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The Surprising Truth About Old Capacitors

Do all old capacitors need to be replaced? Just a quick video to report on something surprising some of you may find useful or interesting. Today I was working on an old Dynaco PAS-3X preamp I’m restoring. It’s in pretty good shape, but apparently got plenty of use as all the 12AX7 tubes tested as barely breathing. The preamp dates back to 1971, so at almost 50 years old, I assumed that along with the tubes, all the coupling capacitors would need to be replaced as well. I started, by making a quick drawing of the two PC boards showing the positions, alignment and values of the caps I planned to replace. For fun, I tested the first one I removed for leakage, and was amazed to find that it tested perfectly. I then tested it for capacitance and again, it was spot on. I removed five more capacitors from the boards and every single one tested perfectly. At that point, I determined that all of the capacitors on the board I had planned to replace were not of the paper variety, but of a newer mylar style similar to today’s modern capacitors. I scrapped all plans of replacing them and decided to reinstall the ones I removed, and not bother testing the rest. If there are any problems once the restoration is complete, I’ll troubleshoot components on an individual basis instead of doing a blanket replacement. Speaking of blankets, I’d hate to make a blanket statement that all capacitors that look like these are reliable and don’t need to be replaced. I did a little research on these Black Cat caps made by Cornell Dublier and I couldn’t find any consistent information about how they’re made. They appear to be mylar, but that doesn’t mean all Black Cats are. They may be, but again, I’ve read too much conflicting information to be absolutely certain. Black Beauties are another type of capacitor made by Sprague which seem to bring similar confusion. The point of this video, really, is that you should test capacitors before blindly replacing them, even if you’ve read otherwise. The exception would be capacitors that are clearly made from paper, such as these much older wax/paper caps. If you see these, you really are going to be better off just replacing them and any tests you do will very likely show they’re leaky or out of spec. Ceramic caps are generally fine, but if your device isn’t working correctly, a ceramic cap will occasionally be to blame so don’t be surprised if you run across a bad one. And of course, electrolytic caps should always be tested as there’s a higher likelihood of failure there. All of the electrolytics in the PAS-3X will be replaced along with the selenium rectifier. I’d also caution you to not buy or use capacitors because of some supposed sonic benefit. Some people pay ridiculous money for capacitors, even crappy old wax/paper ones, because they mistakenly believe they’ll make things sound more transparent or pure. That’s just baloney. If two capacitors measure the same, they’ll sound the same. Don’t be fooled into buying New Old Stock capacitors because someone told you they possess some magic antique sound. If you have old capacitors that test as new, use them. If they test poorly, replace them. And when buying capacitors, get new ones of a high quality from a reputable manufacturer and seller. Capacitors don’t possess magic sound qualities, but there are genuine differences in tolerance, build quality, temperature ratings, voltage ratings and longevity. If any of these things cause a capacitor to be out of spec for the circuit, unpleasant audible problems can be created. Time for me to get back to restoring the PAS-3X. Look for a video in the future where I’ll show its restoration in complete detail. The PAS-3X will be part of a 60s/70s Hi-Fi I’m putting together which will include a Dynaco ST-70 I built, a Dynaco FM-3 tuner I’m going to build, this modified Thorens turntable, and a pair of Dynaco A25 speakers. All of which will also be featured on my channel. #capacitor #radiorepair #radio #electronicsrepair #stereorepair