Service manual

STP 11-25R13-SM-TG
J - 1
APPENDIX J – HIGH PASS FILTER GUIDELINES
J-1. Introduction.
In a high pass filter (HPF), as shown in Figure J-1, the capacitor will pass high frequencies. At low
frequencies, the capacitor will block any current flow through it; thus no current is allowed to pass through
the load.
Figure J-1. High Pass Filter with an Open Capacitor
J-2. Open Capacitors.
An open capacitor cannot charge; therefore, there will be no charging current. Because the charging
and discharging current is used to develop the output signal, there will be no output signal developed.
a. If the capacitor value is large enough (0.01 microfarad or larger), an ohmmeter may indicate
whether it will charge. An ohmmeter may not detect charging action for a smaller capacitor. Applying a
test signal to one terminal and observing the other terminal with an oscilloscope may test them.
b. The presence of an output signal indicates a good capacitor, and the absence of an output
signal indicates an open. To prevent false indications, ensure the test signal used has a high enough
frequency so that the reactance of the capacitor is not so high as to stop the signal from being passed.
J-3. Shorted Inductors.
If a HPF contains an inductor in parallel with the load, a complete short across the inductor will short
out the load device, resulting in the same frequency response curve as seen in Figure J-lB. If the short is
only partial, the rise time of the frequency response curve will increase.
J-4. Shorted Capacitors.
A shorted capacitor does not block DC, so it may cause other components to fail. When a capacitor
is suspected of being shorted, an ohmmeter provides a fast and reliable test of the capacitor.
a. Sometimes a capacitor develops leakage, a resistive path through the capacitor, which can also
be detected with the ohmmeter.
b. Small-value capacitors should indicate infinity when charged by the ohmmeter's battery.
Indications of less than infinity indicate leakage. This is true only if the capacitor is not in parallel with
another component.