Replacing Faulty Capacitors

Article :  Andy Collinson
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Overview
This article is for general repair work and highlights the problems associated with small electrolytic capacitors. The faults and diagnosis will be different in each case, but knowing some of the symptoms arising from a faulty capacitor may help you make effective repairs and save time and money. Please read the disclaimer below:
Disclaimer:
Undertaking the repair of any electronic item will instantly invalidate its guarantee. If the product is still under warranty then it should be returned to the manufacturer. If the item is out of warranty and you feel suitably confident, and can solder and de-solder, then this article may help you repair your product. Extreme care should be taken when working on any apparatus as some components can store charge. You should NEVER work on any item "live" that is connected to your electrical outlet. The author assumes no liability or responsibility for damage to any equipment, please read this sites general disclaimer.
Fault Symptoms Arising from Bad, Leaking or Blown Capacitors
The diagnosis for any faulty apparatus will be different in each case. A quick check of the fuse should be the first step, but if the fuse is good, then the problem lies elsewhere.

A capacitors job is to block dc, allow ac currents to flow and provide filtering. A good capacitor will hold its charge, have very little leakage, and generally look in good condition. A failing electrolytic capacitor will start to leak electrolyte, which can be seen as a brownish discharge on the PCB and quite often the case will bulge. In extreme cases (shown right) on this motherboard the can can blow apart leaving a roll of dielectric material.

In audio systems a failed capacitor can quite often produce a hum usually at line frequency.
On a computer motherboard, a failing capacitor can cause random reboots and intermittent start or shutdown problems. If the capacitor fails completely then the computer will most likely not power up at all.
All the photos below are from a blown capacitor in my DAB radio. The electrolytic capacitor was part of the power supply, and first symptom was a small hum, which became louder over time, associated with difficulty turning the radio on and off.

Fault Diagnosis
My small DAB radio worked ok, but produced a small audible hum, (at line frequency 50Hz ) whenever it was on. Over time, the hum became louder, and one evening, following a power cut the radio would not work at all. The DAB Radio was several years old and out of warranty, so I decided to have a go at repairing it. After unplugging and removing case the power supply is shown left (click to zoom).

CAUTION As all capacitors store charge it is always advisable to leave any item unplugged for an hour or so to make sure all charge has dissipated before working on it.


A close inspection of the radios circuit board revealed a bulging capacitor with some discolouration. This is shown on the right hand image and I have circled the suspect area to make it clearer. The foreign matter is electrolyte which often appears on the top or bottom of an electrolytic capacitor.


To remove this capacitor some further dis-assembley is required. The pcb was held in by four screws and some connectors. Take care with all wire connectors. Only apply gentle force and sometimes a small screwdriver blade can assist. If you have to pull wires, then pull all wires on a cable and not individual wires, to share the strain.


To locate the faulty capacitor you need to view the PCB from both sides. I have circled the problem component in blue which needs to be desoldered. Desoldering is easy with a spring loaded pump tool. This article on Wikipedia shows a typical desolder tool and gives details on other methods to desolder. Also take note which way around the capacitor is connected. The minus terminal usually has a white line and marked on the capacitor.


After desoldering the faulty capacitor can be seen (click to zoom). It shows signs of leaking electrolyte on the base and very small holes in the case can also be seen. It is important to take notice of the value and working voltage on the case. The replacement needs to be the same value and working voltage. A higher working voltage is ok, but physical sizes may differ, so if working in small spaces on a pcb, make sure the new component will fit.


I already had a suitable replacement, although it was physically a little larger than the faulty capacitor. This was soldered back into the PCB and board screwed into the case. The rest of the connectors were put back together and case reassembled.


Other Faults
A simple visual inspection of a PCB will reveal the tell-tale signs of a bad capacitor. Other faults are not so obvious. Resistors that go open circuit are a good example of a component that looks perfectly normal. In circuit tests with a meter may give different results to when a component is tested out of circuit. Sometimes a component has to be de-soldered or tested on a breadboard to make sure it is functional.

Final Words
As a capacitor ages, its electrolytic series resistance (ESR) increases. If the capacitor forms part of a power supply then higher ESR results in worse smoothing and the ripple voltage increases. Higher levels of ripple will be heard as "hum" at line frequency. Line frequency is 50Hz for UK and Europe and 60Hz in North America.

A capacitor that leaks electrolyte will have signs of a bulge of the casing or chemical residue on the sides. A capacitor is stressed most when charged as it has to deal with the inrush of current. Large electrolytics are most likely to fail at switch on, or if a power surge hits your electrical supply.