Reforming Electrolytic Capacitors

"P•11111•1,- By RODNEY CHAMPNESS, VK3UG ---4111111PP" Reforming electrolytic capacitors rect polarity. By contrast, bipolar or Capacitors are the most troublesome parts in non-polarised electrolytic capacitors vintage radio receivers. This month, we look can be connected into circuit either way around, however they are seldom at the various capacitor types and describe found in radio receivers. an easy-to-build circuit that can be used to Note that the capacitance values marked on electrolytic capacitors are reform electrolytics. only approximate. In practice, they and can vary from about 10% low to TN "VINTAGE RADIO" for October in bias circuits) should be replaced as much as 50% high. So don't get _Land November 2004, we looked at if leaky. too upset if the measured value of a paper capacitors and described the In this article, we'll look first at elec- nominal 161.IF capacitor turns out to problems that they can cause. Those trolytic capacitors and describe how be anywhere between say, 14µF and articles also described how paper ca- they can be reformed (or re-polarised). 240. pacitors could be tested for leakage and We'll then take a look at mica, poly- described the circumstances under ester, styroseal (polystyrene), ceramic Main problems which they should be replaced. and air-dielectric capacitors. Electrolytic capacitors suffer from In practice, the decision whether or two main problems: (1) loss of ca- not to replace a leaky capacitor often Electrolytic capacitors pacitance and (2) excessive leakage depends on where it is located in the Electrolytic capacitors are usually current. The first problem, that of circuit. In many cases, leaky capaci- used as power supply filters and as reduced capacitance, occurs because tors in non-critical positions (eg, with bypasses in valve receivers. They are the electrolyte inside the capacitor low voltages across them) can be left also used as coupling capacitors in tends to dry out over the years. As a in circuit, as they will have negligible low-impedance sections of transistor- result, the capacitance of a nominal effect on performance. By contrast, ised receivers. 161.ff power supply filter capacitor capacitors with high voltage across Polarised electrolytics have positive may reduce to virtually zero. This will them or in certain critical positions and negative terminals and must be result in hum and/or "motorboating" (eg, AGC bypass capacitors and those connected into circuit with the cor- in the audio output of the receiver and replacement is the only answer. As for the second problem, electrolytic capicitors always have some gf 6 leakage — usually be less than 1mA. However, an electrolytic capacitor stored for a long period of time can become depolarised. As a result, it will draw considerable current (greater than 40mA in some cases) until it is reformed (by applying a polarising voltage across it). So how do you reform an electrolytic capacitor? There are three different methods and I'll describe the pros and cons of each. Note that some capacitors will not respond to the reforming process and will need replacement. Electrolytic capacitors are commonly used in valve receivers for power supply line filtering Reforming method 1 and as bypasses. Regrettably, some vintage radio col- 98 SILICON CHIP siliconchip.com.au • Polyester capacitors became available in the late 1960s, Mica capacitors usually have relatively low values and towards the end of the valve era, and are very reliable. are typically used as RF bypasses, in tuned circuits. lectors try the brute force method of and observe the rising voltage as the as decidedly risky. While initial re- reforming electrolytic capacitors — by set is turned on for a brief period. This sistance checks may indicate nothing giving the set a "smoke" test without period is around 20 seconds for a set amiss, when the voltage across a sus- first checking the power supply and with an indirectly-heated rectifier and pect capacitor rises to a critical value, for faults on the HT line. In many just a few seconds with a directly- the leakage current may suddenly cases, this is exactly what does hap- heated rectifier. increase or it may become short-circuit pen — smoke appears as soon as power In practice, I let the voltage rise to which can immediately damage the is applied. about a quarter of the expected HT rectifier. If the capacitor then suddenly Often, a set will have been put aside voltage and then turn the set off. If the leaks all over the chassis, you then because it has a fault and subsequently rectifier shows any sign of distress (red have a major clean-up job. And the stored in less than ideal conditions colour on the plates or sparks inside smell is something you will remember which leads to further deterioration. the works), I turn the set off immedi- for the rest of your life! Finally, an This makes it extremely risky to turn ately and recheck for shorts. any old set on before checking it thor- After about a minute, I then repeat oughly. There may be shorted capaci- the procedure, this time letting the tors or capacitors that are so leaky that voltage rise a little higher. If the elec- they may explode after a short time. trolytic is reforming, the voltage across In the process, they may destroy the it will rise to the expected HT voltage rectifier and perhaps even the power after a few cycles of this procedure. transformer. Note that it's necessary to check the A leaky paper audio-coupling ca- second filter capacitor as well. I've pacitor could also cause the audio sometime found that one capacitor output valve to draw excessive cur- would reform but not the other. Note rent, destroying the valve in the proc- also that more modern electrolytics ess. In short, turning a set on without don't seem to need much reforming. checking it can produce some rather If an electrolytic capacitor shows any expensive smoke. signs of overheating, it should be discarded as it obviously has far too Reforming method 2 much leakage. Over the years, I have often used What are the advantages of this a method that some people consider method? It will successfully reform risky when it comes to reforming elec- capacitors over a period of a few trolytic capacitors. First, I check that minutes of on-off switching. It has the there are no short circuits on the HT advantage that no capacitor has to be line and that the minimum resistance removed from the set to do the reform- from the HT line to chassis is at least ing. If used with care in the manner 10142 (the actual value will depend on described above it would be rare for the circuit). In addition, if an initial any damage to occur in the receiver. physical check shows that any capaci- What are the disadvantages? It is a tors are bulging or leaking electrolyte, bit harsh and if care is not taken the I replace them. end result will be damage similar to That done, I connect a multimeter that which occurs with the previous via insulated short jumper clip leads "smoke test" method. across the first electrolytic capacitor (Editor's note: we regard this method siliconchip.com.au OCTOBER 2006 99 Styroseal capacitors became available around the same Early ceramic capacitors were not very reliable but later time as polyester capacitors and are quite reliable. types gave few problems. exploding electrolytic capacitor poses When the capacitor has reformed, They are usually quite reliable but an extreme risk to your eyes!) the voltage across it will be at the they can develop faults that give some selected reforming voltage, while the strange effects in receivers. For exam- Reforming method 3 current will have reduced to a fraction ple, local oscillators can drift or jump Method number 3 is much more of a milliamp in most cases. However, in frequency, while the audio output benign and involves using a special DC if the current remains at about 10mA can have annoying crackles in it. power supply. This supply should be and the voltage doesn't risen to the se- A high-voltage tester will usually re- voltage regulated (so that the applied lected reforming voltage, the capacitor veal if a mica capacitor has noticeable voltage can not exceed the peak volt- is suspect and should be replaced. leakage and if this leakage resistance age rating of the capacitor) and should You can get a good idea as to just fluctuates. Most mica capacitors were feature current limiting. how well a capacitor is holding a made as a "stack" interleaved with In operation, the capacitor is con- charge by disconnecting it from the sheets of tin foil and mica clamped nected to the output and the current supply and observing how quickly the together and then encapsulated. limiting set to 10mA. This current voltage across it decreases with just a Sometimes the contact between some limit applies whether the voltage digital multimeter in place. metal foils and the clamps becomes across the capacitor is 5V or 500V (Be careful though — a capacitor intermittent and so the capacitance (or what ever the maximum working charged to a high voltage can deliver will vary. voltage happens to be). a fatal shock. Always make sure that If you don't have a high voltage a capacitor is fully discharged before tester, the easiest way to test whether touching it). a mica capacitor is at fault is to replace Forming Electrolytic This method of reforming has a it and see if this makes a difference.

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