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John Bau suggests in his interview with Oswaldo Martinez at EnjoyTheMusic.com that replacing the electrolytic capacitors with newer units is a good idea. Electrolytic caps do deteriorate and Bau suggest after 10 yrs that these may need to be replaced. There are several posts on SLUG where caps have physically opened up, much like a leaky battery. When this happens, you most likely will hear a difference and know there is a problem. It may even appear you have a bad driver, when all that is wrong is a dead capacitor. Caps may sound different as they age and deteriorate, but it may not be obvious when listening and there may be no visual queues.
Bau also suggests that metalized polypropylenes capacitors would be a good way to go. They don’t deteriorate and sonically they are very neutral. While this is technically a good idea, it can be difficult to get polypropylenes with the characteristics needed for the existing circuit.
In addition to the labeled value, capacitors have electrical characteristics that affect the way they interacts with other components in the crossover circuit. Metalized polypropylenes have dramatically different (ESR) than their electrolytics counterparts. This means that the crossover point, notch filter, etc will each need new values to maintain the phase alignment and frequency response of the originals. My advise, stick with the electrolytics, as all the original circuitry has been optimized for these.
There are some mylar caps on the boards, they are the smaller values and look different than the electrolytics which appear more battery (or can) like in construction. You should not need to replace the mylars for age reasons.
Capacitor Values
While caps have a stated tolerance from 1% up, Spicas used the less expensive 10% variety. Apparently, Bau used this to his advantage hand measuring and then binning like values. As with caps the drivers are also built with a tolerance range. The best practice is measure each cap you have (you will need a capacitance meter), and replace those with ones that measure the same. Caps that are connected in parallel combine electrically appear as a single value. Adding values together (1.5 uf + 0.5 uf = 2 uf) can help you achieve the more exact results. I suggest you measure each electrolytic and the mylar that parallels it as a unit and use that as your target replacement value.
Some Capacitor Ordering Points
If you are just looking to repair your Spica's I would match and replace the existing caps with as close a match as possible. Spica used a lot of Bennic caps which are still widely available. Electrolytics come in polarized (for DC applications like power supplies) and non-polarized (for AC applications like speaker crossovers). You will want non-polarized (or bi-Polar) caps. There are also differing lead configurations for caps, you will see “radial” and “axial” varieties in the cap listings. Radial have both leads extending from one end of the unit, Axial have a lead extending from either end. While electrically identical, you will want Axial. leads. The factory electrolytics caps were rated at 50 volts (50V) so stay with those rated at 50v or greater.
In addition, to voltage and capacitance, most part listings will also include the physical dimensions of the cap and the lead length. You need to be sure the caps are short enough to fit the existing space and not so large in diameter as to interfere with adjacent components.
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