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Findings behind AR-7 grille


Guest Vesa

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I'm puzzled.

I started work on a pair of AR-7s a couple of days ago. Theyre quite nice, only a bit beat up. They're the birch veneer early 70's version, not the vinyl. Tons of scrathes and dents tho, so ill try to carefully remove the lacquer and maybe apply oil. (ideas welcome)

Taking it easy i went through removing the grilles and am in the process of cleaning the original cloths.

I observed the 8" woofers quite closely and found that one was bare paper/cardboard, the other was soaked in what seems like a sort of wood/ soft material glue i just used to attach some new foams on each. Not just that, when the old very deteriorated foam was completely scraped off, i found that the glue-soaked woofer cone was about 8mm (about 1/3 inch) narrower. Also on the basket (? the metal ring around the driver) i noticed that the narrower cone had had a narrower foam. As i had carefully peeled open the dust caps i saw that the one on the narrow cone had two similar dust caps glued onto another, where the other just had one. When observed from the side the two cones are clearly different in shape too, the other being more convex, numbering doesnt match even nearly etc. The magnets are only 3 digits apart though, and the cabinets are #02559 & #02587.

Now i came to think that maybe i shoulda cut down the foam on the narrow cone to achieve better performance as it had had a differing foam. But the glue dries quite fast under that lamp. ;D

So i was wondering why they didnt use similar parts for all drivers, since these were quite high end stuff back then. One would think they could have. Also i'm guessing the extra glue/whatever + extra dustcap on the smaller cone is there to increase mass to make it match the bigger one better, but then again i do not build good sounding speakers nor do i understand them...

Anyone else had similar experiences with similar (early 70's era) ARs? (if anyone's still reading :) ) Could be this is common knowledge but still surprising to me..

Also i have noted that few, if any of the people redoing their speakers are talking about glueing the original dust caps back on. Is there a reason why i shouldnt since i managed to peel them off with quite minimal clearance from the cones, and so it could be done... Just thought it would be nice to have them as original as possible all the way...

-Vesa

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The magnets are only 3 digits apart though, and the cabinets are #02559 & #02587.

Also i have noted that few, if any of the people redoing their speakers are talking about glueing the original dust caps back on. Is there a reason why i shouldnt since i managed to peel them off with quite minimal clearance from the cones, and so it could be done... Just thought it would be nice to have them as original as possible all the way...

Hi Vesa

What are the numbers stamped on the magnet on the woofers? These numbers (last four digits) tells the manufacturing date of the driver; could be that one driver has been replaced?

I think I can remember a post by Tom Tyson, where he informs that the cone on the original AR-7 is narrower than the one used in AR-6. What shape are the dustcaps on your woofers, flat or spherical?

I would re-use the dust-caps if you have managed to remove them without damaging them.

BRgds Klaus

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I think I can remember a post by Tom Tyson, where he informs that the cone on the original AR-7 is narrower than the one used in AR-6.

I found Tom Tysons post on the AR-7 woofer:

"Dave,

You are right that the original woofers were different. Locating original AR-7 woofers (original AR Part 10708-1) would not be easy unless you came across a pair of damaged AR-7s on eBay, cannibalized the woofers from that pair, and then got them re-foamed. The AR-7 woofer was special to that speaker (it even had a smaller-diameter half-round surround) when the speaker was introduced; AR later substituted a generic woofer (AR Part 1200001-1) which will work satisfactorily in the speaker, but not exactly as the original.

--Tom Tyson"

BRgds Klaus

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Hi Klaus

The numbers on the magnets are 538 7247 and 538 7250. These have been my father's speakers since the early seventies when they were carried out of the shop, and at least he has not opened them. And he says they havent been tampered with. The numbers being so close it would feel to me like they were assembled and used quite simultaneously.

Those digits might suggest the year 1972. But i dont think i can decrypt that any further, can you?

The dustcaps are both flat. And similar in material and otherwise. And glued back on by now btw.

So it seems that of the components used only the cones differ. And of course the amount of dustcaps.

-Vesa

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