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Tintinabulum

Capacitor Tolerance

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I'm a newb. I'm recapping some old amp boards in an active speaker. I'm doing this because they are old, it's a bit of fun and I don't know any better. I've treated myself to a cheap piece of plastic which measures capacitors, seems to function OK having checked random capacitors, new and old and looks "functional". In all this I'm wondering what tolerance is acceptable or designed in for in amp boards. Looking at spec, tolerance can be + - 20%. Is that real or just what they say? Using my plastometer, should I be concerned about say 10% off? Being a newb, I'm also receptive to hints and tips...

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Re caping is one of those mythical audio things people like to do. Yes electrolytic capacitors get old and dry out and go high impedance, yes paper in oil go leaky, yes some film caps of a certain age can do strange things, this is from my limited experience.

Anyway maybe it's cooler in the UK and they last longer. Tolerance on caps are not very good, especially electrolytics.

When describing test equipment it is nice to know make model. I have one that was £16,(probably a close relative of  "LCR-T4 Component Tester Kit 9V with 12864 Green Back light LCD Display" on ebay) and it is very good but not very accurate on passives, I notice there is no tolerance Spec'ed on the above. Buy a few 1% caps and check its accuracy of you don't trust it.

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The board should be designed for the components on it(unless it's been messed with). If that is +-20% or even +80-20 then so be it, and unless there is other reason to suspect problems caps falling inside that range don't need changing, although you may wish to of course
You need to replace with same value, tolerances within the acceptable range, and same or higher voltage. It also helps/ may be essential to replace caps with new ones of the same constructional type (electrolytic, mica, film &c) and orientation if polarised.
Often with electrolytics you can increase the size, even double it in power supply filtering, but this is entirely circuit dependent so difficult to advise.

Sent from my BLA-L09 using Tapatalk

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Thank you. These are 22 years old. I was wondering if I took off a 4700 uF and it tested at 4000, does the 700 difference mean that it wasn't performing correctly in an audible way. No way to know without considering what it was doing and in what company I guess. My sense it from what I'm reading, maybe not.

Also I think I got some capacitors off eBay some time ago they are rated 1000 uF but test at 900 uF, within tolerance but all other capacitors were within a couple of percent. I guess these were possibly fakes? Consistent fakes though, all 4 reading 900. One day I'll know more... How do I start (learning more)?

(plastometer - Jingyan M6013)

Thanks All

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They all sound fine (probably), but you need ESR (equivalent series resistance) to be truly useful, especially for power supply caps. I think tolerance is only important in filter circuits (as a gross generalisation). My cheap one, which I think was based on someone's home project, and then mercilessly cloned by the Chinese, measures ESR at the same time as capacitance.

Edited by dave
Inaccurate statement.

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ESR is quite awkward to measure in high voltage (valve) circuits and the cheap testers don't really help much. I do use one however, a DER EE model and supplment it with a measure of charge/discharge rate and charge retention. If the 4700uF cap was a supply filtering cap then you would want to change it if the amp was starting to hum through the speakers, if not I'd think it was OK for purpose.

Value measured within 10% is about bog standard for caps IME, often ones that have been on the shelf will measure a bit low and pick up a bit after reforming and/or use. Leakage and ESR are more useful for electrolytics than absolute value IMHO.

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