128H Re-surround Problem-Advice needed

[evans224]

After doing maybe a dozen re-surrounds,I have run into my first problem (other than a messy job the first time around). I got a pair of 4412's cheap with bad surrounds on the 128H's. One of these came out just fine. The other I noticed had a rub in the voice coil area before I started. I figured that it would go away after re-surrrounding and centering the cone. While I was running the test tone after gluing it up, it was fine at the required volume. All good, right?

Wrong-I installed the drivers the next day, excited to fire up these things for the first time. I put in Blue Traveler's "Four", and turned it up-there was a rattle, big time at high volume. I could press on the surround to make it stop. Not good.

Well, what I did was use a blow dryer and a butter knife to gently pry up the surround from the frame. I commenced gluing it down again with the test tone playing, being as careful as I could to center the cone.

After everything dried, I could still feel a rub when I gently pushed up on the cone. So it was no surprise when I tried it out that at high volume, there was that rattle again.

Do I need a re-cone, or should I try to re-re-re-surround again? By the way, I really like these speakers! Even with the rattle, they are excellent. Very precise, excellent imaging, and they can move some air at the bottom end.

[Allanvh5150]

The last pair of these that I re-foamed and re-spidered, on one speaker there was a rub at first. When I took the cone out to put the new spider on I looked into the gap and there were some small peices of metal in there. I removed those and all was good again. The voice coil was in good condition.:)

[evans224]

I'm going to try to resurround and hope for the best. I may be looking for a 128H around here soon.

[boputnam]

I got a pair of 4412's cheap with bad surrounds on the 128H's. One of these came out just fine.


I'm going to try to resurround and hope for the best. I may be looking for a 128H around here soon.I don't think it is the surrounds causing the problem.

Your first post indicated both woofs were affected. Do you know if the prior owner had them reconed, er, badly?

I suspect it is the spider that is misaligned. You need to loosen and reposition it. Proper surround positioning cannot compensate for a poorly aligned spider.


Do I need a re-cone...?Yup. Sorry...

[evans224]

Thanks Bo. I have not had this problem before, and I suspect you're right. I'm going to try again, though. This is the second set of "deals" in a row that are going to turn out to be not such great buys. The L100T's I picked up for $85 are going to cost me around $400 if I want to do them correctly with new veneer, and the modified x-overs (I'm leaning toward parting them out. The wife is tired of looking at them with half of the veneer removed). These 4412's will run about the same if I recone for $200 or so.

[evans224]

Re-re-resurrounded-no go. Needs a recone. How would I know if the cone was rubbing before I bought them? Or is thqat for a different thread?

[grumpy]

Tough question. Assume the worst if the foam is gone... pay accordingly.

Sometimes you're pleasantly surprised. That's what I do.

[boputnam]

How would I know if the cone was rubbing before I bought them? You've done too much to these to now go back and question their condition on sale to you. Sorry.

Just move on and get them serviceable! :yes:

[grumpy]

Ah, .. I read the question as being for future reference.

[evans224]

Ah, .. I read the question as being for future reference.
It was for future reference. I've purchased a half dozen pairs of speakers that had woofers in worse shape than these and had no problem. I just wondered if there was a way to check next time.

[grumpy]

If you have local access (vs asking a seller to test remotely), one
-could- carefully check for rubbing as well as looking for changes in
voice coil resistance while exercising the suspension while keeping care
to keep the cone 'flat' in relation to the moving axis (more difficult than
it sounds). Other suggestions have been made in other posts, e.g., looking
for sunk or asymmetric spiders (what centers the voice coil). ...

Still not a 100% guarantee, which is why I assume the worst.
Hope that helps some.

[boputnam]

I just wondered if there was a way to check next time.Ah, it WAS for future!

Do a sweep, or at-least, get a 30Hz tone run into the driver at low voltage drive and listen. If the voice coil is ruint or out of shape in anyway, it will rub - you will be able to hear it. :(

[4343]

Ah, it WAS for future!

Do a sweep, or at-least, get a 30Hz tone run into the driver at low voltage drive and listen. If the voice coil is ruint or out of shape in anyway, it will rub - you will be able to hear it. :(

Usually, but:

I just refoamed a pair of 122A's that turned out to be a lot of fun.:biting:

The first one DID rattle with the tone, and I was able to get the still-wet glue apart before it hardened, then took the cone out to check the coil. It had a half turn of wire sticking out the back! I was able to unwrap the rest of the turn and fix it, so went on to the next one. No problems at all with the tone at low or medium levels, so after drying the two woofs go back into the 166's for some listening. About an hour in, I decide to test them at a "High" level for the first time, and heard a popping sound. Turned out the coil looked just like the other one, just the wire was not sticking out so far...

[delshadowrio]

I have refoamed many 128H woofers. I have had pairs where the vent foam broke down in the magnet vent hole and gummed up the voice coil. I was successful in cleaning them by using deoxit spray into the voice coil from the front with the dust cap removed and from the rear through the magnet vent. Spray and gently move the cone up and down. Be patient and do this as many times as it takes. Gently vacuum between sprays. It always seemed more on 128h with the white cones than the 128H1 with the black cones.

This is if that is the problem. You can see it when the dust cap is off. After that cleaning, I have not had a problem at any volume.

MM

[SMKSoundPro]

Is it just me, or does anyone else advocate removing the dust cap to refoam?

I see where the removal of the dust dome is important to assertain the quality of the VC, crap in the gap, blistering...etc.

In this manner, the VC can be properly shimmed and held in the proper level in realtion to the top plate, then new foam installed and left to dry. Do not fuss with it.

Then, when all is dry, sweep with sine wave test tone, if okay install dome, if not okay try again, or recone with new kit if available.

I understand the idea of Rick Cobb's style of 30 hz tone and not removing the dust dome, but I have seen too many blistered coils in my day to recommend this process to everyone.

Scotty.

Picture below from 2225 with a broken loose ferrite. Used as a view of a vc on pole piece.

[boputnam]

I see where the removal of the dust dome is important to assertain the quality of the VC, crap in the gap, blistering...etc.Scotty, I've gone both ways. ;)

In general, I find that removing the cap actually usually ends-up with more crap in the gap. Then, it must be cleaned. Plus, I suck at getting a "Giskard Bead" on regluing, so there is a real reluctance on my part to enter that trouble.

I run a sweep - if I hear no problems, then I don't cut the cap. Hell, a refoam is basically a lousy repair solution to begin with, but it can add life at low cost. If the sweep suggests deeper problems, then I cut.

Almost always now-a-days, with increased experience and knowledge, I opt for recone if the kits can be had. :)

[Goodwill_HiFi]

......Then, when all is dry, sweep with sine wave test tone......

How does a person go about doing that? I've heard people mention it before, but have never asked....... so I'm asking now :-)

I have an idea how to do it, based on what I did in school today, but don't know specifics. What kind of voltage would I run through the driver? What would I look for on the Oscilloscope..... assuming that's what I'd be using. How do I "sweep"..... I assume you are referring to 20-20K?

Do you know of any tutorial online that would give details?

I have a driver I wouldn't mind checking out, since if I tap on the cone it rings a bit...... I've wondered if the coil is in poor shape.

[grumpy]

If you have a look at this as an example:

http://www.jblproservice.com/pdf/Studio%20Monitor%20Series/4412LR.pdf

for the in-system sweep of the woofer (128H-1), you'd run a sweep
generator manually from 20Hz to 1.2KHz at 6.0V (not clearly stated,
so I'd conservatively assume peak-to-peak until someone here says
otherwise), and -listen- for any non-fundamental related noises.

Note that the tweeter sweep range doesn't go below 2KHz... you
could easily damage a driver by exceeding it's lower operating range.

You might need a line-level generator and a small amplifier
to get to the 6V level. I'd suggest you start at lower levels when
initially practicing and if testing an unspecified driver.

----------------

I should add that if testing outside of a cabinet/system, keep the levels down
as 1) there's no cabinet loading so excursions can be large enough to damage
things even if nothing much can be heard (rear/front cancellation at low freqs).
2) the input levels shown in the tech manuals will produce a very loud sound...
so bring up the level gradually.

[evans224]

Almost always now-a-days, with increased experience and knowledge, I opt for recone if the kits can be had. :)

I'm very hesitant to try cutting the dust cap. If I do, and I mess it up, I'm just going to re-cone anyway, right? I mean, I haven't made anything worse, have I? There must be a tutorial on this somewhere here.....

[SMKSoundPro]

I'm very hesitant to try cutting the dust cap. If I do, and I mess it up, I'm just going to re-cone anyway, right? I mean, I haven't made anything worse, have I? There must be a tutorial on this somewhere here.....

Well, basically yes. If a recone kit is available, and if they are, may be a couple hundred bucks, for each driver. If you recone in pairs, which has been suggested by other forumites, can be cost prohibitive to some.

So, the cheaper half-vast option is to re-foam the original cone, hoping there are not any other cone problems.

In my experience here, a used speaker is just that...used or really used up. I am talking about mainly JBL Pro drivers. Home hi-fi drivers probably don't get worked that hard by real audio guys and gals. So, the prevailing business is to re-foam what you have, since the speaker manufacturer is long out of business. That is the beauty of JBL which really services their line of products, within reason.

If you choose to do a refoam of a particular driver, do it. There are many tutorials. If you want a real brand new "tire" to put on the rim, take it to a JBL service center and have it reconed with a fresh kit. It will last for another 40 years and you can amortize that cost over 40 years and it works to a few pennys a week.

Are you getting this? Recone = perfectly new! Refoam = new foam, old VC, old spider, old driver with a questionable past. (insert Driving Miss Daisy reference here)

Bottom line:
If a recone kit is available, a full recone by a JBL trained reconer is the way to go!!!

If a recone is not available, re-foam is an option that can provide fairly good results, but I consider it a band-aid fix at best.

I consider ALL JBL drivers on Ebay just a basket. Not a working driver. I have bought too many "perfectly working" drivers and found they had dropped coils, blistered coils, shattered diaphragms, shifted pole pieces...etc. So, I know i am only buying a basket that will need to be reconed. Only in this manner am I insured a real, true JBL spec'd product.

Scotty.

[Goodwill_HiFi]

If you have a look at this as an example:

http://www.jblproservice.com/pdf/Studio%20Monitor%20Series/4412LR.pdf


What if I don't have specs available? ...... suppose just keep it low.



for the in-system sweep of the woofer (128H-1), you'd run a sweep
generator manually from 20Hz to 1.2KHz at 6.0V (not clearly stated,
so I'd conservatively assume peak-to-peak until someone here says
otherwise), and -listen- for any non-fundamental related noises.
So I'm listening, not reading an instrument?

I guess I'm going to need more specifics..... at least I have a few words I cant search now!

[boputnam]

What if I don't have specs available? ...... suppose just keep it low.

So I'm listening, not reading an instrument?

I guess I'm going to need more specifics..... One very useful device for this purpose is the Function / Pulse Generator 2001a (http://www.globalspecialties.com/2001a.html) by Global Specialties. You can select the wave type, frequency (or sweep) and voltage drive. This is a fine alternative if you don't have other need for an oscilloscope. And yes, you are listening... :)

[Goodwill_HiFi]

One very useful device for this purpose is the Function / Pulse Generator 2001a (http://www.globalspecialties.com/2001a.html) by Global Specialties. You can select the wave type, frequency (or sweep) and voltage drive. This is a fine alternative if you don't have other need for an oscilloscope. And yes, you are listening... :)

Yes, a Function Generator! That's what I used briefly the other day at school, but couldn't think what it was called!

So what do I listen for? I've missed rubbing coil before on drivers that had no surrounds...... new acquisition, and could wait, and didn't hear anything bad, at first.

This test is starting to sound kind of subjective.

[boputnam]

I've missed rubbing coil before on drivers that had no surrounds...... new acquisition, and could wait, and didn't hear anything bad, at first.Really?

It's not a certain test, that's for sure, but I've had no trouble discerning mechanical rubbing from cone flutter. Keep the voltage drive down, and do this in a dead quiet space. Slowly sweep the planned band pass of the driver. However, if there is rubbing you will hear it at/near 30Hz, almost for certain.

[Goodwill_HiFi]

Really?


Yeah, I was listening to them for awhile the day I got them, and after a bit I started hearing it when the bass would go down low. On the other had I have a pair of drivers where it's difficult to get them to not rub, while pushing on the cone with my finger. I've ordered surrounds, but I'm waiting for warmer weather before I break the MEK out to clean up the old glue. Will MEK remove paint? The first set it won't matter since the frames aren't painted...... but second set is.



It's not a certain test, that's for sure, but I've had no trouble discerning mechanical rubbing from cone flutter. Keep the voltage drive down, and do this in a dead quiet space. Slowly sweep the planned band pass of the driver. However, if there is rubbing you will hear it at/near 30Hz, almost for certain.
Cone flutter?

I suppose if I'm paying better attention in a quiet setting it'll be different.

[evans224]

Are you getting this? Recone = perfectly new! Refoam = new foam, old VC, old spider, old driver with a questionable past. (insert Driving Miss Daisy reference here)
Scotty.

I got it, loud and clear. As I said earlier, I have refoamed successfully a dozen times, this is my first problem. I have heard of cutting the dust cap, and shimming to center the VC, but I've never tried it.
Recone, here I come. I really like the 4412 sound, and I can pass them on to my son when I'm done with them in 40 years.........
I

[SMKSoundPro]

:applaud: Now you're getting it!!!

Scotty.