sign in sign up
<<
Home , Electrolytic capacitor, Frequency changer

"P•11111•1,- By RODNEY CHAMPNESS, VK3UG ---4111111PP" Reforming electrolytic

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 types and describe found in radio receivers. an easy-to-build circuit that can be used to Note that the 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 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- 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 filters and as reduced capacitance, occurs because tors in non-critical positions (eg, with bypasses in valve receivers. They are the inside the capacitor low 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 - result, the capacitance of a nominal effect on performance. By contrast, ised receivers. 161.ff power supply filter capacitor capacitors with high 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, elec- trolytic 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 electrolyt- ic 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 proc- ess 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 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. . 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. Capacitors couple advantages. First, provided Mica capacitors can really cause it's done properly, with the voltage headaches because they can produce So what is this "forming" process? increased in stages, no undue stresses very obscure symptoms in a receiver. Basically it refers to re-forming the alu- are placed on either the capacitor or In fact, it's not uncommon to find that minium layer on the foil the test instrument. Second, it shows the faulty component is nowhere near in the electrolytic capacitor. In just how good a capacitor is and gives where you expected to find it but is in essence, the aluminium foil is the posi- an indication as to whether it should a different section altogether. Faulty tive electrode and the he used or not. local oscillator grid coupling capaci- layer is the dielectric of the capacitor. What are the disadvantages? If the tors have led me up the garden path The conductive electrolytic in conjunc- capacitor is "new old stock" and is out more than once. tion with another small aluminium foil of circuit, there are no disadvantages. and the aluminium can then forms the However, if it is in-circuit, it may need Polyester capacitors negative electrode of the capacitor. to have one lead disconnected. Polyester capacitors are usually In applying the "forming" current to A simple and very effective repo- available in 160V, 400V and 630V DC the capacitor we are setting up a con- lariser/reformer test instrument is ratings and take the place of paper trolled chemical process between the described later in the article. capacitors. The most common style conductive electrolyte and aluminium became available in the early sixties foil to re-anodise or oxidise the alu- Mica capacitors towards the end of the valve era. minium surface.This heals any breaks in Mica capacitors usually have rela- I don't think I have ever had to the oxide layer (the dielectric) and thus tively low values and are typically replace one of the yellow-coloured reduces the leakage current. used as RF bypasses, in tuned circuits Philips units — they are just so reliable. and as vibrator buffer capacitors, etc. In fact, it's a pity they weren't avail-

100 SILICON CHIP siliconchip. corn. au able many years earlier - valve radios would have been so much more reli- Photo Gallery: Peter Pan GKL 4-Valve Radio able without paper capacitors. "Greencaps" and MKT capacitors are also polyester types. However, their voltage ratings can differ from those quoted above. Styroseal capacitors Styroseal (polystyrene) capacitors became available around the same time as polyester capacitors and from my experience, are quite reliable. They have been used have been used in much the same way as polyester capacitors and also in tuned circuits to some extent. Ceramic capacitors Some early ceramic capacitors were not considered particularly reliable, whereas later types gave few problems. They generally come in two types. One type is used more as a bypass where the exact value is unimportant, whereas the other type is more precise in value and is often used in tuned circuits. MANUFACTURED IN 1946 by Eclipse Radio, South Melbourne, the GKL was In addition, ceramic capacitors can a compact 4-valve reflex housed in a bakelite be manufactured with negative, zero cabinet.These sets were produced in a number of colours, the pink example (NPO) or positive temperature coeffi- shown here being quite rare. cients, so that frequency drift in tuned The valve line-up was as follows: 6A8-G frequency changer, 6B8-G reflexed circuits can be compensated for with IF /1st audio amplifier/detector/AVC rectifier, 6V6-GT audio output changes in temperature. and 5Y3-GT rectifier. Photo: Historical Radio Society of Australia, Inc. Ceramic capacitors come in a range of voltage ratings from 50V up to several thousand volts. However, stator and the frame and then operat- This should clear the problem but keep they are not usually used in valve re- ing the tuning. As the unit is tuned, in mind that you are playing around ceivers, with some exceptions. I now any shorts will soon become evident here with a high voltage, so be careful. commonly use 47nF (0.047µF) 50V on the meter reading. If you don't understand exactly what ceramic capacitors on AGC lines as By placing a light behind the gang you are doing, then don't do it! replacements for leaky paper capaci- and looking along the plane of the Another problem that commonly tors. They are small and can often be plates, it should be obvious which occurs is the rotor shaft not making hidden which helps keep the set look- plates are touching each other. The good contact with the frame. This ing original. shorting plates can then usually be can cause jumps in frequency as the carefully bent back to where they receiver is tuned. It can also cause Air-dielectric capacitors should be to clear the shorts. crackles and the set may stop operat- The air-dielectric capacitors we see Sometimes, the meter may show ing. Most, if not all, tuning capacitors in vintage radios are the tuning and that a short is present but no evidence have either a metal spring bearing trimmer capacitors. And although of plates touching can be seen. In this onto the shaft to the gang frame or these items do occasionally have prob- case, there is probably a small sliver other spring-loaded contacts to ensure lems, the faults are easily detected. of metal that is shorting the gang. The good contact is maintained between The problems to look out for are best method to deal with this problem the frame and the tuning shaft. If any usually just mechanical. In tuning is to burn the short out. of these are missing, erratic tuning is gangs, for example, the rotor (movable) First, check that the gang is still almost a certainty. plates may have been bent slightly isolated from the circuit, then connect There is one last problem and that so that they scrape against the stator a 471d2 1W from the receiver's is where the gang has virtually fallen (fixed) plates. This will show up as er- HT line to the stator. That done, turn to pieces. This occurs with very old ratic tuning and crackles as the tuning the set on and rotate the tuning control gangs that have been made from poor gang is operated. It's easy to track the from one end to the other and if there is quality metal and the only answer to problems down by removing all con- a small sliver of metal causing the trou- this problem is replacement. nections to the stators, connecting a ble, there will be some intermittent OK, now let's take a look at the re- multimeter (set to ohms) between the sparks between the capacitor plates. former circuit. siliconchip.com.au OCTOBER 2006 101 D2 1N4007 charge position. This discharges the CURRENT K A capacitor so that it is safe to handle. ® Note that the discharge resistor is only rated at 1W even though the peak DI 1N4007 REG I LR8N3 0 lk 2 0+ A C) K 1 2 dissipation in the discharge mode IN OUT 1 0 0 03 ADJ is around 16W. However, this is for • S ol 3 3.3k such a short time that no damage is sustained. The high-tension (HT) DC input 100k 180k ELECTRO .1; voltage can be obtained from a work- TO BE in VOLTS ing receiver. This receiver MUST USE a REFORMED + 100k mains transformer. Do not even think of I 0µF O o 450V connecting the reformer to a transformer- UP TO 560k 10µF 10k 4.5 0V less mains-operated set - not if you want 2 390k 560k "" 450V 1W DC ld to live, that is. The reformer should be 52 connected to the receiver's HT supply 270k via high-voltage leads and an insulated terminal block. NEON©LAMP (Editor's note: if you want to build a 270k self-contained unit, the 12AX7 Valve 130k Preamp Power Supply described in November 2003 can be used to provide o- the HT. As described, this delivers a REG1 PACKAGE OPTIONS S1 S2 HT voltage of 260V but you can set this 1 - REFORM 1 25V 4 250V higher by reducing the 471(1/ resistor SOT-89 TO-252 (D-PAK) 2-OFF 2 63V 5 350V 3 DISCHARGE 3 150V 6 400V next to trimpot VR1. Alternatively, ei (TAB) 2 AB) you could modify the Valve Preamp 2 3 Power Supply to do the complete job 3 2 1 111.iF by having switchable in the Fig.1: the circuit is based on an LR8N3 3-terminal regulator. Power comes feedback network, so that various from an external high-voltage DC source - eg, the high-tension (HT) line of output voltages could be selected. a valve receiver or from the 12AX7 Valve Preamp Power Supply described Note that current limiting using a in November 2003 SILICON CHIP. suitable resistor would be required and you would need to fit a discharge circuit, to discharge the capacitor after By RODNEY CHAMPNESS reforming). A Reformer For The author's prototype reformer was built on Veroboard and housed in a small plastic case. If you build the device, remember that it works at Electrolytic Capacitors high voltages, so keep a liberal spacing between the various parts. Simple electrolytic capacitor reformer is easy to build and has A plastic case is necessary because six switchable output voltages ranging from 25V to 400V DC. of the lethal voltages present in this device. For this reason, be sure to use This simple circuit is based on an divider connected between the output Nylon screws to mount the board (no LR8N3 voltage regulator which has an and adjust terminals of the regulator metal screws should protrude through input voltage rating of 450V DC and (REG1). Switch S1 is selects between the case). An external insulated termi- a maximum current output of 20mA Reform, Off and Discharge. nal block was used for the metering - all in a TO92 package. The output current is monitored by points and I simply tighten down the Fig.1 shows the circuit details. measuring the voltage across a 1142 screws to hold the probes in place. The input to the reformer is powered resistor. In operation, each milliamp from up to 450V DC and this can be through the resistor registers as 1V Safety improvements obtained from a suitable valve receiver. on the meter. The voltage across the Editor's note: instead of using a Diode D1 provides reverse polarity capacitor itself can be measured using terminal block, we strongly recom- protection, while a neon indicator in a digital multimeter. mend using recessed banana sockets series with a 5601d/ resistor across the When reforming is complete, S1 for the metering points. These can be supply line warns users that a high, is switched to the Off position. This mounted on an internal bracket and potentially fatal, voltage is connected allows the operator to observe how suitably recessed inside the case to to the device. quickly the capacitor discharges. The eliminate the risk of user contact with The reforming voltage (ie, the volt- slower the voltage decreases, the less high voltages. It's then just a matter of age applied to the capacitor) is set by leakage there is in the capacitor. making up some high-voltage meter switch S2 which adjusts the resistive Finally, S1 is switched to the dis- leads with matching banana plugs. 102 SILICON CHIP siliconchip.com.au

The author's prototype used an external terminal block to provide the voltage and current metering points. A better (and safer) scheme is to use recessed banana sockets instead, along with some suitable test leads — see text. Similarly, use recessed banana sockets for the high-voltage output terminals and make up some output leads with banana plugs at one end and fully-shrouded crocodile clips (with high-voltage insulation) at the other end (see text).

6O 350 03 -11{) arson.. Where To Purchase The LR8N3 or s(i-AaGE The LR8N3 featured in this article can be purchased from Wagner Serv- ices (WES), 140 Liverpool Rd, Ashfield, NSW 2131. Orders can be phoned through Similarly, we strongly recommend connected to the set's HT line for that to (02) 9798 9233 or faxed to (02) 9798 that recessed banana sockets be used matter). 0017. The part number is LR8N3-G and it for the high-voltage output. A pair of Now switch to the reform position is priced at $4.98 plus postage and pack- high-voltage output leads (one red, and apply power to the reformer. Ini- ing. Payment may be made by cheque, one black) can then be made up, fit- tially, the current will be about 12mA money order or credit card. ted with matching banana plugs. The but will quickly drop as the LR8N3's other ends of these output leads should be thermal protection circuit kicks in. If fitted with fully insulated (fully shrouded) the capacitor is reforming, the voltage The procedure for reforming an crocodile clips (also called "safety croc across it will slowly climb until it electrolytic capacitor out of circuit is clips"). reaches the reforming voltage. virtually the same. Make sure that the You can buy fully shrouded croco- Finally, when reforming is com- capacitor is securely located on an insu- dile clips with high-voltage insulation plete, turn off the power to the re- lated surface, preferably inside a plastic from RS Components (www.rsaustral- former and switch S1 to the Discharge container). The whole process can take ia.com). WES may also have them. position. This will discharge the cap- up to around three minutes, depending DO NOT use conventional crocodile acitor and make it safe to remove on how much reforming is required clips with exposed ends (and minimal the leads but you should always use and the size of the capacitor. insulation), as shown in the photo. a multimeter connected directly to One limitation of this unit has is that Remember — we are dealing with high the capacitor's terminals to confirm the reforming current isn't very high voltages here. that it has indeed discharged before but if the capacitor can be reformed, touching it. it will get to the selected voltage in Using the device Don't simply rely on the discharge time. It also can not handle 525V and A HT filter capacitor in a receiver circuit — if the discharge resistor goes 600V electrolytics but can only reform that's being restored can be reformed open circuit, the capacitor will still them to about 400V (depending on the in the following way. First, remove all be charged. applied HT voltage). SC valves from the receiver and check that there are no shorts or bleeder resistors across the HT line. Alternatively, you MaJdffail can simply disconnect one lead of the This electrolytic reformer circuit operates at lethal voltage. DO NOT build capacitor from circuit. or use it unless you are experienced at working with high voltages and That done, switch S1 to discharge, understand exactly what you are doing. connect the reformer to the capacitor and select the appropriate reforming Note that the leads to the capacitor operate at high voltage and that a fully- voltage (it must not exceed the voltage charged capacitor can deliver a potentially fatal shock. Always discharge rating of the electrolytic that's being the capacitor before disconnecting it from the reformer and use your reformed — or any other capacitors multimeter to confirm that it has indeed discharged before touching it. siliconchip.com.au OCTOBER 2006 103