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Old 29th Nov 2016, 1:30 am   #1
DKW F102
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Default '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 !! Many thanks.
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Old 29th Nov 2016, 1:41 am   #2
dave walsh
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Default 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!

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Old 29th Nov 2016, 1:42 am   #3
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Default 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.
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Old 29th Nov 2016, 2:00 am   #4
DKW F102
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Default 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 !
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Old 29th Nov 2016, 4:02 am   #5
Refugee
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Default 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.
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Old 29th Nov 2016, 4:50 am   #6
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Default 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.
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Old 29th Nov 2016, 8:25 am   #7
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Default 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.

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Old 29th Nov 2016, 9:40 am   #8
David G4EBT
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Default 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.
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Old 29th Nov 2016, 9:43 am   #9
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Default Re: 'THAT' Capacitor. What is it ?

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

Lawrence.
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Old 29th Nov 2016, 9:55 am   #10
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Default Re: 'THAT' Capacitor. What is it ?

Agreed,it is just a grid coupling cap.
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Old 29th Nov 2016, 10:30 am   #11
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Default 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.
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Old 29th Nov 2016, 10:32 am   #12
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Default 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.
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Old 29th Nov 2016, 10:42 am   #13
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Default Re: 'THAT' Capacitor. What is it ?

Quote:
Originally Posted by Dickie View Post
It's more than silly, as shown above it's opaque and confusing if you're not in the know.
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.

Last edited by ms660; 29th Nov 2016 at 10:55 am.
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Old 29th Nov 2016, 10:53 am   #14
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Default 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.
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Old 29th Nov 2016, 11:54 am   #15
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Default 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.
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Old 29th Nov 2016, 12:09 pm   #16
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Default 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
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Old 29th Nov 2016, 12:13 pm   #17
Ted Kendall
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Default 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.
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Old 29th Nov 2016, 12:20 pm   #18
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Default Re: 'THAT' Capacitor. What is it ?

Quote:
Originally Posted by Ted Kendall View Post
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.
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..
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Old 29th Nov 2016, 2:33 pm   #19
Ted Kendall
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Default 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.
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Old 29th Nov 2016, 2:49 pm   #20
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Default Re: 'THAT' Capacitor. What is it ?

Quote:
Originally Posted by Dickie View Post
It's more than silly, as shown above it's opaque and confusing if you're not in the know.
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!
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