While capacitors may last decades on a circuit board, they’re subject to age, manufacturing defects and damage from voltage spikes. Sometimes a bad capacitor will give visual clues you can spot with a careful look. If it appears to be good, you must still test it both in and out of the circuit. A defective or damaged capacitor will reveal itself when you check it with an oscilloscope or multimeter.
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Things you need
- 25-watt soldering iron
- Desoldering pump
- Circuit schematic
Examine the capacitors on the circuit board. Normally, they should have a smooth, symmetric shape. If you notice cracking, bulging or dark streaks on the capacitor, or if you’ve seen smoke coming from it, replace it.
Examine the circuit schematic to find the power supply paths. Connect the oscilloscope to the circuit board’s DC power by first connecting the grounding clip to power supply ground, then hooking the probe onto a point that has positive or negative DC power. Set the oscilloscope to DC coupling. If the oscilloscope shows excessive AC ripple or reads a below-normal DC voltage, an electrolytic filter capacitor probably needs replacing.
Disconnect all power from the circuit board. If the power has been on, give the capacitors a few minutes to discharge.
Desolder one lead of a suspected capacitor by heating the solder joint with a soldering iron, then vacuuming the liquid solder away with the desoldering pump. This should free the lead. Pull that side of the capacitor out of the board. If your multimeter has a capacitance checker, use that setting to test the capacitor. If you get a good reading, resolder the lead into the circuit board using fresh solder. If the meter doesn’t give a normal reading and the capacitor is within the meter’s capacitance range, replace the capacitor. If your meter has no capacitor checker, set it to read resistance. Touch the probe wires to the capacitor leads and watch meter’s behaviour. If the capacitor is polarised, match the polarity to the probe leads. If the capacitor reads a dead short or a constant resistance, the capacitor is bad. If it reads low initial resistance, then increases to infinity, the capacitor is probably good. You can solder it back into the circuit.
Tips and warnings
- Desoldering braid may be used instead of a desoldering pump.
- Do not test a powered circuit board unless you know how to work around hazardous voltages. Some circuits, especially those with vacuum tubes, have voltages higher than 110 V.
- When some capacitors go bad, they emit a black, foul-smelling smoke. Avoid breathing the smoke directly. Let it dissipate before working on the circuit.
- It’s important to let the capacitors discharge before testing them with a multimeter. Voltage left over in the capacitor can damage the meter.
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