Service manual

STP 11-25R13-SM-TG
H - 1
APPENDIX H – CAPACITOR GUIDELINES
NOTE: If an unexpected breakdown occurs, locate and correct the fault by using
careful troubleshooting methods. The goal of troubleshooting is to analyze the
symptoms and get to the source with a minimum of delay.
H-1. General.
Capacitors can short, become leaky (a partial short), or open completely. Electrolytic capacitors
normally allow a small amount of leakage current. Resistance readings of about 50k ohms or higher are
generally current. Resistance readings of about 50k ohms or higher are generally satisfactory for
electrolytics. Most other capacitors (such as mica, ceramic, or paper) should give resistance readings
approaching infinity.
a. Capacitors can also change capacitance value. Many capacitors are sensitive to temperature.
A temperature change will result in a change in the capacitance. Some capacitors increase capacitance
with an increase in temperature. These capacitors have positive temperature coefficients. Other
capacitors decrease in capacitance with an increase in temperature. These capacitors have negative
temperature coefficients. Therefore, it is important not to interfere with the normal airflow around a
capacitor. Also when replacing a capacitor, ensure that the new one has the same temperature
coefficient as the original.
b. Variable capacitors (such as trimmers or tuning gangs) may become shorted. At times they
may short only at certain settings. Shorts can be detected by measuring the resistance of the capacitor
while varying it through its capacitance range. A very high resistance reading (about infinity) should be
obtained at all settings of such capacitors. A low resistance (possibly zero resistance) is an indication of
a short circuit.
c. Mishandling of variable capacitors may cause the plates to bend and short together. In some
cases, straightening of the plates may effect a repair. If the bending is severe, then replacement may be
necessary.
H-2. Voltage and Resistance Checks.
When making voltage checks across capacitors, the voltages should compare favorably with the
manufacturers or as indicated in technical data on the equipment being checked. If the voltage readings
are incorrect, then perform an ohmmeter check to see if the capacitor is good or bad. When the leads of
the ohmmeter are first connected across the capacitor, the meter indicator will quickly move toward zero
and then increase to infinity as the capacitor charges to the power supply voltage of the ohmmeter. If
there is no movement of the meter indicator whatsoever, then the capacitor is probably open. A partial
short, complete short, or open indicates that the capacitor is defective and needs replaced.