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 Post subject: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Tue 05, 2017 5:10 pm 
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Posts: 18
Lets assume most of us don't have an expensive Cap tester or maybe in this case it wouldn't have helped.
Lets also assume that on an old tabletop tube radio that most times a hum indicates a bad cap or two.
I had this 50ish Emerson tube radio with a typical through the speaker hum.
The suspect part was a dual 50uF/160v paper cap. I checked the voltage on each side and it was within
the percentage allowable. When I checked each side the(out of circuit) capacitance was also within the allowable.
I replaced the cap with (2) 47uF/160v caps and suspect hum was gone.
We all know that on vintage radios caps should be replaced anyway and on this one it also had a small discharge(all the more reason).
Out of circuit I tested this further with a Tenma cap tester and it tested within limits
I also have a ESR Meter that I tested it with and it gave me the same reading as a new cap
I don't mean to take up so much space on an item that the simple answer is JUST REPLACE IT but curiosity has
got the best of me. Is there a way to test a filter cap that by simple indications tests good but is performing badly.


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Tue 05, 2017 5:46 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1080
Location: Tucson AZ
Your testers aren't feeding the same potential to the suspect cap, therefor, you are not able to see the AC riding the rectified DC that is apparent with the suspect cap in the circuit.

When you checked the in circuit voltage, did you look at the AC level?

What ESR meter did you use?


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Tue 05, 2017 6:24 pm 
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Joined: Apr Wed 03, 2013 2:38 pm
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Thanks for getting back on this question as it's about time I find out more than whats going on
below surface testing. I did not look at the ac level and I'm assuming this is whats leaking through
due to the slight deficiencyin this particular cap.
I just have a handheld ESR made by Creative Electronics

So then the true test of a filter cap can only be established by being in-circuit????


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Tue 05, 2017 6:36 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Albuquerque, NM 87123
I find it difficult to believe that a 50 year old electrolytic will have the same ESR as a new capacitor, especially when it is obviously bad.

Maybe it's time for a new ESR meter, preferably not made in China.


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Tue 05, 2017 8:13 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
There are three things that can go wrong with electrolytic filter capacitors that could cause a radio to hum. One, the capacitance could be lower than required. Two, the AC loss (dissipation) could be too high. And three, the DC leakage current could be excessive. I think that if you carefully tested just about any original, 60-year-old consumer grade electrolytic, you'd find it out of spec in at least two of the three ways.

It sounds as though you used a modern digital capacitance meter to measure the capacitance. Problem is, most simple C meters use a ramp-and-sample technique to see how fast the capacitor charges up. Problem with that is, the results can be affected by AC loss and DC leakage. So you don't know if a cap is really good, or if it has a combination of defects that makes the meter give the correct reading. And ESR meters are designed to find open caps or ones that have developed excessive internal resistance. They will give perfect results on shorted caps!

Leakage current in electrolytics is not linear or ohmic. A good electrolytic always has some leakage current; the exact amount depends on the size in uF's, voltage rating, and age. Consequently the only way to know for sure if it is within spec is to measure the current through it at or near the rated working voltage using a current-limited DC power supply.

If you are patient, it is possible to find eye tube capacitor testers like the Heathkit C-3 or the Eico 950B for no more than a new ESR meter costs. Less if you throw in the cost of a digital C meter. The old eye tube testers could make all three of the tests mentioned above: capacitance on an AC bridge, AC loss (usually expressed in power factor), and DC leakage current at various voltages. Basic as they are, those old bridges are perfectly capable of telling you why the filter caps in an old Emerson cause it to hum. There are enhanced versions of these testers around such as the Sprague TO-6/A, but they tend to be more spendy.

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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Tue 05, 2017 8:59 pm 
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Chris108 wrote:
There are enhanced versions of these testers around such as the Sprague TO-6/A...
The TO-6A is what I use for testing electrolytics.

That was before I realized the old ones are all bad, and not worth wasting my time on.
That happened maybe 30 or 40 years ago. Old caps haven't gotten any better since.

I still have the TO-6A, but only use it occasionally, and only for testing new caps.

- Leigh

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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Tue 05, 2017 10:48 pm 
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Location: Tucson AZ
I have an Eico 950b and a Solar CE tester.

I like using the Solar more than the Eico, but on either, if I want to actually watch the leakage down to the 10's of microamps, I'll run a Triplett 630 with it.


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Tue 05, 2017 11:26 pm 
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Easiest and best way to tell if it's bad:
Throw it in the trash can. If it makes a sound when it hits the bottom, it's bad.

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To be a man, Be a non-conformist, Nothing's sacred as the integrity of your own mind.
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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 12:08 am 
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Pbpix wrote:
Throw it in the trash can.
If it makes a sound when it hits the bottom, it's bad.

A time-honored technique...

and 100% accurate.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 12:33 am 
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Location: Long Island
A mind is a terrible thing. Some of us are curious why particular circuits and test instruments act the way they do, in which case testing old capacitors is not necessarily a waste of time even if the caps are going to get canned anyhow. And if you gain some experience with your test equipment on bad capacitors, you know what to watch out for when QC'ing brand-new ones.

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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 12:47 am 
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Chris108 wrote:
Some of us are curious why particular circuits and test instruments act the way they do, in which case testing old capacitors is not necessarily a waste of time even if the caps are going to get canned anyhow.

Certainly, testing of bad caps might be educational, and even instructive.

My comments were in the context of a technician repairing an old radio.
Testing old caps in that context absolutely IS a waste of time.

It's based on an assumption that some might still be good.
In the case of old electrolytics, that's a dream worthy of the best high-priced stuff.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 1:07 am 
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I think the real reason ppl try to test , double check and try to over analyze those crappy failing or dead old electrolytic caps goes beyond educating themselves. I believe it's more like a way to try to just avoid admitting it's bad and then maybe having to spend a few cents to buy a new one.
These are the same people who save string and aluminum foil and paper towels.
Sort of like the frustrating feeling of having a nice new tube that you just blew the filament on. You know the filament is open... but you still want to check it somehow to avoid actually having to toss it in the trash bin. But it's like spilled milk.. nothing left to do!
Yeah I know, it is a frustrating feeling with a nice, special, hard-to find tube. So I guess i can understand that a tiny bit...even though it's an exercise in futility.
... but when it comes to old e-caps... once you've ever tested one of them, then there is nothing more to ever gain by testing others of the same ilk... esp once you can see it's 50 years old... lol
..and especially since it's only a few cents.

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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 1:33 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Tucson AZ
Please don't try to assume that people who do things like that, that you apparently don't approve of, are doing it because they are tight a$$es trying to save a penny or two...

What a garbage generalization.

I've tested caps for giggles knowing that the cap was going right in the trash can just because I had nothing better to do at the time. What's it to you what anyone does with their free time?

And if you think that testing caps that were going to be replaced is futile, then why would you have such an expensive piece of test equipment to test them if testing them was a waste of time?


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 1:35 am 
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stratozen wrote:
Is there a way to test a filter cap that by simple indications tests good but is performing badly.

Tubes can exhibit the same symptoms as well. The only true test for either component, is in-circuit.

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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 1:57 am 
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fifties wrote:
Tubes can exhibit the same symptoms as well.
The only true test for either component, is in-circuit.
There's a very big difference.

Bad tubes are only likely to behave badly, or not at all.
It's pretty unusual for a bad tube to cause collateral damage (but not impossible).

Capacitors, on the other hand, usually fail by exhibiting high leakage, sometimes shorted. That definitely CAN cause collateral damage.

It is NEVER a good idea to apply power to any set having its original caps.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 2:02 am 
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Joined: Feb Sun 17, 2008 11:36 pm
Posts: 1909
Location: Dayton, Ohio
Sometimes a capacitor of any age can go bad. Instead of wasting time ordering and waiting for a new capacitor to arrive, when it might not be the cause of hum.

If you have a o'scope, monitor the ripple. Bridge the suspect cap with a known good one, even if it is of a lower capacitance, the ripple should reduce significantly, if the suspect cap is bad. (if the suspect cap is good, and the bridging cap is close in value, the ripple will only be moderately reduced).

If you don't have a o'scope, a voltmeter set to *a.c. volts can be used.

Charlie

P.S.
*some meters will require using a d.c. blocking capacitor!

C.


Last edited by tinwhisker on Dec Thu 07, 2017 10:05 pm, edited 1 time in total.

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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 2:24 am 
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xwarp wrote:
And if you think that testing caps that were going to be replaced is futile, then why would you have such an expensive piece of test equipment to test them if testing them was a waste of time?

Ah ha...That's exactly why I don't!
I do not have an expensive piece of equipment for testing them... for that exact reason. It would not make sense.

Years ago maybe, when capacitors were more expensive like back in the in the 30s or so ... It made more sense to find ways to test them because it might have made sense trying to salvage them.
But these days at a few cents or less then a $1 to replace them , any costly test equipment no longer makes sense.

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To be a man, Be a non-conformist, Nothing's sacred as the integrity of your own mind.
-Emerson


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 2:30 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1080
Location: Tucson AZ
Pbpix wrote:
xwarp wrote:
And if you think that testing caps that were going to be replaced is futile, then why would you have such an expensive piece of test equipment to test them if testing them was a waste of time?

Ah ha...That's exactly why I don't!
I do not have an expensive piece of equipment for testing them... for that exact reason. It would not make sense.

Years ago maybe, when capacitors were more expensive like back in the in the 30s or so ... It made more sense to find ways to test them because it might have made sense trying to salvage them.
But these days at a few cents or less then a $1 to replace them , any costly test equipment no longer makes sense.


Good for you, but your specific generalization about why people test old caps is out of line.


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 2:52 am 
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Location: Florida
Waste of time? The whole purpose of a hobby is to waste time in an enjoyable manner.

RRM


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 Post subject: Re: How to Tell if a Filter Cap is Really Bad ????
PostPosted: Dec Wed 06, 2017 3:12 am 
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Joined: Feb Sun 17, 2008 11:36 pm
Posts: 1909
Location: Dayton, Ohio
The exercise should be to troubleshoot and find the defective part. Changing a good but old capacitor or part, falls under preventive maintenance.

You may want to use the same order blank or trip to the jobber, to order the faulty part(s) as well as a replacement for the old capacitor.

Charlie


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