   change   with   different meters. A    "good"    controlled    rectifier    will    have    both forward and reverse resistance from anode to cathode of 30,000    ohms    to    300,000    ohms    but    they    must    be approximately the same. To make a test of the gate circuit of a controlled rectifier,  connect  the  positive  and  negative  cables  of  an ohmmeter to the gate and cathode terminals.  Read the value  of  resistance.    Reverse  the  cables  and  read  the value  again.    A  shorted  gate  will  have  an  indication  of zero  resistance  in  both  directions.    An  open  gate  will have an indication of an infinite (maximum) resistance in both  directions.    A  ''good"  controlled  rectifier  will  have  a forward  (gate  to  cathode)  and  reverse  (cathode  to  gate) resistance  that  are  about  the  same  and  approximately 10 to 200 ohms. To  make  a  test  of  the  "turn-on"  characteristic  of the controlled rectifier use the continuity tester (8S4627). Put   the   positive   cable   of   the   tester   on   the   stud   end (anode) and the negative cable on the cathode terminal. Temporarily put a wire between the terminals of the gate and anode.  The tester light will come on and be on until one of the cables is removed.  If the light does not come on, the rectifier is bad.  Install a new rectifier. The  stud  used  to  install  these  rectifiers  has  two purposes: to give an electrical connection and a method of  taking  the  heat  away  from  the  rectifier  through  the heat sink. Be   careful   during   the   installation   of   a   power rectifier or controlled rectifier on the aluminum heat sink. The  threads  on  the  rectifier  and  the  contact  surfaces  on both the rectifier and the heat sink must be clean.  Apply a  small  amount  of  5P8937  or  5P92  10  Thermal  Joint Compound to the contact surfaces. NOTE:   A   pound-inch   (N· m)   torque   wrench   check accuracy  in  the  respective  torque  range)  must  be used     for     installation     of     power     rectifiers     and controlled rectifiers. The    replacement    of    rotating    rectifiers    (CR1 through CR6) can be done more easily by removing the heat sink assemblies from the cooling fan.  Positive heat sink  (E1)  has  a  mark  "POS'"  and  the  edge  has  a  red paint  mark.    Negative  heat  sink  (E2)  has  a  mark  "NEG" and the edge has a black paint mark. RECTIFIER TIGHTENING CHART Tightening Torque Rectifier Rectifier Base Pound-Inches Pat No. Stud Size (N-m) 3N1482 No.  10 - 32 12 to 15 (1.3 to 1.7) 3N1483 1/4 - 28 20 to 25 (2.3 to 2.8) 3N1484 1/4 - 28 20 to 25 (2.3 to 2.8) Controlled Rectifier 1/4 - 28 20 to 25 (2.3 to 2.8) 3N1481 76