b. Troubles in a capacitor are indicated as follows:
(1) When the ohmmeter reading is immediately zero and stays there, the
capacitor is shorted.
(2) When the capacitor shows a charging action
but
the
final
ohmmeter
reading is less than normal, the capacitor is leaking.
(3) The electrolytic capacitor must be checked by taking a normal reading,
then reversing the ohmmeter leads and taking another reading.
The higher reading
indicates the true condition of the component.
(4) If the capacitor shows no charging action and immediately indicates a
high resistance, it is open.
3.
Inductor checks should be made with the component disconnected from the
circuit if we are to set a true indication. The most common trouble in coils is an
open, which is indicated by an infinite reading on the ohmmeter.
a. Less common troubles are a short between turns, a short between primary
and secondary turns in a transformer, and a short to an iron core.
b. A coil has a DC resistance equal to the resistance of the wire used in the
winding. For RF coils with inductance values up to several millihenrys, the 10 to
100 turns in the coil have a DC resistance of 1 to 20 ohms. Inductors for lower
frequencies have several hundred turns and a range in resistance from 10 to 500
ohms, depending on the wire size.
4.
When checking a transformer with four or more leads, check the resistance
across the two primary leads, and across any other pairs of leads for additional
secondary windings.
a. For an autotransformer with three leads, check the one lead to each of the
other two. When an open is indicated in a coil, the connection from the external
terminals to the coil should be checked.
Often these can be re-soldered to make
the coil reusable.
b. Shorted turns cannot be definitely checked with the ohmmeter because a few
shorted turns will only slightly reduce the DC resistance. When shorted turns are
suspected because of a reduced resistance, the unit should be replaced.
Excess
heat across the short can eventually create an open in the coil.
5.
The resistance between separate windings in a transformer is normally
infinite. If the ohmmeter is connected between the primary and secondary windings
and reads a low resistance, this indicates a short between the primary and the
secondary.
Similarly, the resistance between the winding and the core or frame
should be infinite. If a low reading is shown between these points, this indicates
a short.
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