Voice coil replacement on vintage Wharfedale 12"

[flyingtele]

I recently got given a pair of 12" Wharfedale vintage drivers, both with blown voice coils, so I thought I would post pictures of each step in the process of voice coil replacement. I carry out a lot of repairs on speakers, so will make similar threads for different repairs. (If this is in the wrong section feel free to move it to the correct location).

PLEASE NOTE: This is written only as an explanation to the process, and to give some helpful advice to those wishing to try it themselves. It is not a comprehensive how-to guide, and I accept no responsibility for and damage or injury resulting from this article. If anyone has any questions, I will be more than willing to help where I can

First up are the basic tools you will need: Scalpel, solvent and eyedropper, adhesive and some vernier calipers. I use toluene for the solvent, as it quickly softens most glues (not epoxy though) and doesnt harm most speaker materials. Please note it is not suitable on some types of plastic or foam surrounds, in that case a heated knife is a safer option.

Calipers are essential for accurate measurement of the voice coil dimensions for when you buy your replacement. The important dimensions are former height, winding height, overall thickness, and inside diameter.
The scalpel must be sharp and it is used for slicing through adhesive and separating different components.

I use Araldite for securing the coil to the cone and spider. I use Ados or similar for gluing the spider to the basket, as well as rubber surrounds. For dustcaps and foam surrounds, I use a white PVA based adhesive, which is similar to wood glue but more tacky.

[flyingtele]

Here is a photo of the driver in question. Has no model numbers, just says "Wharfedale, England". The have a very light cone, thin rubber surround, and an aluminium damping ring attached to the cone beneath the dustcap. This is to add mass, and I imagine to raise the drivers Q. From looking at the driver they would have been in a sealed cabinet.

[flyingtele]

The first step is to remove the speakers software, ie. the cone, surround, and spider. To do this, I apply a small amount of toluene to the edge of the spider where it is glued. And depending on the type of surround, I either apply toluene to that too, or carefully cut the adhesive holding it to the frame with a scalpel using a curved blade. Care must be taken not to slip and slice through the surround or your own hand.
Another option is to use a heated blade, this works better with delicate surrounds such as foam as it slices through the adhesive but doesnt damage the surround if you work quickly and efficiently.

Once the adhesive is softened, you can work the scalpel between it and gently prise it upwards, and you can use the back edge of the blade or a blunt butter knife to help separate the surround or spider from the basket.

[flyingtele]

After that you remove the dustcap, and remove the cone assembly from the basket. To remove the dustcap on some drivers you can apply a few drops of toluene around the egde and remove it intact, but I prefer to carfefully cut around the edge as otherwise you can tear the cone.
Use a scalpel blade that is straight and has a sharp tip. Then its simply a matter of de-soldering the two braided lead wires from the terminals and the cone should come out freely. If it doesnt want to come then dont force it, find what is sticking and gently free it.
In the upper pic you can see the aluminium damping ring glued to the inside of the coil.

[flyingtele]

Now it comes time to inspect the voice coil and take the necessary dimensions. What you need to know is the height of the former, the height of the actual windings, the thickness of the windings, and the inside diameter of the coil. If any of these are not correct then the coil may not fit in the gap, or it may rub, or alter your X-max. Sometimes it is possible to deliberately use a different coil, for example if you want a longer X-max then fit a longer coil, but check the depth of the gap first or it may hit the back plate.
As you can see, this coil is well and truly busted. It is only on a paper former, and the adhesive had probably dried and perished over time.

After you have taken your measurements, carefully cut the coil free. I use the sharp straight blade, and go in from the front. In the second pic, note the scalpel blade, carefully work your way around the circumference of the coil. Better to go slowly and not slip and be accurate, than to rush and slice through the cone. The coil should slide out freely and leave a neat and tidy hole.

[flyingtele]

Here is a pic showing all the separated components: The cone/spider, the old faulty coil and its former, and the damping ring.
The damping ring could be omitted if one wanted to customise the driver and experiment, but I will be re-installing it to keep the drivers as close to original spec as possible. In this case I was able to keep the cone attached to the spider which is preferable, but not always possible. Always try to keep the hole as neat as possible and close fitting, as it will aid the centering of the assembly when it comes to re-assembling it.

This is all for now, until I source some new coils for the drivers. Then I will continue to update

[flyingtele]

I have managed to find some replacement coils and am now just waiting for them to arrive.
On drivers of this age, the paper degrades and can lose strength. These ones were also starting to go a bit fluffy on the surface, as the glue holding the fibers deteriorates.
What I did was mix up some PVA wood glue and some water, to a consistency that was runny and easy to brush but still white. It doesnt take much water.

I just give both sides of the cone a liberal brushing, and let it soak into the cone. When it dries it restores strength and keeps it looking fresh.
In some places the rubber surround had just begun to start to split. I open the split up, and gently apply a small amount of ados and smear it with my finger,
this holds it together and it is barely noticeable from the front.

Now I just have to wait for the coils to arrive and then re-assemble. More photos to come!

[speakerdoctor]

Great thread flyingtele. Here's a link to a Snell Model A woofer's VC replacement thread.
Model A recone challenge - The Classic Speaker Pages Discussion Forums

Looking forward to the remainder of yours. IMO, alignment of the new coil in the old cone is the most difficult part so it doesn't rub afterwards.

[Pano]

Very nice thread. Thanks for posting. This is real DIY!

[flyingtele]

Well the coils have arrived! The replacement coils are on a polyimide former, so these drivers will ow have a higher power rating as the previous coils were made from paper. Now the fun part, re-assembly.
The first thing I do is mark on the coil its correct height. There are various methods to do this, and it i essentially the same process as calculating the drivers X-max (linear excursion, for those that don't already know).
I measure the coil winding height, in this case its 10 mm. Then I measure the thickness of the top plate and subtract that value from the winding height. In this case it is 5 mm, so that leaves 5 mm. Then I divide that by two, to give the amount of winding overhang either side of the top plate, which will be 2.25 mm. I set my calipers as close as possible to 2.25 mm, in the photo it shows 2.27, but 0.02 mm will not make a difference.
I then put a piece of masking tape on the coil to mark where it should sit in the gap.
Next up is to find some shims of suitable thickness that can firmly hold the coil in place, but loose enough not to distort the coil and allow adjustment. On these drivers, I found a pair of cardboard playing cards to do the job nicely.