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 Post subject: AR speaker crossover cap question
PostPosted: Jun Mon 24, 2019 11:54 am 
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Since I'm removing and fixing the tweeter pot in these AR 4X speakers and the cap is now out of circuit, I want to test it. It should be 20 uF but measures 31 but how do I do the leakage test on my Sencore Z meter. Would this be a non-polarized electrolytic? The cap is a big thing, like a thin deck of cards. So is the leakage test for "electrolytics" or "all others".


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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jun Mon 24, 2019 2:09 pm 
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A picture of the capacitor might help but it is probably a non polarized electrolytic. Common in crossovers. But really it “could” be a Mylar or paper or regular electrolytic as well

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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jun Mon 24, 2019 2:13 pm 
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I'd have to remove it to take a picture....let's say it's a non-polarized cap (schematic doesn't show +/-), how do I do that leakage test?


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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jun Mon 24, 2019 2:30 pm 
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I cannot help you on how to use your Sencore for that. To do a proper leakage test you'd either need a capacitor tester, or a power supply, some resistors, and a voltmeter.

If the Sencore does this test, the instructions should tell you how. You could do a go/no go test with just an ohmmeter on high ohms range, but it may take it a while to "charge" the cap to read resistance that way. Start on low ohms and while still connected, just click up through to the higher ohms scales pausing on each range to let the cap charge. Although it may immediately rule the cap BAD if it's reading significant resistance and you can just stop there and replace it.

If in doubt, just replace it anyway. Not that expensive.

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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jun Mon 24, 2019 7:32 pm 
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I've got a couple of tubular film caps here that are rated 20 uF at 100V. Tested good on 4 parameters on the Sencore. They are used so the leads are only half an inch but I could splice buss wire on to them. They are dated 1987.

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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jun Tue 25, 2019 4:03 am 
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do not waste your time testing it.

it is most likely reading high due to leakage already. i've seen it happen here many times.

replace all capacitors in vintage speakers. the tweeters will be extremely happy.

i have seen too many tweeters be damaged and/or blown from leaky electrolytic crossover capacitors. some of the tweeters were one of a kind types for mounting and size.

i use the "Dayton" film capacitors from parts express.

steve

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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jun Tue 25, 2019 12:20 pm 
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The original crossover caps in the AR 4X speakers were paper in oil, not electrolytic. They were rated for 20-uF, 50-V and were specially made for this application.
They are generally known for getting leaky with age. Most digital capacitance meters get thrown off by leakage and indicate a higher uF value than is actually there. You can use the Sencore to test for leakage; just set the applied voltage to 50 and select "all others."

If I were fixing the speakers, I probably would not use an electrolytic for the replacement unless I had to economize. The frequency response would never be the same. Instead, I would look for a polypropylene cap of the same ratings with a high AC current spec (not an overpriced Aduiophile cap). If you decide to use an electrolytic, it should be a non-polarized type.

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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jun Tue 25, 2019 8:53 pm 
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Quote:
do not waste your time testing it.


I'm retired now, no time is wasted :D ....I test them just for fun.....like "Wow, still good" or "Wow, super leaky".

Quote:
You can use the Sencore to test for leakage; just set the applied voltage to 50 and select "all others."


Super leaky.


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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jun Fri 28, 2019 9:01 pm 
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These presumably mylar but some kind of plastic, film caps read zero leakage at their rated 100VDC. They are out of a piece of avionics.

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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jul Wed 10, 2019 1:20 am 
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Quote:
I've got a couple of tubular film caps here that are rated 20 uF at 100V. Tested good on 4 parameters on the Sencore. They are used so the leads are only half an inch but I could splice buss wire on to them. They are dated 1987.

It would be helpful to see photos of the old original AR crossover capacitors and of the newer 20uF caps you have on hand.
Speakers only receive AC voltages. No DC. But because the impedance of a speaker is quite low (4 to 8 ohms), the ESR of the crossover capacitors must be much lower than that. Preferably only about 0.1 ohm when tested with an ESR meter.
Paper caps don't develop high ESR when they age. But for reasons I don't yet understand, old paper capacitors almost ALWAYS measure 20 to 50% higher in uF than their rated uF. I don't think this occurs because of resistive leakage. Rather, I suspect the paper dielectric itself gets "thinner" or somehow its dielectric constant gets larger.
Electrolytic caps definitely dry out and routinely end up with high ESR. Their uF capacitance may still measure OK. But if the ESR is above about 1 ohm, that's too high for a speaker crossover and they should be replaced.

Parts Express is a good place to get parts for speaker crossovers.

But, hopefully, the 20uF capacitors that you have on hand will work.


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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jul Wed 10, 2019 1:56 pm 
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Quote:
Paper caps don't develop high ESR when they age. But for reasons I don't yet understand, old paper capacitors almost ALWAYS measure 20 to 50% higher in uF than their rated uF. I don't think this occurs because of resistive leakage.


What you get out of a capacitor depends on what you measure it with. What you are describing sounds like a typical result when a digital capacitance meter is used. Most of the smaller, simpler ones apply AC waveforms or pulses through a resistance to the capacitor under test, then try to determine the capacitance from the charge/discharge times. Any leakage or ESR in the capacitor will increase the apparent time it takes for the capacitor to charge to a certain point so the cap will read as being larger than it really is.

Bridges are not as likely to get thrown off this way as most have the means to resolve capacitance separately from the losses. Unless a paper capacitor is has so much leakage it is practically shorted, you can still determine the capacitance and ESR or dissipation to a fair degree of accuracy using a bridge. Capacitance bridges are inherently AC devices so they are not much affected by leakage in capacitors (which is ohmic or a DC phenomena) unless it is overwhelming.

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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jul Thu 11, 2019 12:19 am 
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Chris108 wrote:
Quote:
Paper caps don't develop high ESR when they age. But for reasons I don't yet understand, old paper capacitors almost ALWAYS measure 20 to 50% higher in uF than their rated uF. I don't think this occurs because of resistive leakage.


What you get out of a capacitor depends on what you measure it with.

In my workshop I have several different items of test gear that can measure capacitance in uF. This includes an AC bridge.
All of my testing devices that give a result in uF actually read within about +/- 10% of each other when testing old paper capacitors.

I'm beginning to experiment with different ways to test the DC leakage of old paper capacitors. I don't have any of the nifty 1940's/1950's era "capacitor checkers" that apply a variable DC voltage from 0 to 400 volts to test capacitors for leakage. Maybe I will look for one of those.

What I do have is a variable DC bench power supply with a rated output of 0 to 1500V. I plan to use it to apply the rated DC working voltage to the capacitor under test while monitoring the DC voltage drop across a 1 megohm series resistor. Using Ohm's law, a 1V drop across the 1 megohm resistor corresponds to 1uA of leakage current. I will test and compare a whole bunch of capacitors, both old and modern.

To the best of my knowledge, modern capacitors in the range of 0.001uF up to 1.0uF generally have unmeasurably low DC leakage currents. But I may find something different with my "high sensitivity and high voltage" DC leakage test setup. I'll post my results.

Regards, EB


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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jul Thu 11, 2019 2:59 pm 
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The power supply-resistor-meter method is really the best way to measure leakage in capacitors. On the simpler service and kit capacitor testers, the eye tube simply opens at some predetermined current like 2-mA. Any typical electronic grade capacitor that leaks that much is bad, but it is hardly a quantitative measurement. Better testers give you meters to measure the leakage current and sometimes resistance, but most of the older ones measure in milliamperes so they are tough to use on modern caps where leakage is down at the microamp level.

This should give a little bit of insight: comparing old parts to new parts can lead to false conclusions. Those meters and dial scales on the old cap bridges are telling us that old paper and electrolytics had at least 10 time more leakage current or 1/10th the leakage resistance of modern ones when they were fresh out of the factory. Maybe at least some of them have not changed as much as we think they have. Circuit designers of the day knew about the leakage and factored it into their calculations. Then we wonder why all the voltages in a radio are higher even after taking line voltage and meter differences into account, after all the old caps are ripped out and modern ones are installed.

As for saying the caps are all 10% out of spec, that is probably true if you are measuring them on a bridge. Again, that’s more than likely the way they came from the factory. Unless specially ordered and selected (at higher cost), paper capacitors were 20% tolerance components. They were wound on semi-automatic machines with human intervention, so there was lots of variation. And with the quality control prevalent until the 1970s, manufacturers tended to err on the high side of the bell curve so even the worst case components would still be acceptable. Nobody complained if a bypass or coupling cap was a little larger than it needed to be, but they sure did squawk if it was too small.

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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jul Thu 11, 2019 5:59 pm 
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These were tested with an LC102. I don't know what test frequency it uses right off the top of my head, but after I zeroed the tester it said ESR 0.0 and DA 1%. They had short leads from PC mount, so I trimmed them to equal lengths and soldered on buss wire lead extensions. I suppose that if someone knows Dearborn specs, they can look up the part number or something. I assume at some high frequency that their inductive reactance might become evident. I have some big square blue film caps that are only rated at 3.3 uF/250V but their volume looks close to what these are.


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 Post subject: Re: AR speaker crossover cap question
PostPosted: Jul Fri 26, 2019 3:39 pm 
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Those should work fine.


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