(e) If the reason the pc control board is being checked, and no substitute board exists, is insuffi-
ciency of output voltage with proper AC secondary voltage at the main transformer, check for
proper pc board voltages
(See 3-3),
if no oscilloscope is available. If the readings are good, the
problem may be one or more open SCR devices. The best test equipment is an oscilloscope but
the use of a crude SCR tester with the input power off can verify open SCR devices but not hard
to fire ones. One type of SCR turn-on tester is a battery power circuit tester with a light bulb in se-
ries with the battery and two leads. With input power off, the positive lead is connected to the an-
ode of the SCR under test and the negative lead is connected to the cathode of the same SCR
device. The light should stay off until the gate lead of the same SCR has the positive voltage ap-
plied to it. If the light turns on and remains on after the gate lead voltage is removed so long as
the anode and cathode leads stay connected, the SCR is probably good. If the SCR stops con-
ducting as soon as the gate lead is removed, an anode to cathode open or very hard to drive
faulty SCR is probable. If any SCR devices are found to be faulty, the printed circuit board may
not be faulty unless the voltages in 3.3 of the pc board test aren’t achievable. Then both compo-
nents may be faulty.
OM-2010
April 10/89 Revised 3-2
Page 7
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