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Old 2nd Dec 2016, 5:15 pm   #41
julie_m
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Default Re: 'THAT' Capacitor. What is it?

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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.
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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.
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Old 14th Dec 2016, 9:59 pm   #42
DKW F102
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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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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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Old 17th Jul 2019, 10:50 pm   #44
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Default Re: 'THAT' Capacitor. What is it?

Slang and acronyms may help communication amongst some people, but hinder it with others. I started my working life as a computer consultant, and have been an environmental consultant for the past 29 years, so have always had to take care when communicating with people who are not "insiders."

Why even bother testing any old paper and foil capacitors?

So many old resistors are far off of the correct value, so I often also replace all of them. Many are connected to the same locations which are de-soldered to replace the capacitors, so replacement adds very little additional effort and time.
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Old 18th Jul 2019, 5:47 pm   #45
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Default Re: 'THAT' Capacitor. What is it?

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Why even bother testing any old paper and foil capacitors?
I've noticed a different cultural ethos between the UK and USA in TV/Radio restoration, particularly when watching Youtubers like shango066. We seem to be more obsessed with maintaining originality, and sometimes have an almost reverent respect for these old chassis.
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Old 19th Jul 2019, 8:30 am   #46
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Default Re: 'THAT' Capacitor. What is it?

I agree with FStephenMasek. I'm in the process of repairing a Bush DAC90A, and have already replaced all the waxies, without bothering to test them-they are the best part of 70 years old and will all be electrically leaky. The resistors I have checked, and will replace out of tolerance ones. When undertaking these kind of jobs I metaphorically wear my 'service engineer's hat' i.e. I replace faulty components with the nearest equivalent modern ones. Waxed paper caps. I replace with yellow LCR or similar poly. wire-ended types, resistors usually with 2 watt metal film, since these are similar in size to the old carbon composition type. I don't re-stuff old components unless I'm working on a particularly rare radio.
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Old 19th Jul 2019, 9:03 am   #47
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Default Re: 'THAT' Capacitor. What is it?

That's fine if you know what you're doing and have fault finding skills, but many newbies don't. They're convinced that if they change all the capacitors in a set they'll get it going without the need for so much as a test meter. Often they'll change capacitors in the RF and IF sections having values of a few pF, spoiling the alignment. There's also great scope for fitting the wrong value capacitor and introducing wiring faults. I've done both myself

"In changing a capacitor I dislodged a wire. Where does it go?" is a frequent plea for help.

The main fault may have been something as simple as a dodgy OFF/ON switch, which is easily repaired or it might have been an open circuit transformer winding which could render the set BER. These faults could easily have been found with a meter without the need to apply power to the set.

It surprises me that some newbies are willing to change any component because they think it may be faulty, but won't take a few resistance or voltage readings.

As for record players, the simplest two valve amplifier seems to defeat some people even though they're given sound advice on testing and repairing it.
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Old 19th Jul 2019, 9:09 am   #48
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Default Re: 'THAT' Capacitor. What is it?

They want 'instant' results with everything or it goes for 'recycling'.. John.
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Old 19th Jul 2019, 9:37 am   #49
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Default Re: 'THAT' Capacitor. What is it?

Points taken, Graham and HCS. In my defence I'll point out that I have been a service engineer since 1971, and before that a hobbyist for 3 or 4 years, none of which means I'm immune to making mistakes-far from it-but over all those years if I've learned nothing else it's when to spend time testing & checking and when, by visual inspection,to simply replace things. I still replace parts such as capacitors, one at a time, and check after so doing that I've wired things correctly.
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Old 19th Jul 2019, 9:43 am   #50
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Default Re: 'THAT' Capacitor. What is it?

No need to defend yourself. I have no doubts concerning your technical ability and skills.
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Old 19th Jul 2019, 11:19 am   #51
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Default Re: 'THAT' Capacitor. What is it?

It's ironic that the replaceable parts are the ones that usually stay with any radio I attend to, whereas the there-for-life, wired-in components, are the ones which get replaced.

I have several radios where I have needed to rewind the mains transformer, and replace waxies, but the plug-in, life-limited valves are still originals!

Whether faulty or not, 'that capacitor' gets replaced as a matter of course. Modern polyester capacitors are so good, they'll see me out - and the next generation too. 40-year-old Mullard C296's would be left in place, but so far I have not dealt with a valve radio new enough to have been fitted with them from new!
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Old 19th Jul 2019, 11:25 am   #52
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Default Re: 'THAT' Capacitor. What is it?

I don't _always_ replace "that capacitor" - some Philips radios from the 50s and 60s use a ceramic capacitor there, which never seems to fail.

(watch out though for some strange Philips 'things' that incorporate two capacitors and a resistor into one unit - these are often used as IF-decouplers in detector circuits. The resistor-component can be a source of an annoying background 'rustle' sound when it goes intermittent)
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Old 19th Jul 2019, 12:12 pm   #53
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Default Re: 'THAT' Capacitor. What is it?

"You should replace that capacitor"...."Which capacitor?"...."That capacitor"....."Do you mean that one there?"...."No, that capacitor"....."Ah, got it now, this capacitor?"...."No, no, that capacitor"...."I see...that capacitor?"....."No, no, no, this capacitor"...."But you said to replace that capacitor?".... "I did but it's not that capacitor you need to replace it's this capacitor"...."So....this capacitor there then?"...."Yes"..."But you said replace that capacitor which is this one which isn't that one?"...."I did but it's that capacitor"...."Got it now, you mean replace the grid coupling capacitor?"....."Yes"...."No problem"

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Old 19th Jul 2019, 1:38 pm   #54
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Default Re: 'THAT' Capacitor. What is it?

This forum requires a post-like feature for Lawrence's post alone... (that post, not this one or those ones!)

Never forget some kit doesn't have that cap but those caps!)
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Old 22nd Jul 2019, 4:53 pm   #55
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Default Re: 'THAT' Capacitor. What is it?

Nice one Lawrence, that just about sums up the stupid annoying term

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