Service manual

STP 11-25R13-SM-TG
I - 1
APPENDIX I – LOW PASS FILTER GUIDELINES
I-1. Introduction.
In electronic circuits, defective capacitors and inductors must be isolated and replaced. Faulty
resistors can usually be identified using an ohmmeter, but this is not always the case with capacitors and
coils. Small values of inductance or capacitance will read as constant shorts or opens, respectively. In
the field, often the best step in troubleshooting is a physical inspection of the circuit that is malfunctioning.
An open resistor or capacitor is often burned, and shorted coils often have a distinctively melted
appearance.
I-2. Troubleshooting.
There are two types of troubles that most often occur in electronic circuits containing inductors and
capacitors. These are opens and shorts. When one of these malfunctions occurs there are several
checks, both static and dynamic, that can be made to determine which component is at fault. The
process of performing these checks and locating the faulty component is known as troubleshooting.
Troubleshooting should be performed in a logical sequence to arrive at your final conclusion.
a. Low Pass Filter (LPF). The LPF, as its name infers, will be able to pass low frequencies onto
another circuit (the load resistor), but it will reject, or stop, high frequencies from ever getting to the load
resistor.
b. Open Inductor. If the series inductor in the LPF in Figure I-1 opens, this will break the circuit,
preventing current flow. Since the inductor is located in the series portion of the circuit, the circuit
operation will stop completely and the output voltage will fall to zero. A quick test for an open inductor
can be performed with the ohmmeter. Depending on the size and number of turns in the coil, a good
inductor will indicate continuity with a small amount of resistance. Very high or infinite resistance
indicates an open.
Figure I-1. Open Inductor or Shorted Capacitor
c. Shorted Capacitors. A LPF that contains a shorted capacitor (Figure I-1) will yield the same
frequency response curve as an open inductor would. This is because the shorted capacitor would short
out the load device, leaving the inductor as the only opposition in the circuit. As with any short, the
current being drawn from the source would increase. At low frequencies, the inductive reactance could
be low enough to allow the excessive current to burn something out; a visual inspection might give the
technician a clue as to the source of the trouble.