Recording - December 2009 - 34

Figure 1 shows a typical power supply; C1 and C2 are the main filter capacitors. Note that there are usually two of them (for the positive and negative supplies), and note especially that one end of each Linear faders are usually easy: poke the schnoz capacitor is marked with a “+” sign. through the slot in the top. Give a very short schpritz, This is important; most electrolytic capacitors are work the fader up and down a few times (leaving it in polarized, meaning they only work properly when the a different position than you started), and finish up “+” terminal is sitting at a higher DC voltage than the with another very short schpritz. Wipe any stray “–” terminal (in a power supply, one terminal is ususpray off the surroundings. ally at zero volts, or ground). If you hook up an elecRotary faders are trickier. Often they have an open trolytic cap with the polarity reversed—say, with posarea on the side of the can where the terminals cone out; poke itive voltage on the minus terminal—it’s liable to explode with a the schnoz into that opening. If it’s not there, look for small gaps BANG and frighten the cat. on the sides of the can, where two parts are fitted or crimped How can you make sure your replacement cap is connected together. By putting the schnoz up against these gaps, you can properly? First and foremost, check the old cap before you get most of the spray inside the pot. As with linear faders, do a remove it. Some old caps have the “+” and “–” terminals very quick schpritz, work the fader a few times, leave it in a new labeled, some just the “+”; many modern capacitors only label position, and do another very quick schpritz. the “–” terminal. In any case, note which holes are the “+” and Rotary switches are easy, if they’re open-frame; you can usual- “–” terminals on the circuit board, and make sure your new cap ly reach the moving part with a Q-Tip and apply DeoxIt and is mounted the same way. PreservIt. (As always, exercise the switch a couple of times and What about size? Capacitors typically have two major specifiapply a bit more to the areas that were covered on the first go.) cations: the capacitance, which is expressed in microfarads (µF), For less accessible switches, or those in closed cases, CaiLube is and the DC working voltage (DCWV). The latter spec tells you the your buddy; poke the schnoz into each maximum voltage the cap will tolerate; Desoldering Tricks layer via any available opening. put on more than the specified voltage, Desoldering is trickier than soldering, because and again the capacitor will go BANG, Toggle, rocker and pushbutton switches often have very small holes in their out- there’s always the danger of applying too much filling the air with bits of foil and paste, side casings; poke the schnoz against heat and lifting the trace off the circuit board. I and further traumatizing the cat. usually avoid using Solder-Wick® for that reason; them, just as you did for rotary pots. How big are the capacitors you’re instead, I remove capacitors, resistors and the like replacing? If you don’t have a schematInstant recap by heating up the solder on one terminal while I ic, it’s usually possible to read the value Okay, not instant. A fact of life with gently pull or tilt the component. This pulls the from the side of the cap (although you electronic gear is that it usually con- lead out of the hole; heating up the other terminal might need a magnifying glass and a tains a bunch of capacitors, many of usually lets the part fall or pop right off the board. flashlight). The markings are sometimes This leaves the circuit board pad filled with sol- cryptic; “100/35” means “100µF, 35 the type known as electrolytic. These contain rolled-up aluminum foil layers der; heating up the pad and using a solder-sucking volts”. If worse comes to worst, desolseparated by a paste, which helps device helps, but often the hole remains blocked. der the capacitor—after marking the them store charge. Unfortunately, To clear it, I heat the pad, then poke a round wood- polarity!—in order to read it. when caps get old, the paste dries up, en toothpick through the hole from one side or the When rehabbing old gear, it’s a and the caps need replacing. (Dried- other.When the solder cools I withdraw the tooth- good practice to replace power supply up caps in the power supply make old pick.—PJS capacitors with the same values of gear hum excessively.) capacitance and voltage—but what if How old? A good rule of thumb is that caps over 10–15 years you can’t find exactly the same values? In this case, err in the old should be replaced wholesale, a practice known as “shot- direction of up. If you can’t find a 470µF/35V capacitor, it’s fine gunning”. It’s an annoyance, but there’s a silver lining. Over the to use 1000µF/35V, or 470µF/50V. (Go in the wrong direction, last couple of decades, manufacturers have steadily improved like 220µF or 10V, and you risk hum and bang, respectively.) the quality of capacitors along with their efficiency. That means What type of electrolytic capacitor should you choose? There you can get equivalent capacitance in a lot smaller package (or are many variations, and your choice will partly depend on the more capacitance in the same package); it also means that, if physical dimensions of the old capacitors. “Snap-in” capacitors the caps are passing audio, they’ll affect it a lot less. have a standard pin spacing of 0.4" (you can recognize them by There are three broad categories of electrolytic capacitors the largish mounting holes); “radial-lead” caps have two wires found in most solid-state gear: coupling capacitors, decoupling capacitors, and power supply capacitors. Let’s take them in reverse order. New (or maybe better) Good As Power supply capacitors Power supply capacitors, to simplify a bit, are usually the biggest ones in the box. Their job is to filter hum out of the DC rails that power the audio circuits; when they get old, they don’t filter very well, and the equipment hums. Decoupling capacitors are also part of the power supply, but they’re typically smaller than the big supply caps, and they come after the voltage regulator chips.RECORDING DECEMBER 2009 Fig. 1

Recording - December 2009

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