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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 2nd Dec 2016, 5:15 pm   #41
Join Date: May 2008
Location: Derby, UK.
Posts: 7,226
Default Re: 'THAT' Capacitor. What is it?

Originally Posted by DKW F102 View Post
I bought an E.S.R meter when I started geting my kit together but could not realy understand what it was supposed to be for, so have not used it since.
Equivalent Series Resistance (hereinafter abbreviated) is a property of any electronic component, and represents the fact that the connections are never perfect conductors. An electrolytic capacitor makes contact via the electrolyte, which can be a very imperfect conductor if the capacitor is old and worn out. In this state, its decoupling ability is reduced, since an AC voltage can exist across the equivalent series resistance; and there is now a positive feedback loop causing the capacitor to run warmer than it should, only accelerating its own demise.

An E.S.R. meter is designed to work at a very low voltage, too small to turn on a semiconductor junction, and a high frequency so the capacitive reactance is close to zero; so it can be applied to a capacitor still in-circuit without fear of false readings. It would normally be usedf or testing the capacitors in a switched-mode PSU, which are required to have a low E.S.R. for correct operation. But a high E.S.R. is a problem in any decoupling or smoothing capacitor; and also in a loudspeaker DC blocking capacitor, since it ends up stealing some energy that should have gone into the speaker.

Since a capacitor which has started to go high-E.S.R. probably also has begun failing in other ways, it can be useful for identifying faulty electrolytic capacitors in any circuit. I haven't got one yet, but I would definitely purchase one if I needed to do some repair work on a PSU or amplifier with many electrolytics. (But I probably would not buy one this close to Christmas, in case there was already one in Santa's sack ..... )

Capacitors used for DC blocking can develop an unwanted parallel resistance. This sometimes only begins to conduct at high voltage, so may not show up on a test meter using only a low voltage at the probes -- or even in real life, in a transistor circuit using only low voltages.
Originally Posted by DKW F102 View Post
I measure my resistors with my AVO 8 and my capacitors with a Greyshaw CR 50 C/R Bridge. Both the AVO and the bridge have been overhalled /aligned by a kind member of this forum. Am I missing something I should realy be paying attention to ?
No, that's right. The AVO 8 was for many years considered the "gold standard" test instrument, until it was finally displaced in the 1980s - 90s by digital instruments such as the Fluke 77; and an AC bridge measures capacitance in a way that is less likely to be affected by DC leakage resistance than a modern, digital capacitance meter. The latter type work by measuring the time taken for the capacitor to charge enough to produce a certain voltage change, at a known current. A parallel resistance will take some of the energy that was meant to get stored in the capacitor; meaning it will charge more slowly than expected, and discharge more quickly. Most digital capacitance meters only time the charging phase, and ignore the time taken to discharge; so they will tend to read artificially high when testing leaky capacitors. An AC bridge is measuring the capacitive reactance (which is inversely proportional to capacitance) directly.
If I have seen further than others, it is because I was standing on a pile of failed experiments.
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Old 14th Dec 2016, 9:59 pm   #42
DKW F102
Join Date: Oct 2012
Location: Emsworth, Hampshire, UK.
Posts: 33
Default Re: 'THAT' Capacitor. What is it?

Hi Again. I'm going to have to sit quietly and read this carefully. It seems like I have more to learn about capacitors. I always wondered why I was geting odd readings when I tried to check electrolytics. I understood when a wax paper type was leaky, but fought shy of Electrolytics because of the confusion.
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Old 22nd Dec 2016, 1:59 am   #43
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Location: Manchester, UK.
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Default Re: 'THAT' Capacitor. What is it?

I started by building TRF valve receivers when I was at school, did radio and TV at collage, worked in the TV repair industry on valve sets throughout the '70s and into the '80s, but I've never come across the term "That capacitor" until I read this thread. Now it has been explained to me I fully "get it".

I get worried, paranoid almost, about this capacitor, and the damage it can do if it goes leaky.
Good example is my old Yeasu FT200 transceiver, "That capacitor" is a well known stock fault, and the 6JS6C, finals are practically unobtainable.
Hence the paranoia.
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