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Old September 8th, 2012, 08:45 PM   #1 (permalink)
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JBL/Altec voicecoil rub aluminum dustcap

I have recently noticed a slight metallic buzzing/ghost note type of thing going on at low volumes as the note decays with some JBL D series speakers I have. One is a D123 which clearly had coilrub since being damaged in transit, I had found it serviceable when mounted with 1 side up though. The other is an 8" D208 I have in a 1957 Tweed Princeton amp. This D208 has a DCR of 3.2 ohms so I would hate to think the coil is going...but it has become quite an annoyance and does not seem to be reduced by shifting the mounting direction.

Is this a sure sign of voice coil rub...or is it a phenomena with alumkinum dome speakers perhaps? I dont notice it on other JBL's I have, but I am new to them compared to many here...

Much obliged! Thomas

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Old September 8th, 2012, 09:26 PM   #2 (permalink)
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OK lets see whats up. D123 was damaged , what was the nature of the damage ?

D208 DCR 3.2 is low , should be about 5-6 , it sadly sounds like coil rub however alli s not lost. Sometimes you can get magnetic junk in the gap that will abrade the insulation on the coil. JBL gaps are small compared to most and as a result any small particles can and will cause rubs. These are 40 year old drivers so things wil happen over time..

Try this , take the driver out and lay it on the magnet , cone up. Use your forefinger behind the cone edge and your thumb on the front and gently move the cone in and out. Do this in 90 deg segments rotating the driveras you go. You may well hear the coil rubbing. If you do hear this find where you can aggravate the rub and basiclly try to tamp down the particles of junk so it will not do it any more.
If you are lucky ther will have been a small piece of c##p and your manipulation will have displaced it. You can check this with a meter.

You have been doing the good thing by rotating the drivers.

Check also for un-glued parts anywhere..
Lets hear how you get on

M
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Old September 8th, 2012, 10:15 PM   #3 (permalink)
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Much obliged for the advice Macaroonie...
It will take me a couple days to try this out since my scedule is packed now.

The D123 had been dropped in transit I guess, the frame wasn't exactly cracked, but it had a large spot chipping away. The box had been torn trhough by the weight of the speaker falling right where the chipping is. It has significant rubbing when pushing the cone by hand.

I have heard of folks telling about a few 4 ohm D208's marked as usual 8 ohms and was hoping this was the case with the one I bought. JBL had a history of mismarking speakers intentionally so I thought it plausible. It is in great shape and sounded perfect until lately...I have not pushed it too hard, only through the 6 watt Tweed, some square wave forms with that dirty tone though I guess?
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Old September 8th, 2012, 11:04 PM   #4 (permalink)
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I am a bit confused...where do I "tamp down" to attempt to dislodge particles? On the dustcap? On the frame or magnet? How do I use a meter to check it? See if it returns to a typical 5.9 DCR?

I pulled out a spare D208 I had which has repaired cracks (by me) and a smushed dustcap but no "ghost notes" like the former...sounds pretty good.

I was saddenned to notice a small tear in the surround where I must have scraped the bolt trying to remount the speaker / shifting recently. I put some glue on it, but that is enough to bum a homebody like me out!

Good news is I thought I heard something drop out of the speaker when I was shaking it around...I will test it DCR when I come home for lunch.
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Old September 8th, 2012, 11:05 PM   #5 (permalink)
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Whats the best modern equivelent to JBL D/K series speakers guys?

Is reconing old Ampex and JBL 8" drivers thru Weber worth the time and money...???
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Old September 9th, 2012, 01:44 AM   #6 (permalink)
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Whats the best modern equivelent to JBL D/K series speakers guys?

Is reconing old Ampex and JBL 8" drivers thru Weber worth the time and money...???
it would be to me if it was mine.

i believe they say a weber california is supposed to be like a jbl d120f. but most say it is not a good reproduction.

you also might check out an ev12L for a brand new speaker.

for me i think ev and fane make the best speakers available today.

you could buy a 12in jbl d120f for what you pay for a ysed jbl d120f.

weber says the d120f is his favorite speaker of all time. you just have to be careful and check them out because different years had different wattage ratings. between 30-60 watts i believe.
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Old September 9th, 2012, 06:52 AM   #7 (permalink)
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I am a bit confused...where do I "tamp down" to attempt to dislodge particles? On the dustcap? On the frame or magnet? How do I use a meter to check it? See if it returns to a typical 5.9 DCR?

I pulled out a spare D208 I had which has repaired cracks (by me) and a smushed dustcap but no "ghost notes" like the former...sounds pretty good.

I was saddenned to notice a small tear in the surround where I must have scraped the bolt trying to remount the speaker / shifting recently. I put some glue on it, but that is enough to bum a homebody like me out!

Good news is I thought I heard something drop out of the speaker when I was shaking it around...I will test it DCR when I come home for lunch.
OK here's the deal. If there is a bit of metallic junk in the voice coil gap it can cause the symptoms you describe and can in time wear through the insulation causing a partial short circuit.
If you manipulate the cone as I described , start by trying to move it as straight up and down as you can. Rotate the speaker 90 deg and do it again etc. At some point you will encounter the rub or whatever it is. Once you find this try to aggravate the rub by skewing the cone a little with your fingers in the position I described. Basically you are trying to make it worse by skewing the coil setting. Hopefully this action will dislodge the offending junk and when you release the cone to its natural position it will be clear of the piece of whatever is causing the problem.
In all likelyhood it will be a tiny piece of metal that is held in place by the magnetic attraction , all you are trying to do is shove it over on to its side rather than it standing up and tickling the coil as it moves to and fro.
I have done this many times with about a 75% success rate.
Sometines also the insulation on the coil can bubble up if the driver has been run hard at some time in the past. Again the action I suggest just abrades the oversize insulation and reduces the coil size back to normal. A bit like removing hard skin.
Let us know how you get on.

If you want the best 8" JBL made try and find one of these in good shape

http://www.lansingheritage.org/image...le8t/page1.jpg

They get good money because they are in demand for the Hi Fi market

http://www.ebay.co.uk/itm/Vintage-JB...item19d3045bf0

Can you post any pics of the damage to the D123

Cheers M
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Old September 9th, 2012, 08:05 AM   #8 (permalink)
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Interesting on the LE8T. I had watched these but thought the thicker white cone and the rubber or foam surround might not be best suited for guitar/production applications...and they were too pricey to test drive. I think I have seen a couple different letter suffixes, and perhaps some yellowish rubber looking surrounds on this model (or its brother)? I have a D280 which has a thicker white cone, but its lower efficiency and/or 8 ohm impendance made it a poor match for my Champ, didn't try the tweed. Similar cone, I tried a 123A early model which sounded fantastic during a test in my SF Princeton but it was just not efficient enough to be practical...dimed it was still apartment bedroom levels.

Indeed I have an EV12L I love. My only D120F has a repaired tear. I have a couple D131's as well which can be nice.

It is tough to try and find vintage JBL's in good condition...so many folks selling worn out drivers as exc condition b/c of the premiums they command.

Much obliged guys, I will report on the turn out. Great to know about the massaging technique Macaroonie...I always loved those "cookies"...not too sweet.
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Old September 9th, 2012, 08:36 AM   #9 (permalink)
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You can get D/ K/ E 110 easily enough and at not silly money. They are much more commonplace. That would fit the champ if you made a new baffle board.

Here's one in a dirtbag 50 buck chinese 25w soid state combo.

http://www.youtube.com/watch?v=RmRjT7HnYik

http://www.youtube.com/watch?v=oJylH...hannel&list=UL


The 123A is a bass driver as you will no doubt know.
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Old September 9th, 2012, 09:37 AM   #10 (permalink)
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i put 4 jbl e120's in my sound city cabinet and the sound and projection is unreal. it is twice as loud as it was.
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Old September 9th, 2012, 10:43 AM   #11 (permalink)
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Just get rid of that 8" JBL and stick an Altec 755A in there.

I know guys who stick JBLs in everything. They're not a "do it all" speaker and besides, most of 'em are gettin' long in the tooth.

They used a paper voice coil former. That's '50s tecnnology but that was how you got a light efficient motor assembly. I think most of the difficulty is keepin' that big ol' voice coil round and centered.

Want a demonstration? Next time you hit bottom on a roll of toilet paper throw the cardboard tube in the toilet. Don't flush. Watch what the tube does when it gets wet. There's not a lot of difference between a toilet paper tube and a voice coil former in an old JBL, unfortunately. Get it damp or go the other way... overheat the voice coil. Bad things happen.

I spent a little time workin' at a papermill back when we still had papermills. The papermills that are still there make something special. For instance Crane makes all the money paper for the government. We bought Nomex fiber from Dupont and made tubes out of that. There's gotta be a joke there somewhere, Nomex toilet paper rolls...
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Old September 9th, 2012, 12:46 PM   #12 (permalink)
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i also have 2 x 15 altec 418-8h that i run with my 4 x 12 jbl e120's. i also like altec and have reccomended them but usually no response.

santana recones his jbl e120's with hempcones. that is what i will do when the time comes.

i had the altecs since new....1974 and have never had 1 problem from them.
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Old September 9th, 2012, 09:09 PM   #13 (permalink)
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i also have 2 x 15 altec 418-8h that i run with my 4 x 12 jbl e120's. i also like altec and have reccomended them but usually no response.

santana recones his jbl e120's with hempcones. that is what i will do when the time comes.

i had the altecs since new....1974 and have never had 1 problem from them.
That lot should make a big noise. What are you running into all this as a matter of interest. Q includes guitar amp and type of music.
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Old September 9th, 2012, 09:17 PM   #14 (permalink)
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Good thoughts. I had adored Altec 755's but thay seem to go for BIG bucks, and I had not heard of them being used in amps. I love the 425 Altec I have. I also have several D/K/E 110's I accumulated for my Tremolux, so that is a very attractive idea.

Problem is my champ has the glued baffle to switching is not an easy proposition. Does anyone know of a way to maintain the original baffle while installing a 10"? Front-mounting it somehow maybe?? Its a SF so extra holes are not an issue for me.

The D110's have been fetching ridiculous prices on ebay lately...but ebay has gotten absurd for trendy items, the K/E's are usually under $100.

As for the D208, I could not find any rubbing in performing the cone inspection described. It still measures 3.2 ohms. I wondered if sticking a magnet down there to collect stray metal would be successful? There is no screen protecting the abyss.

I will retest the @208 and inspect the D123 tomorrow. Since the frame was damaged on the D123 and it has extremely tight gaps I am guessing the frame has bent causing the rub. It was present on 3 of 4 sides when I inspected it originally.

Last edited by tatittle; September 10th, 2012 at 12:28 AM.
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Old September 9th, 2012, 09:22 PM   #15 (permalink)
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Point taken re: the paper formers and aging components. I suspect this causes greater discrepencies in reproduction than is percieved due to the lack of an original to compare. I wasn't even alive when these speakers were fresh, so I cannot even use my memory as a guide, only recordings and subjective taste.
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Old September 10th, 2012, 05:14 AM   #16 (permalink)
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Don't stick a magnet down there. It may never come out.

Download this http://www.nch.com.au/tonegen/index.html to your computerend either burn a disc or get an audio output that you can fire into your amp. If you use a 30 HZ tone you can generally hear any rubs or rattles.
You can use a sweep tone to cover the entire audio spectrum and that will shpw up any rattles or fizz from loose formers or unglued parts.
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Old September 11th, 2012, 10:03 AM   #17 (permalink)
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VERY NICE! It appears that the ghosting squeel is gone...at least at upon my 1st listen to the remounted speaker. Much obliged for your tips Mac'...

I will try the D123 later...though that one I am less hopeful about.

Of course while playing the other D208 in the interim I found it is on the way out more seriously...cutting in and out on certain frequencies when pushed harder. So the point about aging components/paper combined with unscrupulous/ignorant resellers is one to be mindful of for imprudent tone chasers like myself....
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Old September 11th, 2012, 10:12 AM   #18 (permalink)
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One other thing to remember with guitar amps and cabs , you can get rattles and such that come from loose screws / baffles / tube rattle can be sympathetic with the speaker and so on. Sometimes these noises can be elusive to say the least.
Glad you are fixed up.
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Old October 11th, 2012, 06:35 AM   #19 (permalink)
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Just for posterity...I have seen a JBL D208 - 4 ohm impedance model. Not in the flesh, but in a picture of a tweed champ. Like the 4 ohm D130's they are out there, perhaps even rarer, and rarer by the day thru attrition.
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