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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 25th Aug 2008, 4:14 pm   #1
Alan Stepney
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Default "Change all capacitors...."

That is the usual advice when restoring old equipment.
But, is it really required?

I have a Windsor (Taylor) 65c signal generator, that my father bought when they were the latest model.
From memory, around 1950 or so.

It has been used regularly ever since, sometimes often, then perhaps many months, even a few years, lying idle on the shelf.

Today I had it running and decided to check the frequency.
Absolutely spot on, or as close as one can read off the dial.

In all that time, it has had NO repairs. The only thing I have done is replace the mains lead, some 30-40 years ago.
So, all componenets, valves, etc are as the maker fitted them.

Has it stayed working BECAUSE it is used regularly, or just by sheer luck?

(The case, however, is very tatty, so my next task is to find some dark blue crackle paint, and respray it.)
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Old 25th Aug 2008, 4:39 pm   #2
Mike Phelan
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Default Re: "Change all capacitors...."

Alan
You will get all sorts of answers across the spectrum from "leave them all" to "replace them all".

I'll err towards leaving well alone, as generally test equipment uses better components and everything is under-run for long life and reliability. HT voltages tend to be very low.
(I remember a Smithy and Dick episode about this, years ago!)

Maybe just check HT electrolytics by looking at ripple on a 'scope, and/or seeing how they are physically.

Regular use is good. I've had a couple of Advance E2 sig gennies and my current B4A which I use, and no component has ever been replaced; they all came up spot-on against a counter. The valves all looked original as well.
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Old 25th Aug 2008, 5:14 pm   #3
paulsherwin
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Default Re: "Change all capacitors...."

I agree, test equipment tends to use much better quality caps than normal domestic radios and TVs.

The advice to change all wax paper caps is given because they all tend to leak sooner or later. It's difficult to test them reliably out of circuit and they can cause all sorts of weird faults.

There is normally no need to change RF caps, which generally use mica or ceramic technology and don't leak (they're often not exposed to much voltage anyway).

Paul
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Old 25th Aug 2008, 5:24 pm   #4
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Default Re: "Change all capacitors...."

If it's working, leave it alone! A common engineer's saying.

This 'change all the caps' and switch on mullarkey has problems all of it's own, so after changing 'That Capacitor' in a radio and doing a few checks, switch on. Find the faults first and then do a change of the rest if you want to. And then change one, test, change one, test.... If you change them all and introduce a fault, it could be anywhere. That way lies insanity!

Another enginner's maxim: do what you have to and no more.

Cheers,

Steve P.
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Old 25th Aug 2008, 5:41 pm   #5
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Default Re: "Change all capacitors...."

Regular use will keep the electrolytics in working order. Paper capacitors will probably be leaky but what I do is to look at the circuit and see what effect that has. It is surprising how few capacitors are in a situation where a little leakage actually matters.

In most radios it is often just the AF coupling capacitor to the output stage where leakage has dire consequences. The rest will often get away with being sub-optimal.

You may well find that your signal generator just doesn't contain any capacitors of the type that usually fail. My old Advance SG62 is unmodified and works as well as the day it was made.
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Old 25th Aug 2008, 6:05 pm   #6
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Default Re: "Change all capacitors...."

Quote:
Originally Posted by GMB View Post
In most radios it is often just the AF coupling capacitor to the output stage where leakage has dire consequences. The rest will often get away with being sub-optimal.
Capacitors on the AGC line can affect performance if they are leaky too. This is high impedance so a leak of a few megohms will have an effect.

Any waxies across the mains input will need replacing with proper class X types. They will probably have blown themselves to pieces anyway.

The other one to be wary of is tone correction capacitors across the output transformer or from the anode of the output valve to chassis as these are subjected to high AC voltages at the audio frequencies, especially at higher volume levels. If they look to be in good shape and it all works OK though then they could probably be left.
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Old 25th Aug 2008, 6:29 pm   #7
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Default Re: "Change all capacitors...."

Yep....agree with all that. My Advance generators are as they left the factory some 50 years ago (with the exception of a valveholder and main smoothing cap in one). All the other capacitors are of the 'metal clad' type and are perfect (having checked them on my Marconi bridge (and not the ones in the RF box).

I am one that advocates changing the waxies in a mains radio however. These are not of the same class as those found in test equipment. No doubt if the Advance generators were full of waxies, they would be far from perfect now. Changing all waxies is one of personal choice however. That's the way I do things. There have been occasions where I haven't changed all the waxies in a radio.

What I sometimes do to assess a radio (say for an estimate) is replace the audio coupling and any HT decouplers. Any on the AGC line and across cathode resistors can be left in as they will only have a very low voltage across them. Afterwards they can be changed for reliability. It is worth noting that any old waxies left on an AGC line can cause problems with sensitivity...a leak of only 10 meg can cause problems here. A cathode decoupler leak of 10 meg across a cathode resistor of only a few hundred ohms could probably be ignored but a leak of 10 meg in an audio coupling capacitor could spell disaster, not only for the output valve (think of the expensive UL41) but also the output transformer. The other most important one is to remove the capacitor across the mains (if fitted). If this is not done, they can explode like a rifle shot and may damage the on-off switch (in the DAC90A) and these controls are now getting difficult to replace.

If you restore televisions as well, most waxies in the line timbase and field (frame) stages will need to be replaced to avoid disasters and the boost capacitor can produce an amazing amount of wax as it bubbles away.....!

My practical advice would be to automatically change the audio coupling capacitor (as a MUST), the one across the mains (although this can be removed for testing purposes) and HT decouplers. The others can be replaced as you work through the set. Remember though that some capacitors can be a real pig to get at and if you have to remove other components to get at the one you want, you may as well replace others while you are there.







Rich.
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Old 25th Aug 2008, 6:39 pm   #8
Alan Stepney
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Default Re: "Change all capacitors...."

I'm pleased to see that some people think as I do.

If it is working leave well alone!
If not, I treat any piece of equipment as a repair, and that means to find the fault and put it right. Then, and only then, change parts that might give trouble in the near future.

As for the wax capacitors, at least they gave an easy way to stick screws to the end of a screwdriver!
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Old 25th Aug 2008, 7:42 pm   #9
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Default Re: "Change all capacitors...."

To add to my earlier comments (and I'm sure I've said the same thing before in these forums), I don't like keep taking vintage radio's apart no matter how much I enjoy working on them. Screws can get worn and there is a danger that some all-but-irreplaceable part could get damaged. Also replacing some parts can mean a major dismantling job which may have already been done at some time. When I restore a radio, I like it to keep working well for as long as possible so waxy caps in awkward places will get the chop anyway!

There is no way that I want to keep taking my prized Superinductance set from 1934 to pieces. That is why I painstakingly rebuilt ALL the paper capacitors to look original along with the main smoothing cans. It's a major job getting to some of these especially the four or five that are buried inside a metal can...



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Old 25th Aug 2008, 8:38 pm   #10
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Default Re: "Change all capacitors...."

Hello.
Yes there is two thoughts on this. I have to agree with Richard though, why keep dismantling a set when you can do the full job at one time?
Some people stress changing the Audio coupling cap and stopping there, but as Paul so rightly says caps on AGC lines are also known to give trouble, tone correction, frame oscillators, line o/p coupling, boost and boost decoupling caps etc.
I'm sure it must be easier to do it all in one go then once you have boxed up your radio or tv when there's a collectors gathering you know that you can switch on your prized items and they will work!

Trevor
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Old 25th Aug 2008, 11:49 pm   #11
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Default Re: "Change all capacitors...."

Well, I've replaced most of the capacitors in a DAC90a this year.

THAT capacitor had already been replaced. Very soon after buying the radio (sold as working), the across-the-mains waxy failed. Snipping this and replacing the fuse restored operation, but I found the AVC capacitors measured a few megohms only of insulation resistance. The IF / RF cathode decouplers measured the same, and although this is non-critical, I figured that there must be moisture in there and in time, there would be internal electrolysis and the capacitance would eventually disappear. The tone corrector (0.01uF) measured 0.05uF at 100Hz, and smaller values at higher frequencies, so that went.

The wax-covered mica capacitors all measured OK - right capacitance, no leakage, so they could have stayed (I've replaced with polystyrenes or epoxy-dipped silver micas). The capacitors in the IFT's also stayed - the alignment was pretty bang on so I didn't suspect them. The HT electrolytic is OK so that stayed. And the 2-gang variable capacitor has also stayed.

The reasoning behind all this is, exactly as others feel above, when I put the radio back together I'd really like not to have to take it apart again for 10 years, so I want to do a good, permanent job.
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Old 26th Aug 2008, 8:12 pm   #12
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Default Re: "Change all capacitors...."

Quote:
Originally Posted by Richard_Newman View Post
I don't like keep taking vintage radio's apart no matter how much I enjoy working on them.
....and today was a prime example! I was cleaning the valve pins of the I.F amp in my FB10 today (the 6BA6) when I put it down on the bench.....and it rolled off and hit the floor with obvious results !! Typically I don't have a replacement valve so that's another order.....

Should have cleaned the pins first time round.....


Rich

(Sorry for drifting off topic).
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