I’ve read that I should replace any capacitors measuring 10% under spec – is this correct?

No, this is misinformation typically spread by folks lacking the knowledge needed to parse it.

Reality Check

I need to preface this FAQ by noting that with all technical concepts, the real answers are layered. Understanding the layering comes with education and experience. Just as most people lack the experience and knowledge needed to rebuild their car’s engine or diagnose human illnesses, so it is with working on complex electronic equipment. Common sense, to the sensible.

With this in mind, DIY attempts to replace electronic parts that probably measure well within spec are a recipe for trouble. Not only are these attempts unlikely to resolve faults but they dramatically increase the risk of damage and the introduction of additional faults, as well as the risk of electrocution.

While this may not be a popular message, I’m a science educator and technician interested in passing on factual information and keeping classic hi-fi equipment running well. People will either think they know better or be pleased to have found me and it’s all good, but we’re here mostly for the second group!


The Dunning-Kruger effect is when people over-rate their level of understanding of topics they understand little about, yielding the classic: “A little knowledge is a dangerous thing”, the Darwin Awards, etc. It plays out when folks read something in a forum for example and then believe themselves to be experts based on a snippet of likely incorrect information that they don’t realise is incorrect.

The following example concerns misinformation about capacitors, a misunderstood topic in general, using a reader’s comment on an article I wrote about the wonderful Kenwood KD-500 turntable. This commenter presented something he thought he understood well but didn’t and again this is no big deal but a learning point from my perspective.

We need to be critical of what we read and the sources we trust. We also need to be mindful that time spent reading comments in forums does not an expert make. Too much time spent in the wrong places can prevent legitimate learning through missed opportunities and the need to unlearn incorrect stuff later.


The comment, made about speed issues with a turntable that were unrelated to capacitors:

A simple fix can be changing any electrolytic caps.. Always good to remove then and check there [sic] capacitance. 10% below should be changed.

Mark, commenting on one of my KD-500 articles

Mark’s statement is incorrect in several ways and I replied as follows:

Thanks for your comment, Mark. I appreciate those trying to assist others, but given that your comment highlights general misinformation regarding technical electronics repair and only encourages capacitor replacement rather than finding and fixing the issues, it’s helpful if I explain why.

Capacitors are much maligned and generally poorly understood. Capacitors should be checked, but rarely cause speed issues with these decks. New caps are typically specified to be within +/- 20% of rated capacitance, like these excellent Panasonic parts for example https://industrial.panasonic.com/cdbs/www-data/pdf/RDF0000/ABA0000C1209.pdf.

A reading of within +/- 20% is within new part specs and measurement error. Replacing capacitors that measure within 10% of spec is not only a waste of time and money, but it will not fix anything unless those parts also have a measurably very high ESR.

Most end-users also don’t have the test equipment needed to make these measurements accurately, or the experience to interpret the results and therefore should definitely not remove or change parts they cannot properly measure and assess.

Liquid Mike

Did You Know..?

Did you know that brand-new capacitors that have been sitting around for a while typically measure low for C and high for ESR? There’s nothing wrong with them, they simply need to reform, a process that happens when they are charged. According to the theory above, even brand-new capacitors that have just been sitting for a while should be thrown away and this is, of course, nonsense.

Capacitors can fail and must be replaced when they do. They can also be replaced, with the right technical understanding, to significantly improve performance, depending on where they are in a circuit. However, a 10% variance from rated spec does not constitute failure and may well be within the measurement error of the typically ordinary test gear most owners will have access to.

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That being said, I always replace ‘Swell-Long’, ‘Hung-Long’ and ‘Long-Dong’ brand caps…


All too often though, perfectly good parts are binned and replaced with inferior parts that don’t fix anything, because they weren’t broken! ESR is a more important measurement than C in terms of capacitor health. Most don’t have the equipment needed to properly test capacitors, or interpret the results even if they did.

Replacing parts can cause circuit board damage and introduce new faults. This is especially true for those working with cheap hobbyist soldering and re-work tools and lacking the experience and skill needed to work neatly. Again, a glance through the Hall of Shame shows that even people calling themselves technicians damage boards, all the time.

As always, the best advice I can give is: if you’re not sure, leave it alone, unless you are prepared to lose it!

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