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4x Woofers


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In a recent thread a question as to whether AR-4x woofers ever had foam surrounds came up...

In the past few days I have had the opportunity to dissect and examine a previously unopened pair of later 4x's; serial numbers 374,073 and 374,209. Dates and date codes on various components indicate they were manufactured in the last weeks of 1971.

It is quite apparent that foam surrounds were used at that point...see attached photos.

It should be noted that the woofers are identical in every other way to the cloth surround variety original to another pair I have on hand. These were manufactured in May/June, 1971; serial numbers of 363,970 and 358,017, suggesting a changeover to foam surrounds occurred sometime in the later half of 1971.

I have forwarded these and other details to John O'Hanlon (johnieo) to include with the excellent data he has been collecting on this model.




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Guest orionkc


I knew I wasn't imagining this...One of mine had a cloth surround (Serial # 380725) and the other had a foam surround (Serial # 390851) which was shot...Mine were 1172 apart and had a date code of 45th week of 1971 on the cloth surround. The foam surround numbers were not ledgible but were stamped in ink. The cross hatch woofers I replaced them with that I won on an ebay auction were a perfect replacement.

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Hi Rick,

I thought I had seen 4x's with foam surround woofers auctioned on Ebay from time to time, but wasn't sure if they were for real until the subject came up in your 4x cap thread. Coincidentally this pair became available and I grabbed them to take a closer look.

The Sprague "Compulytic" caps in mine have date codes indicating manufacture in the 48th week of 1971, but the woofers, like your foam surround specimen, are not dated. Only one tweeter date can be (somewhat) read, and I believe it says Dec 5, 1971. Another observation is that #374,073 has the wooden framed grille and #374,209 has the masonite type.

It appears that AR was using both types of surrounds for awhile. The woofers of that era appear otherwise identical, and the crossover and cabinet stuffing remained the same.


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>Roy, are you saying that AR-4x's were using Sprague

>"Compulytic" caps in 1971, while other models were

>still using Chicago Industrial units?

Hi Brad,

It is hard to say when the Sprague Compulytics began being used and the Chicago Industrial Condenser Corp caps completely disappeared. Several 3a's I have worked on had 50uf and 150uf Compulytics as mid and woofer caps along with an Industrial 6uf block cap for the tweeter.

I have a Compulytic here from an early '71 4x marked as being manufactured in the 3rd week of '71, and an Industrial dual 20/24uf block cap from a 4x (#358017) dating to late May '71. Other 4x's #'s, 363970, 374018, and 374209 all had Compulytics.

See attached photo of the dual cap from #358017. The unused black 24uf lead was cut flush with the cap. This was the first time I have seen one of these in a 4x.

It should also be noted that the Compulytics in the (3) 4x's above (as well as many other earlier Compulytics I have measured in the past) were well within spec. The Industrial Cond. cap in the photo was bad.

AR must have phased out the Chicago Industrial caps very gradually.



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>It should also be noted that the Compulytics in the (3) 4x's

>above (as well as many other earlier Compulytics I have

>measured in the past) were well within spec. The Industrial

>Cond. cap in the photo was bad.

Recently I replaced over 350 electrolytic capacitors in an electronic organ mfg. 1960; a few dozen had been replaced in '75. The caps were manufactured by a number of companies. Their condition today appeared dependent on the material used for their housing and most importantly, the type of hermetic seal. The primary failure mechanism of an electrolytic is outdiffusion of water from the liquid electrolyte. The oxide layer in the capacitior is an anodic oxide formed by a +V applied to the aluminum layer. It is in dynamic equibrium - both oxidizing and etching at the same time. If not used, the oxide becomes thin and C increases. After leaking for some years, pH decreases; ESR and C increase. Suddenly application of a large dc voltage to such a power supply cap has caused many an explosion.

The electrolytic caps that failed first used simple end caps, which dried and allowed all the water to evaporate. A typical 25-uF cap measures 0.04 uF 47 years later! In the 1980s Callins encapsulated their units in PVC tubes with either PVC or hot glue end plugs. Not many of these lived beyond 25 years, because the seal between the plug and tube delaminated. Chicago Industrial encapsulated their units in wax- or plastic-coated paper boxes with no extra effort to seal the joint around the wires. Many of these failed with dissipation factors as high as 1.

The best encapsulation and seal I observed was used by both Sprauge and Mallory. The attached cartoon illustrates why these were so good. First, the can is solid, drawn aluminum. Second the seal consists of two Bakelite "flanges" on either side of a soft elastomer gasket. This assembly is pushed into the end of the aluminum can, rolled, and crimped tightly as sketched. The elastomer was tightly squeezed against the can edge and the electrical feedthroughs. Result: a long-lived capacitor. Modern electrolytics also contain buffers to hold the capacitance constant, but have not improved upon this seal.

Even so, I would recommend changing Sprague Compulytic capacitors. The caps in my amplifier power supply filter measured within 10-20% on a hand-held C meter, but the amplifier had a great deal of 120-Hz hum. When replaced, the hum disappeared. The hum is caused by ESR.

(When opened, very little electrolyte was found.) Fifty-dollar capactiance meters don't measure ESR, because they don't measure phase of the current. The capacitance they measure may appear to be in spec, but the ESR will have increased.



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As always thanks for the capacitor insights, John :-)!

The 20uf section of the Chicago cap in my photo above measured 26uf and the DF was .14. There was a significant gap in the wax where the black lead 24uf had been snipped.

The Compulytics I mentioned above measured between 20 and 20.5uf with DF of .04. I'm going to leave a couple of them in one pair for awhile to do some comparison listening, but I completely agree regarding replacing them as a routine matter in restorations. After going to the trouble of dealing with nasty fiberglass and corroded pots, replacing the old caps is simple, inexpensive insurance. If one wants to hedge their bet and get as close as possible to the "old" sound with a new film cap (any brand), a .25 to.4 ohm resistor in series before the cap (to emulate the higher ESR of the old cap) seems to take the edge off IMO.


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