FST repair/replacements

[ert]

I have a CDM9NT with a damaged midrange. The driver has a crease in it and is torn from the foam surround around the crease. Although I cannot actually hear a problem compared to the other speaker, I think I'll replace the driver in case it gets worse. Given the ease of replacing the midrange and the relatively low cost of replacement parts, I think it's a no-brainer. However, I now wonder if a new driver will have a significantly different sound signature from the original. Should I leave this alone or fix it? Should the FST in both speakers be replaced together to help maintain the tonal balance? Thanks for any help or feedback.



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[stuofsci02]

I think if you replace it with the correct part for the CDM9NT you should be good... I don't think you need to replace both.

[JürgenW]

For replacing the tweeters B&W advised to replace both tweeters because of the tonal balance, which could be a bit different when from different productions.

[ert]

Thanks for the input. I removed the driver and upon closer examination, there is only a very small tear in the foam surround. I will try to repair with foam surround glue first and see how it holds up. Since there is not much movement in this driver, I don't expect it is under a lot of stress. My concern with replacing only one driver is that the change in tone from an old to new driver would be greater than any changes caused by the crease.

[aarsoe]

Don't! You should not be using glue to try and fix this. Especially since the cone could get glued by accident.
Contact B&W and you can most likely get that foam ring as a spare part.

[ert]

Don't! You should not be using glue to try and fix this. Especially since the cone could get glued by accident.
Contact B&W and you can most likely get that foam ring as a spare part.

Looks like ordinary foam surround. Using glue designed for speaker surrounds is a common repair. There's nothing complex here...

[aarsoe]

The one big difference is that the foam acts as a support for the cone without being attached to it and if you somehow change the, in lack of a better word, "softness" the you could make the cone wanting to move in a non liniar move and thereby increase wear and tear over time. I would be surprised if B&W did not have it as a spare part that you could get either free or at next to nothing. So why take a chance?

[madmac]

Is the speaker box ported?. If so, don't touch the driver, especially since the tear is small. If it's a closed box, you are now letting some air into the box chamber which will change the sound and should replace the driver. Buy one and see if there is a tonal difference (probably not) and if so, then replace both sides. Don't even think about changing the foam surround. That will drastically change the tonal quality of the driver. It won't be the same as it was. Tried it once myself with dubious results and the speaker never sounded the same after.

[ert]

The one big difference is that the foam acts as a support for the cone without being attached to it and if you somehow change the, in lack of a better word, "softness" the you could make the cone wanting to move in a non liniar move and thereby increase wear and tear over time. I would be surprised if B&W did not have it as a spare part that you could get either free or at next to nothing. So why take a chance?

I'm not sure if there's confusion over what I am describing or it's a language issue . The foam surround around the FST driver I have is glued to the cone and to the driver cage, just like any other driver. By using a small amount of glue to repair a small 1cm separation from the driver and the foam surround, I will restore the driver to the original condition. B&W does sell replacement drivers for $130, which I might end up getting anyway...

[azazel]

I'm not sure if there's confusion over what I am describing or it's a language issue . The foam surround around the FST driver I have is glued to the cone and to the driver cage, just like any other driver. By using a small amount of glue to repair a small 1cm separation from the driver and the foam surround, I will restore the driver to the original condition. B&W does sell replacement drivers for $130, which I might end up getting anyway...

Its best to replace the driver if you want your speakers back to perfection. You only need to replace the one driver as they are all the same. You could ask the B&W serice department what adhesive they used on the surround so as to repair it so you have a spare in future or you could sell it to help recover some cost of the new one.

I have a ASW 4000 and the voice coil was coming away from the cone. I asked B&W what glue they used and they said it was super glue, the loctite one. It seems to have fixed it well and saved me alot of money on replacing the driver and B&W would not repair it so I did it myself. I was disapointed that such a high end product should be failing. John Bowers would turn in his grave!

I doubt super glue is the adhesive used on the foam surround on FST as it is too rigid. It looks like someone has poked the cone with little fingers to me BTW.

[aarsoe]

Maybe I have misunderstood something, but I was under the impression that FST meant frameless and that the cone should not be attached to the frame. At least it is like that on the 800 midranges.
But as long as you are happy..

[madmac]

I'm not sure if there's confusion over what I am describing or it's a language issue . The foam surround around the FST driver I have is glued to the cone and to the driver cage, just like any other driver. By using a small amount of glue to repair a small 1cm separation from the driver and the foam surround, I will restore the driver to the original condition. B&W does sell replacement drivers for $130, which I might end up getting anyway...

You've got nothing to loose by trying to glue repair it. I've heard that butyl rubber cement is the preferred adhesive for these types of repairs due to it's flexible characteristics. Good luck!! :T

[ert]

Maybe I have misunderstood something, but I was under the impression that FST meant frameless and that the cone should not be attached to the frame. At least it is like that on the 800 midranges.
But as long as you are happy..

The "F" stands for "Fixed." Fixed suspension drivers (FST) do not use a flexible surround, rather a small ring of dense foam to secure the edge of the cone to the driver basket. All of the B&W FST drivers are constructed this way. If the outer edge of the cone were not suspended, it would just flop around uncontrollably.

You've got nothing to loose by trying to glue repair it. I've heard that butyl rubber cement is the preferred adhesive for these types of repairs due to it's flexible characteristics. Good luck!! :T

I shall try! It's actually easy in this case since the foam surround in this driver is NOT supposed to move.

[ert]

I ended up just buying a factory replacement since I was concerned that if I damaged the driver during repair, I would not have a speaker until the replacement arrived. I have documented the process of replacing the FST. Thumbnails go to full size images.

The CDM9NT is the only speaker outside of the 800 series to use a drawbar to hold the FST to the cabinet. On the rear is the same aluminum screw nut seen on the 800 series. I used a coin to loosen the screw which can be done by hand once the tension is removed.



After the rear screw is removed, the driver is still firmly attached to the front of the cabinet. I tried pushing a screwdriver into the rear hole to force the driver out but it did not budge. I needed to gently use a plastic prybar between the face of the cabinet and edge of the driver basket to loosen it from the cabinet. I started on the bottom edge and worked my way up to minimize damage to the wood veneer.

Once the driver is loose, it may be pulled forward. The connectors are standard automotive style crimp spade connectors which are removed by hand. They have different sizes to prevent switching polarity. After the connectors are removed, pull the driver forward being careful not to poke anything with the drawbar behind the driver. There is a large bundle of polyfill wrapped around the drawbar. I removed the driver first and the polyfil next.



Inside the cabinet you can see the midrange has its own chamber. The hole on the back of the cabinet can be seen along with the filter for the tweeter and tweeter mounting bolt.



This is the driver with the drawbar which needs to be removed for use with the new driver. The drawbar was epoxied into the old driver and required considerable force with a wrench to remove. The back of the old driver shows manufacture date. You can see the size of the tear in the foam surround. I am lifting it with my fingernail in this photo to exaggerate the separation.



The replacement driver was packaged very well. This box was inside another with additional packing.



Comparing the old and new driver the color difference is striking. The camera doesnt really capture how yellow-green the new driver is. It's really a florescent green. I actually prefer the golden color of the old driver. The new driver will yellow in time. It's interesting that the replacement driver is only one year older than the original.



The gasket on the old driver is made from a very rubbery material. Before ordering the replacement, I opened up the cabinet to check the condition of the gasket. Rather than risk damaging the old gasket, I simply ordered a new one shown here. The new gasket has a much stiffer backing but has the same rubber stuff as the old one. It's simple to put the gasket on the driver since its stickiness holds it in place.



I attached the drawbar to the new driver and tightly wrapped the polyfil around the drawbar. It's easy to put the new driver into place. Reattach the connectors and tighten the rear bolt. I jiggled the driver around a bit before fully tightening the bolt. If the driver does not fit perfectly centered then the gasket will squish out on one side.



The end result looks very professional. I ran frequency sweeps comparing the L/R speakers before and after replacement and found no differences. The left brother is now a bit jealous so he might be receiving a replacement in the near future