ARR Series Three Phase, Six Pulse SCR Charger
Corrective Maintenance
RS-420, Rev. 7
5-6
cathode for the other diode will be the other anode of the rectifier stack), and a very high resistance from cathode
to anode. A shorted diode will show no or very low resistance both ways. An open diode will show a very high
resistance both ways. Suggested ohmmeter scales for these checks is either X100 or X1K.
Check the silicon controlled rectifiers (SCR’s) for shorts in the same manner in which the diodes are checked (in
the silicon controlled rectifiers; the anodes for each rectifier are the anodes of the stack, i.e. the anodes for the
SCR’s are the same as the cathodes of the diodes; the cathodes for both SCR’s is the positive terminal point of the
rectifier stack). However, a high resistance reading both ways does not necessarily mean the SCR’s are open.
To check for open SCR’s de-energize the charger and pull the printed circuit boards out. Connect a 100 ohm
resistor from the anode to the gate (located on the terminal board, on the positive side of the rectifier stack, refer
to the drawing) of one SCR and turn the charger on. If the rectifier is good, the charger will produce output
current. Repeat this procedure for the other SCR.
To check for resistance of each gate to cathode of SCR’s use X10 scale. Should read from 70 to 200 ohms in
forward and reverse directions. An open or shorted gate will read same as open or shorted diode.
Replacement of Silicon Controlled Rectifier, Silicon Diodes, and entire Silicon Rectifier Assembly.
A. If it is found that one or more of the diodes and/or the SCR’s is/are defective the following procedure may be
used for replacing the defective pieces. Refer to the diagram and drawing to locate the connection points on
the charger.
1.
Disconnect the charger completely from the AC power and the batteries (DC power).
2.
Disconnect the external leads of the rectifier stack and the following points: the positive and negative
terminal points of the stack, the anodes of the rectifier, and the gates of the rectifier (refer to the
diagram for the exact location).
3.
Loosen the hardware used for mounting the stack to the back panel and remove the rectifier stack from
the cabinet.
4.
Remove the defective diode (s) for the defective SCR (s) by:
a. disconnecting the diode or rectifier connections where necessary on the rectifier stack.
b. Defective pieces may now be removed.
5.
To replace the defective parts simply reverse steps one through four.
CAUTION: When replacing the defective part, avoid excessive pressure which may damage either the
case or threads.
