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

No.

People don’t know what they don’t know, but as long as expert sources of information and professionals are sought when needed, this generally isn’t a problem. With that in mind, I’m going to help with this one, but before we proceed, note that DIY electronic equipment repair attempts significantly increase:

1. The risk of equipment damage
2. The risk of the introduction of new faults
3. The risk of personal harm through electrocution

Stay safe, and if in doubt, take your equipment to a professional.

Dunning-Kruger

The Dunning-Kruger effect describes people overestimating their understanding of topics they know little about, leading people to treat cancer with fruit juice, deny the moon landings, visit the Titanic in a home-made submarine and replace capacitors in equipment because a non-subject-area-specialist in a forum somewhere says it’s a “good idea”.

The internet has led to an explosion of ‘recappers’, poorly skilled repairers, and damaged and destroyed hi-fi equipment. The good news is that all this can be avoided by understanding that finding the right people to assist with critical, specialised work is the smartest thing any of us can do.

An Example

A reader’s comment on an article I wrote about the Kenwood KD-500 turntable prompted me to write this FAQ. Mark wrote:

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

Mark

I’ve no doubt Mark was trying to be helpful, but it’s precisely these half-truths and regurgitated opinions that can be so harmful. The danger lies in the knowledge that is missing and/or not understood. Mark has likely not owned or repaired KD-500/550s, and his comments will not only not help the original enquirer fix his turntable, but will likely lead to him damaging it.

Yes, capacitors should be removed and tested, but they should be tested for ESR, leakage and C, and certainly not replaced before finding and fixing the cause of any issues. Your average owner and DIYer cannot measure these things because they don’t own the equipment to do so, let alone to be able to interpret the results.

Capacitors are rarely responsible for speed issues with these decks. The biggest issue is the damage DIY-ing can inflict on gear. DIYers typically lack the test equipment, rework tools and knowledge needed to take on this sort of work. This leads to frequent board damage and even the destruction of equipment.

My reply:

Thanks for your comment, Mark. I appreciate 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 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: https://industrial.panasonic.com/cdbs/www-data/pdf/RDF0000/ABA0000C1209.pdf.

A reading 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, nor the experience to interpret the results and therefore should not remove or change parts they cannot properly measure and assess.

Liquid Mike

Facts (Useful)

I’ve provided a few facts here to help readers, from a specialist’s perspective and a source of factually correct information. Are capacitors important? Sure. Should all capacitors measuring “10% below” be replaced? No.

Did you know that brand-new capacitors typically test low for C, often more than 10% low? There’s nothing wrong with them; they need to reform. Did you know the average cheap multimeter cannot properly test for capacitance, ESR or leakage?

Capacitors can fail and must be replaced when they do. They can also be replaced to significantly improve performance, depending on their role in a circuit. However, a 10% variance from the rated spec does not constitute failure and is within the margin of measurement error of the test gear most owners will have access to.

Summary

All too often, people bin good parts and replace them with inferior parts, trying to solve problems not caused by those parts. You may read that one should replace every capacitor greater than a certain age, but I can assure readers that this is not factually correct

ESR is a more important parameter than C in terms of capacitor health. Proper lab-grade test and measurement equipment is needed to effectively test capacitor parameters; cheap multimeters cannot do it, and most people do not own expensive test and measurement equipment.

Replacing parts often causes circuit board damage and introduces new faults when done by inexperienced folks with poor rework skills and tools. The best way to save and rejuventate equipment is to have an expert do it, someone with hopefully a decade or three of best practice behind them.