'THAT' Capacitor. What is it?
 

Hello. As a beginner to radio repair with only 5 or 6 basic sets done, I have to ask this question. I keep seeing ' that' capacitor coming up in conversation. Will someone please help me by explaining which 'that' capacitor is ???
Most times it is the only description I can see of it. Is it a smoothing capacitor (an electrolytic can) ? is it a mains filter capacitor ('orrible Hunts 'toffee') ? Please help as I will never know what it does otherwise, and my learning is greatly reduced:o !! Many thanks.

Re: 'THAT' Capasitor. What is it ?
 

Just put that capacitor into "search this website" above DKW and you will be swamped with explanations. When the term first appeared on here it was used in a way that assumed everyone knew what it was but it's been such a frequent question that answers abound. It doesn't really need more I suppose but basically failure to check a 2p component can create a cascade effect that blows the output stage. More detail in the archive!

Dave W

Re: 'THAT' Capacitor. What is it ?
 

It's just a figure of speech, used on here to illustrate the destructive capabilities of a leaky grid coupling capacitor in the output stage. It is capable of damaging the output valve and transformer if left in place.

Re: 'THAT' Capacitor. What is it ?
 

Ah, brilliant thanks very much. It means something now. I'm sure I have been checking 'that' capasitor as a matter of course, I shall keep on in the same fashion. Thanks for the enlightenment !

Re: 'THAT' Capacitor. What is it ?
 

Then the confusion starts.
Guitar amplifiers have two of them in there push pull output stages. Larger radiograms also have two as do vintage cinema amplifiers.
Even small record players can have two of them such at the top of the range Pye black box.
Be aware and make sure you look after the output transformers in your sets.
Just look at the capacitor on the control grid of the output valve and test it for leakage or better still replace it with a new one without testing it first.

Re: 'THAT' Capacitor. What is it ?
 

To check it out thoroughly, monitor the HT rail volts, short the side of the capacitor that goes to the anode of the preceding valve to chassis.
If the HT rises at all, its toast, leaking, change it.

Re: 'THAT' Capacitor. What is it ?
 

"that capacitor" is a meaningless and confusing term used by many on this forum. You don't see it mentioned in any of the original service manuals.
I hate the term, and like you, when I first came across it I had no idea what people were talking about, there are lots of capacitors in a radio, which that capacitor do they mean.

The term could refer to any capacitor that happens to be the subject of a conversation. But often the writer is referring to the "audio coupling capacitor" which connects from the anode of the preceding valve (often the audio preamp) to the grid of the audio output valve.
The purpose of the audio coupling capacitor is to allow the audio signal (AC) to pass through it, but to block the high voltage DC found on the anode from getting to the grid of the output valve.
The output valve normally has a negative DC voltage on its grid, to "bias" the valve, and any positive voltage here can cause failure of any of the following; the valve, the output transformer, speaker energising coil, smoothing choke, and possibly the mains transformer.

So it is essential to change the "audio coupling capacitor" to one that has zero DC leakage so it can continue to block the positive voltages getting to the output valves grid.

Some older radios may use an audio coupling transformer instead of a capacitor.

Mike

Re: 'THAT' Capacitor. What is it ?
 

I've attached an example below.

It's part of the Bush DAC90A circuit showing the audio coupling capacitor to the grid of the UL41 output valve from the anode of the preceding stage.

Its role in life is to allow AC (audio) signals to pass to the output valve for amplification, whilst blocking any DC from getting to the grid. If it 'leaks' in the electrical sense, it allows Dc to pass to the grid. If that happens, excess current will be drawn by the output valve, which will pass through the primary winding of the output transformer, which - particularly in the DAC90A - are known to be fragile. It's therefore a good example of what can happen if - as is often the case - someone can't resist the temptation to 'switch it on to see if it lights up'. That's often followed by 'it lights up but I can't hear any sound'. You won't if the output transformer primary has burnt out.

Given that DAC90As generally command prices far above their worth as a radio, and they are quite costly to restore in any event, in terms of replacement capacitors, (and often, the likely need for a rectifier valve and output valve), unless 'that' capacitor is changed, it will become even costlier and more complex to restore. Particularly dispiriting for newcomers, who seem to favour the DAC90A - maybe because they're plentiful as hundreds of thousands were our on rental from about 1948 into the late 1950s, and are seen by some as 'iconic'. (As an aside, on the DAC90A - and other sets that have one - the mains suppressor capacitor across the AC input should be snipped out and either left out or replaced with an X Class capacitor. If left in place, it may self-destruct with an almighty bang. If the suppressor capacitor is removed and not replaced, it's no detriment).

Hope that helps a bit.

Re: 'THAT' Capacitor. What is it ?
 

I think the term "that capacitor" should be abandoned, it's silly.

Lawrence.

Re: 'THAT' Capacitor. What is it ?
 

Agreed,it is just a grid coupling cap.

Re: 'THAT' Capacitor. What is it ?
 

It's more than silly, as shown above it's opaque and confusing if you're not in the know.

Re: 'THAT' Capacitor. What is it ?
 

I agree too. It puts far too much emphasis on one component in the eyes of a beginner to the sport. They think, "Oh, change 'that' capacitor, and I'm away, set fixed" - and ironically it may well work loads better with just that cap changed. But the truth is, there's many more waxies in there that will be affecting performance, passing DC. Ok, not as 'dangerous' as the grid coupling cap on the output stage, but still in need of change for optimum performance. CAWOS - 'Change all waxies on sight' should be the war cry, never mind that single one in the grid output valve. Yes I know there are opponents to that approach (why I don't know) but banish those waxies and in one fell swoop and you remove a large amount of potential problems immediately. I suppose proponents of not changing all waxies are die hard originality freaks (no offence meant, each to their own). But don't forget, open up any old wireless and it's likely you'll find period correct, contemporary repairs whereby new caps are just 'dogged' in place of clipped out waxies. So it begs the question, what's original? A set with contemporary repairs (as is needed to function properly) or a set with brand new caps hidden away inside old ones? Worse still, a set where tired, resistive 'border line' waxies are not changed in order to preserve the way it looks? I know what I'd rather have, a set that functions properly with new caps neatly fitted. BTW, I used to re-stuff caps.

Re: 'THAT' Capacitor. What is it ?
 

Exactly, when folks ask what "that capacitor" means the replies usually explain that it's the grid coupling capacitor, so why not call it that in the first place, there is sometimes another capacitor that can screw up the output transformer etc on a less frequent basis.....maybe that should be called the "that not so bad as that capacitor"

Proper terms are best, less chance of confusion, ditto with "smoothers" in my opinion.

Lawrence.

Re: 'THAT' Capacitor. What is it ?
 

Not so bad as?
If it is not the output transformer it will get at the tone pot and burn that out for you while the audio output transformer is distracting you.
Evil thing.

Re: 'THAT' Capacitor. What is it ?
 

I first saw the term "that capacitor" in Radiophile magazine many years ago.

I agree that it is confusing to those who don't know, but then I suspect there are a lot of shorthand terms in various hobbies and interests that are equally as unclear. I don't think anyone is being deliberately vague, it's just typical short-hand used when people who understand the subject are chatting.

I will make this thread "sticky" (so it stays in the top part of the list) as the term is likely to continue being used and continue confusing people new to vintage radio repair.

Re: 'THAT' Capacitor. What is it ?
 

Thanks, Paul. It is an irritating, (and to my mind childish) expression. Novices have enough necessarily difficult expressions and concepts to deal with without such pointless and vague (to the uninitiated) terms as "That Capacitor".
Time for a dignified burial, I think......Tony

Re: 'THAT' Capacitor. What is it ?
 

The point here is, surely, that this one capacitor can, if seriously u/s, make the difference between a simple repair and a write-off. Reason enough to make a noise about it, I think. CAWOS is fine IF you have the experience to know what you're doing and the dilgence to avoid stupid wiring mistakes in the process - and yes, I've made them too.

Re: 'THAT' Capacitor. What is it ?
 

Well yes, but like I say it can give the impression to the uninitiated that that is the ONLY cap that needs to be changed; too much emphasis. Maybe 'CAWOS - particularly the output valve grid cap' should/could be the mantra. The phrase 'That capacitor' is like a silver bullet, a fix all, but it ain't..

Re: 'THAT' Capacitor. What is it ?
 

As the mathematicians say, the replacement or at least verification of said capacitor is a necessary but not sufficient condition for correct operation.

Re: 'THAT' Capacitor. What is it ?
 

Though, when I first came across the expression (having found a couple of faulty AF coupling capacitors in the past), I knew instinctively what it must mean!

Re: 'THAT' Capacitor. What is it ?
 

CAWOS ? I had a guess, what does it stand for?

Re: 'THAT' Capacitor. What is it ?
 

'Change all waxies on sight' according to post 11.

Re: 'THAT' Capacitor. What is it ?
 

CAMOS, surely: Mouldseals are grim but I haven't found Hunts electrolytics to be worse than anyone else's. But, yes, such acronyms seem best avoided, and this particular family of them refers to a practice I'm never likely to be tempted to engage in.

Paul

Re: 'THAT' Capacitor. What is it ?
 

Though "waxie" could itself need a bit of further explanation for the uninitiated- many mica plate types have a good dolloping of wax on them but are amongst the more trustworthy vintage types. There are always the excusable exceptions after 50-80 years though.... As they are often front end/oscillator/IFT types, sometimes with "odd" values, blanket changing could unlock a host of un-necessary troubles.

Re: 'THAT' Capacitor. What is it ?
 

And conversely there are the American "Micamold" components, some at least of which, the name and moulded lozenge-shaped housing notwithstanding, are actually waxies in disguise, and not very good ones at that.

Re: 'THAT' Capacitor. What is it ?
 

Hi again. Thanks to everyone who has explained about the 'audio coupling capacitor'. As I said in my second posting, I have been replacing the 'audio coupling capacitor' as a matter of course ( not realy knowing what it does) while changing all of the other wax paper capasitors, or to use a new confusing term ' Toffees' (so called as they are toffee colour and sticky too). I geuss that all of the resistors and capasitors should ALWAYS be checked during the restoration of a radio / amplifier. If they are not in spec then it pays to change them. I do wonder if people who talk about 'the valve sound' are used to sets with duff resistors and capasitors making them think that all valve stuff should sound like that. Anyway, I'm of to measure some 'audio coupling capacitors' ... !!

Re: 'THAT' Capacitor. What is it?
 

Just to confuse things, there are sets where the coupling capacitor to the output valve grid does not come from a preceding anode so a little leakage does little harm. We don't want folk searching high and low in the circuit diagram for a cap which is not there! (Example: Pye P45).

Re: 'THAT' Capacitor. What is it?
 

Within the Forum we soon get an understanding of the shop talk slang. I struggled with "that capacitor" initially, with exactly the same question. I can't remember how it dawned on me, quite possibly a previous post just like this one. As Dave says in post two, the ever present search box will provide chapter and verse on this and most other subjects.
Personally, I always think in terms of paper rather than either waxie or Hunts, both of which are ambiguous, for different reasons.
We also sometimes get the statement that "there is no that capacitor" in a radio. The best course of action may be that when replying to a new member, or one that has been honest and made it quite clear that they have no experience, we refrain from using that or any other slang. It still leaves the problem of people reading posts from experienced members, but there is little that can be done about that as with everything.

Re: 'THAT' Capacitor. What is it ?
 

Guess what? If you google "that" capacitor the top few returns point you to explanations on this forum......

Re: 'THAT' Capacitor. What is it ?
 

Great Scott! He's right! And I didn't even use quotation marks!

Re: 'THAT' Capacitor. What is it?
 

... Found even though "That capacitor" is the shortened form, lacking the essential central adjective which is routinely included after the "That capacitor" has been found to have taken out the output and mains transformers.

David

Re: 'THAT' Capacitor. What is it?
 

That's because the Google spider is busy indexing the Forum as we speak. This thread will appear in the list in due course, I've seen it happen within hours.
A quick check shows it reading a thread called "Winter Project" among others. The images spider is viewing "Re-stuff TCC capacitor from a Quad II". MSN is reading "Bush TV22 totally stuck see pics" and there are several others at work too.
Posts from this Forum are usually at or near the top of most related searches.

Re: 'THAT' Capacitor. What is it?
 

I suppose the real question is why more people don't think to use google or the
specific "search this forum" feature that was specially created by Paul Stenning on here to help? Yes it's nice to ask a friendly Forum but only if you're really stuck [in my view]. Most questions have been asked many times already so the answers are already there. When they are not-it's a different matter:shrug:
Dave

Re: 'THAT' Capacitor. What is it ?
 

When you say "in spec", don't just check capacitors for capacitance value, check them for resistance values too. Mainly, that's how wax caps (and other types) are 'bad', they have capacitance and resistance, there should be infinite resistance (approaching if not at, open circuit).

I firmly believe that that is the case - certainly with guitar amplifiers where players (myself included sometimes) are looking for a particularly gnarly, nasty distortion, and the out of spec and faulty components provide that. I remember the late great Rory Gallagher complaining that he had had his vintage Fender amplifier overhauled and it came back with the sound ruined. Doubtless the tech found some leaky caps and resistors out of spec, changed them and the amp was much cleaner afterwards!

Re: 'THAT' Capacitor. What is it?
 

Hi. Stevehertz ... When you say to check capacitors for resistance, how can I tell what the reststance should be ? I've only seen uF/nF/pF and the voltage on them. Is this the E.S.R I suppose ? 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. 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 ?

Re: 'THAT' Capacitor. What is it?
 

I think every subculture has its own slang. When I worked in the industry, diodes did not have cathodes -- they had business ends. And "That capacitor", said in the right tone of voice, carries a delicious suggestion of infamy which will not be lost on anyone who has seen the devastation it can wreak if leaky.

Even if you were trying to use strict scientific terms, it could still equally be described as an audio coupling capacitor or a DC blocking capacitor!

It's common courtesy to define a term when first using it around someone who may not be familiar with it says she, who is recently guilty of forgetting that not everybody has studied physics to quite the same level. But it's not as though this forum is a secret society whose members know one another by their special words. If it was, there would hardly be an open, sticky thread on the subject, would there? ;D

Re: 'THAT' Capacitor. What is it?
 

If you are checking a capacitor for DC resistance, which as mentioned earlier should in theory be near infinity, you should measure with a voltage source close to the working voltage of the capacitor. If the capacitor allows any DC current (fractions of mA's) to pass at this voltage then it is said to be "leaky" i.e. leaking current. (not necessarily leaking its contents)
The degree of leakage allowable depends on the application of the capacitor, but in the case of the audio coupling capacitor, if it shows any leakage then replace.
ESR meters like the PEAK ESR60 are really only useful for higher value capacitors like electrolytics, ideally the ESR should be zero.

Mike

Re: 'THAT' Capacitor. What is it?
 

You can still have poor leakage on a capacitor with good ESR, and vice versa. The two things are quite different.

David

Re: 'THAT' Capacitor. What is it?
 

The simple way is to connect your capacitor to your meter which should be set to read 'resistance', and set to 'high value' ohms range, the highest range it has. If you get any resistance reading - a low value is a definite no-no - but even a high value (100's of K ohms) means that the cap will be able to pass DC and should be replaced. Ideally you want to see a reading that is very high; in excess of 10 or 20 millions ohms (Megohms), or better still, 'open circuit' ie an infinitesimal amount of resistance.

Re: 'THAT' Capacitor. What is it?
 

No. ESR is Equivalent Series Resistance, which is relevant for electrolytic capacitors and irrelevant (being virtually zero) for almost all other capacitors. ESR is irrelevant for a coupling capacitor even when an electrolytic is being used, unless the ESR is astonishingly high.

For coupling capacitors the resistance we are interested in is parallel to the capacitance, so in a sense the opposite of ESR. However, it is conceivable that some cheap capacitor testers could confuse leakage resistance with ESR, as they have the same effect on AC phase shift but for different reasons.

An ESR meter is for measuring ESR.

Re: 'THAT' Capacitor. What is it?
 

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.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.

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.

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.

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.

Re: 'THAT' Capacitor. What is it?
 

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.

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.

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 :dunce:

"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.

Re: 'THAT' Capacitor. What is it?
 

They want 'instant' results with everything or it goes for 'recycling'.. John.

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.

Re: 'THAT' Capacitor. What is it?
 

No need to defend yourself. I have no doubts concerning your technical ability and skills.