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 Post subject: Replacing crossover cap in Acoustic Research AR-4X speakers?
PostPosted: Sep Sun 26, 2021 9:48 pm 
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Joined: Jun Fri 26, 2020 9:35 pm
Posts: 76
Location: 98020
Hello-

Yesterday I acquired a pair of Acoustic Research AR-4X speakers that have not been used in quite a while. Connected them to a Fisher 500B and they really don't have much output at all. I know that the treble pots can get dirty and require cleaning, but these also have 1968 electrolytic caps in the crossover that I assume are no longer operating properly. They are 20uF @ 50VDC, and are almost the size of a deck of cards. The wires coming out are blue and green and are not marked for polarity.

To replace these, I have a couple of questions. Is the old phrase "green is ground the world around" applicable here, and so I should connect negative to the green terminal? How can I be sure of the polarity? Schematic is attached below for reference.

And next question- I do not have any new 20uF caps @ 50VDC, but I have plenty of 22uF @ 500VDC. Obviously 500V is overkill, but it's what I have right here and there is definitely plenty of space in there to fit these in and leave the originals intact. Is it OK to use caps with this much excess working voltage? (I am assuming that +10% of 22uF vs 20uF will have negligible effect on performance.)

Thank you and best regards-

Troy
Edmonds, WA


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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Sun 26, 2021 10:16 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3791
Location: Seattle WA US
Crossover caps are non-polarized. The two leads are equal. You should not replace a crossover cap with an ordinary, polarized electrolytic cap. Most suppliers of speaker builder's parts will have non-polar caps available. You may want to consider using polypropyline caps rather than electrolytic. Using the exact capacitance specified by the designer is important to the resulting sound, you may have to combine several caps in parallel to get 20 mfd. (a change of 10% in C would result in a 10% change in crossover frequency). Rated breakdown voltage of the capacitor should not be a concern, as long as it exceeds 50 volts.

The AR-4 speakers are notorious for failure of the treble adjust potentiometer. I opened mine and found the problem was with the plating on the wiper contact. I cleaned the contact surfaces, but the fix did not last. Next time I open the speakers I will replace the potentiometer with a switch and some fixed resistors.

When you close up the speakers after repairs, be sure to get a good seal between the woofer basket and the cabinet. I believe the original sealing compound on my AR speakers was Scotch Strip Caulk, available from automotive paint shops.

Good luck with your project, may the results please your ears.
-Chuck


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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Sun 26, 2021 11:45 pm 
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Joined: Jun Fri 26, 2020 9:35 pm
Posts: 76
Location: 98020
Hello Chuck-

I have ordered some new caps, of the polypropylene non-polarized value, and I was able to get 20uF exactly. Should be here within a week or so, given that shipping is so slow everywhere now.

Since the original caps are not electrolytic, do they fail with age and/or disuse? These speakers have dates inside indicating production in 1968 and the caps are clearly original. Do they normally die after 50+ years like the caps in radios?

The pots in mine are wirewound; haven't investigated them yet.

Thank you and best regards-

Troy
Edmonds, WA


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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Mon 27, 2021 1:11 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3791
Location: Seattle WA US
Polypropylene caps are among the best we have available, now. But we don't have as many years experience with them as with the older technologies that we have found lacking......
I don't know the details of the caps in your AR-4x or mine. I havent replaced the caps in mine yet, as I have found my problems related to the wirewound pot.

I suggest that you feed a 2 to 5 KHz tone to the speaker, and turn the treble pot thru its range, and listen......

-Chuck


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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Mon 27, 2021 10:01 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 5822
Location: Sunnyvale CA
soloapollo wrote:
Hello-

Yesterday I acquired a pair of Acoustic Research AR-4X speakers that have not been used in quite a while. Connected them to a Fisher 500B and they really don't have much output at all. I know that the treble pots can get dirty and require cleaning, but these also have 1968 electrolytic caps in the crossover that I assume are no longer operating properly. They are 20uF @ 50VDC, and are almost the size of a deck of cards. The wires coming out are blue and green and are not marked for polarity.


As everyone else already pointed out, these are non-polarized capacitors, and a typical industrial-quality poly film cap is perfectly satisfactory. you probably want to match them as close as you can between the speakers.

However, not to be negative, but if the overall output is "low", it is very unlikely that the problem is strictly the capacitors. Usually, that will cause the tweeter output to drop first, but the rest of the output should be normal. Frequently, the woofer receives all the signal, and then the various bits of the crossover are there to just keep low frequencies out of the tweeters. So, it tends to screw up the tone balance, and make it seem very "heavy", but have relatively small effect on the overall volume.

There may be something else wrong with the speaker, or, the claimed "65 watts total music power" = which in reality is maybe 18-22 watts per channel like most 7591A amps, is not enough for the typically inefficient AR-4x. For sure test it with conventional modern speakers with 86-93 db/watt sensitivity, just so you know the amplifier is working properly. The problem with my theory is that there's hardly anything to go wrong in the speaker aside from the crossover. Maybe, the suspension is falling apart, but that usually makes it louder rather than softer (since you have removed most of "spring" force that returns it to center - that spring being the air inside the box).

Brett


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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Mon 27, 2021 11:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3791
Location: Seattle WA US
FWIW, I've never considered my AR-4a speakers lacking, driven by the 7868's in my Fisher 400. My program material does not contain bass guitar, but does include full symphony and pipe organ music.
-Chuck


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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Tue 28, 2021 2:50 am 
Member

Joined: Jun Fri 26, 2020 9:35 pm
Posts: 76
Location: 98020
Hello-

I can say that the amp is working fine; I've been listening to a pair of Bose 501 series V speakers on it for years without problems. I have only taken one of the ARs apart so far (always like to have an intact reference) and found that the treble pot definitely has some flaky/open spots and the resistance also measures over 20 Megohms in places. It also has places in the range that are OK.

But the biggest problem is that the tweeter I have removed measures open, so that is definitely contributing to the problem. It has a big red X in sharpie drawn on the back. Do not know if this is a coincidence. No idea if the other one is alright yet or not.

Thank you-

Troy
Edmonds, WA


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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Tue 28, 2021 3:43 am 
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Joined: Jul Fri 18, 2008 10:02 am
Posts: 2186
Location: near ST Louis Mo 62002
https://www.hifiengine.com/manual_libra ... r-4x.shtml ./

It might be hard to find an exact match for that tweeter , just find a pair that fit the hole and screw holes if possible .
Having a level control will make it possible to get a level match .The cross over point is quite low for a 2 way speaker 1800 HZ .

https://www.parts-express.com/HiVi-Q1R- ... er-297-417

https://www.parts-express.com/Visaton-D ... hm-292-556

https://www.ebay.com/itm/321544463881?h ... SwF1dUMtwS

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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Tue 28, 2021 4:59 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3791
Location: Seattle WA US
Back in the day, many of those tweeters were smoked by solid state amps driven into clipping. The treble output of clipping 2N3055's exceeded anything found in normal program material. The tweeters were designed to handle the power spectrum of music, with high fidelity, not for participation in frat house noise battles.

Because of this vulnerability there has been much discussion of replacements.
Google or Duckduckgo search on < Acoustic Research AR-4 tweeter >

-Chuck


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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Tue 28, 2021 6:55 am 
Member

Joined: Aug Wed 24, 2011 4:35 am
Posts: 5822
Location: Sunnyvale CA
K7MCG wrote:
Back in the day, many of those tweeters were smoked by solid state amps driven into clipping. The treble output of clipping 2N3055's exceeded anything found in normal program material. The tweeters were designed to handle the power spectrum of music, with high fidelity, not for participation in frat house noise battles.


Or, the cap in question got leaky, and fed it lower frequencies with significant power. Blown or failed tweeters are not a surprising discovery after this amount of time, and unfortunately there aren't a lot of easy solutions. You can get lots of tweeters, but getting them to match the original performance is a non-trivial problem.

Brett


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 Post subject: Re: Replacing crossover cap in Acoustic Research AR-4X speak
PostPosted: Sep Tue 28, 2021 3:40 pm 
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Joined: Dec Sun 25, 2016 5:53 am
Posts: 417
This website is the best I know of about Acoustic Research and similar "East Coast" types such as KLH, Advent, Avid, and such. Repair details, parts sources, and just plain good reading. This link is straight to the AR forums.
https://community.classicspeakerpages.n ... -research/

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