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

No, this is just typical of the misinformation so commonly found on the interwebs.

Most people shouldn’t be opening their equipment, let alone attempting to replace electronic parts that measure well within spec. Not only is this a waste of time, it is unlikely to solve faults and it dramatically increases the risk of damage, especially when the work is done by people who think you should replace in-spec capacitors. See where I’m going with this?

I know that this is not a popular message but I’m only interested in passing on accurate and factually correct information. My goal is to keep classic hi-fi equipment running, so please don’t shoot the messenger, just visit the Hall of Shame if you don’t believe me.

Let’s look at an example of the misinformation concerning capacitors, a misunderstood topic in general, using a real reader’s comment left under an article I wrote about the wonderful Kenwood KD-500 turntable.


This commenter was trying to help, but people read something and amplify some element of it, without the knowledge or experience needed to properly filter it. There is often some element of truth to these statements, requiring context to understand, but sometimes they are complete nonsense. The comment was 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.


The Real Answer

This statement is incorrect in various ways, so here is my reply to Mark’s comment immediately after his:

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

New Caps Measure Low

Did you know that brand-new capacitors that have been sitting around for a while will 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, but 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, not true.

As with all things technical, there’s nuance to understand. 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’ 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 in the first place. This happens because most equipment owners don’t have the equipment needed to properly test capacitors, or interpret the results even if they did. ESR is a more important measurement than C in terms of capacitor health. Most don’t even know what it is, let alone how to measure and assess it.

Replacing parts can cause circuit board damage and introduce new faults. This is especially true when non-technical folks are doing this sort of work because many have poor re-work techniques and equipment. 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!

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