2245 questions - buzz

[valbird]

I recently picked up a couple of JBL 4645 boxes with 2245 drivers in them. I'm trying to check things out, and here's what I've noticed:

-surround replacement done less neat than I like
-one box buzzes when the cone is tapped
-almost all the mount screws have their phillips heads cammed out

So I decided to pull the driver on the box that buzzed when tapped. Had to drill out 4 mounting screws. Noticed that the front gasket (thick rubber) was not aligned with screws, and was of two different thicknesses.

For the drill out, I kept a vacuum near the drill bit to capture the shrapnel. It took a while because I didn't want to chance breaking the bit or slipping.

Expected to find a screw or something on the back side, making the buzzing. I did not. I did find a piece of particle board.

Out of the box, the driver made the normal damped thump when tapped. Putting some bass into it, I found that it was distorting significantly with a manually induced excursion of about 3mm. Seems like too little, but I haven't checked Xmax for this driver yet, etc. Since everyone is home, I have stopped my play for the day.

I need to get some more mounting screws, as all but one I would not put back in, and 4 have been destroyed by drilling. As a result, this box will be open for a while.

Does anyone have some suggestions on how to test this, and determine if I have a problem?

As near as I can tell, the less than ideal bad foam job is not skewing the cone off center, but I could be wrong.

I did run this driver through the crossover (DCX2496) and using a cutoff of 100 Hz there was audible buzzing. However when I gave it broadband music, I could not readily detect any buzzing. For now, I will assume that the piece of wood (15mm x 3mm x2mm) may have been captive somewhere making that noise.

Thanks.

[pos]

Putting some bass into it, I found that it was distorting significantly with a manually induced excursion of about 3mm. Seems like too little, but I haven't checked Xmax for this driver yet, etc.

Hi

Xmax is quite higher than that, and I found this driver to be particularly silent during large excursion.
Did you allow the air to escape the rear vent of the magnet during your test?

In my tests, even with lots of room around the rear vent, I did hear the noise from the air in the vent before any other distortion from the moving assembly itself. My drivers had the square foil cal on top of the round vent, which does not help with air noise...

[valbird]

Got some more time with the driver tonight, and reinstalled it in the box. After installing in the box, what I found was that slight pressure at the 12 o'oclock caused the buzzing again. Changing the fold in the foam, I could alter substantially the buzz pattern, so my conclusion is that the VC in not properly centered. I'll order surround and a dust cap, and redo it. If that doesn't work, then I'll put a new VC in it, assuming that there is nothing wrong with the magnet or basket.

The good news is that I have to drive this thing pretty high until it is perceptible, and even higher until it is measurable. So I can use it for softer listening, or I can just bridge the <100Hz together and run it in to the apparently good sub, until I have the chance to fix it.

Thanks for all the ideas.

After I get this fixed, I have to measure the box performance and decide what kind of EQ I might wish to implement That will be several months away, because there are several intervening projects.

Thanks again, everyone.

[valbird]

Venting was not an issue. I guess my concern is that there may be VC problem or an alignment problem. My recall is that Xmax is well over .25 inch, but I have not looked that up yet.

I have not compared it to the other driver, but since that one is in the enclosure, it would be harder to do.

[maxwedge]

xmax is 25mm but but checking that with your hands is not really an acceptable method of testing. You really need to apply a sine wave signal to it either from your pc, a cd or a signal generator. Just hook it up sitting on a table sitting and listen for voice coil rub, which is what it sounds like to me. These aren't long excursion drivers even though 25mm seems like a lot but remember that that is the max as in failure point! I have a pair but don't use them as subs (bottom in 4 way) and when I crank them they really don't move a whole lot, although a lot of air is pushing out the ports.

Edit: I meant xmech.

[valbird]

I agree. My 40 yo Wavetek generator has taken a run somewhere. I need to find generator / sw to use, and about .5 Hz sine with 500 Hz or 1kHz imposed on top of it will help me see what is going on.

[pos]

the calculated xmax is around 7mm one way.
the xmech is much higher than that, but remember this driver is quite fragile.

[valbird]

the calculated xmax is around 7mm one way.
the xmech is much higher than that, but remember this driver is quite fragile.

I'm seeing about half that.

I have another concern, and that is while the re-surround was not the neat job I do, or am used to seeing, the dust cap appears to be original. This has me wondering if the cap was removed and the VC shimmed when re-surrounding.

The previous owner did say that he re-surrounded the speakers with JBL OEM parts (he does business with a large JBL distributor), but that he never tested them afterwards.

Somehow I have the feeling that I might end up re-surrounding this driver...but I'd like to verify what is happening, and if I can why, which is why I am calling on the collective wisdom here. I've re-surrounded probably a hundred or more drivers, never once had a problem, and I have no experience with the 2245 driver.

pos, why do you say the driver is fragile? More so than other drivers?

[valbird]

Subwoof, there is no foilcal on the 2245H. There is a screen.

I cannot determine for certain that the dustcaps came off during the refoam. However, I can say that the dust caps on these are slightly larger than the ones on a known 2245 from 1984.

I can determine that with the existing surround, there is less than a 0.5mm variability in the end of the cone to the basket, which is consistent with what I measure on the other 2245H. Similarly, the cone is symmetrically placed within the basket.

I put a couple of watts of pinknoise, rolling off at 100 Hz, and found that coincident with random higher excursions of the cone, there was raspiness, like when there is a fried voice coil. It is not readily audible at lower levels.

Furthermore, when pressure is placed on the back of the cone at 1 and 7 o'clock, there is immediate raspiness, and at other symmetric positions there is no such raspiness.

I'm thinking that there is some voice coil damage, and that I'll be ordering a cone. Normally JBL dealers won't sell you a cone alone, but I have done probably 100 other drivers, so I'll probably just get a cone kit. Before I do, I would like to prove out the cone. One way that occurs to me is to put a scope on it, and see what happens when things break up (raspiness). If the voltage spikes then, it could be a voice coil, or a connection to it opening up.

I'm still open to any suggestions or ideas. Thanks.