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TM 55-1905-219-14-10
REPAIR (Cont.)
The silicon diode may be a source of trouble. The function of the diode is to allow the flow of current through in
one direction only. If the polarity of the conducting current is reversed, the diode will block the current flow.
Thus, the diode has a low resistance to current flow in the other direction, and a high resistance to current flow
in the other direction. Therefore, a simple ohmmeter may be used to test the diode. The procedure for
checking the silicon diode is as follows:
1. Isolate one end of the diode by disconnecting the wires attached to the nipple (or pig-tail) end of the diode
(only one end of the diode must be disconnected).
2. Clip one lead of the ohmmeter to the nipple (or pig-tail) lead of the diode. Clip the other ohmmeter lead to
the aluminum heat sink. (If a portable multimeter is used, set the switches on Ohms, DC and scale RX100).
3. Note the ohmmeter reading. Then reverse the leads to the diode. Again, note the ohmmeter reading. If
the diode is good, the meter will indicate a high resistance in one direction, and a low resistance with the
leads reversed. If the diode is shorted, the meter will read full scale, or "O" resistance with the leads in
either direction. If the diode is "open", the ohmmeter needle will not indicate or show infinite resistance,
indicating an open circuit with the ohmmeter leads in either direction.
4. All diodes must be checked in the event that more than one diode is defective.
5. If the diode is defective, remove the defective diode from the heat sink and replace with a new diode.
When installing a new diode, be sure to note if the old diode was insulated from the heat sink. If the diode
should be insulated from the heat sink, care shoud be taken so that the mica insulating washer is placed
properly on each side of the heat sink with the insulating bush- ing between the diode mounting stud and
the aluminum heat sink.


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