Wharfedale Dovedale cone rubber repair
Posted 10th October 2013 at 11:28 PM by hbc
After the Magisters went I started missing them. Much irrational thought later I decided that I needed some Dovedales. It may be some left over residual memory of some super 8s I had when I was young, they were very nice despite the lack of tweeter (retrospective young thought). The big Dovedale woofers look similar with the green felt, bit nervous of those strange pink domes though... Anyway, eventually "won" some with mint drivers slightly tatty boxes. Long story short the rubber had got stiff, but not yet brittle.
Sulk, got the Magnums... (must have bass now). After much research and thought, the Dovedales don't seem to be a standard size for foam, and I had got to the point of contemplating cutting away some of the cone to fit a flat foam. I was tidying up the garden when I found a 700 inner tube I had changed recently.
At this point I thought what have I got to loose, so gave it a good wash, (lots of washing up liquid applied neat), chopped out the valve and tested for size. Then cut along the outer circumference and for me along an inner edge leaving all the stretched bits where the spoke nuts stuck in. Then I laid the cut tube into the surround to check fit and to cut to approximate length. With this tube the outside was a snug fit, but the inside was a little wrinkly, being to big from a larger diameter wheel!
Then I got a sharp knife and cut away sections from the old surround, carefully leaving some "spokes" away from the frame for later removal. I also left 3 - 5mm of old surround around the edge of the cone and on the frame. This leaves a support for cone and tube and something to glue on to.
Then I put some rubber solution (from the bicycle repair kit) around the remaining outside surround and also around the outer edge of the inner tube, waited till it had dried a little, and then carefully positioned the edge to line up with the edge of the green felt / old surround, pushing it together with my fingers. Then repeat for the inner. This requires a little more encouragement to make sure all the wrinkles smooth out. Oh yes and glue the join. I had no over lap so it took quite a long time to dry.
I found I had to trim away some excess inner tube around the cone, and also a bit later cut off the supporting "spokes".
And now the best bit, the free air resonance is down to 25 Hz, from around 60Hz, and the cone can move freely again. I suspect the excursion may be now a little less than a new driver, because of the remaining old surround, but I don't have some good ones to compare..
Oh and they sound lovely. Better than I had hoped, that pink tweeter is quite well behaved, although I have some ring radiators on order to try. It is interesting to contrast the Magnums, which are more efficient, but on the near field, are rolling off at about 90Hz, where as the Dovedales roll off below 50Hz.
For the record I tried some "Rubber Renue", and it had no lasting effect apart from distorting the surround slightly, most exciting when the rubber started to swell up!
I have been very pleased with the results of this fix, I was anxious about re foaming as there seems to be a weight in the middle of the voice coil which would make shimming difficult. With the "spokes" this was a non issue, 8 (stiff) spokes held everything in place.
Hopefully posting this will give the courage to try a similar fix (at your own risk) and to get some of these old speakers up and running again.
Pictures:
Before and after 1m on tweeter axis frequency response, before and after impedance, and some action shots, the last one is a comparison of the original and resuscitated driver.
Sulk, got the Magnums... (must have bass now). After much research and thought, the Dovedales don't seem to be a standard size for foam, and I had got to the point of contemplating cutting away some of the cone to fit a flat foam. I was tidying up the garden when I found a 700 inner tube I had changed recently.
At this point I thought what have I got to loose, so gave it a good wash, (lots of washing up liquid applied neat), chopped out the valve and tested for size. Then cut along the outer circumference and for me along an inner edge leaving all the stretched bits where the spoke nuts stuck in. Then I laid the cut tube into the surround to check fit and to cut to approximate length. With this tube the outside was a snug fit, but the inside was a little wrinkly, being to big from a larger diameter wheel!
Then I got a sharp knife and cut away sections from the old surround, carefully leaving some "spokes" away from the frame for later removal. I also left 3 - 5mm of old surround around the edge of the cone and on the frame. This leaves a support for cone and tube and something to glue on to.
Then I put some rubber solution (from the bicycle repair kit) around the remaining outside surround and also around the outer edge of the inner tube, waited till it had dried a little, and then carefully positioned the edge to line up with the edge of the green felt / old surround, pushing it together with my fingers. Then repeat for the inner. This requires a little more encouragement to make sure all the wrinkles smooth out. Oh yes and glue the join. I had no over lap so it took quite a long time to dry.
I found I had to trim away some excess inner tube around the cone, and also a bit later cut off the supporting "spokes".
And now the best bit, the free air resonance is down to 25 Hz, from around 60Hz, and the cone can move freely again. I suspect the excursion may be now a little less than a new driver, because of the remaining old surround, but I don't have some good ones to compare..
Oh and they sound lovely. Better than I had hoped, that pink tweeter is quite well behaved, although I have some ring radiators on order to try. It is interesting to contrast the Magnums, which are more efficient, but on the near field, are rolling off at about 90Hz, where as the Dovedales roll off below 50Hz.
For the record I tried some "Rubber Renue", and it had no lasting effect apart from distorting the surround slightly, most exciting when the rubber started to swell up!
I have been very pleased with the results of this fix, I was anxious about re foaming as there seems to be a weight in the middle of the voice coil which would make shimming difficult. With the "spokes" this was a non issue, 8 (stiff) spokes held everything in place.
Hopefully posting this will give the courage to try a similar fix (at your own risk) and to get some of these old speakers up and running again.
Pictures:
Before and after 1m on tweeter axis frequency response, before and after impedance, and some action shots, the last one is a comparison of the original and resuscitated driver.
Total Comments 5
Comments
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I'm facing repairing Dovedale 3 woof surrounds but suitable replacements appear to (still) not be available so I wonder if the existing surrounds could be softened in situ.
They are made of Flexiprene,a rubber type that's still available and one of several similar types. The hope is that one of the many rubber-softening recipes found on the web could be gently applied until Fs reduced to something of its former spec.
I'm currently experimenting with strips from an old inner tube and painting them with 30% oil of wintergreen in ethanol. This has, after ~20 coats, made no perceptible difference. Maybe a more aggressive solvent may be required but the glued surfaces on the woofs will need to be avoided so gently, gently at first.
Any ideas or experience to offer?
Roj.Posted 11th January 2017 at 10:57 PM by Roj
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Since learned that Rubber Renue is a version of oil of wintergreen in (apparently) aggressive xylenePosted 21st January 2017 at 11:46 PM by Roj
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Thought of a more thorough way to recon the outer surround so I removed it completely. That was a bit of a task as the glue was rather hard. Achieved removal with minor damage which will not matter after reinstalling.
Used
1. 2 large ziplock bags. Big enough to allow the surround to sit flat and stay that way during the heating phase. New bags are best as there will be few pin holes or compromised welds
2. ~20mL Oil of Wintergreen (OoW)
3. ~250mL water
4. Paper towels
5. Rubber gloves
6. Large microwave oven
Placed surround, concave up, into first bag.
Taped the bag's corners inwards to prevent fluid from residing there.
Add the OoW and water and seal.
Placed that into second bag as first was leaking a little. Seal it.
Covered the glass turntable with paper towels.
Placed into large microwave oven but on top of a small plastic lid to raise the centre of the bags and force the fluid to the perimeter.
Set 1min High, 3 min Low and 20 min V.Low and Start.
Checked fluid temp a few times by hand. Always hot but did nor appear to boil.
Removed and allowed to cool.
Removed paper towels containing leaked fluid and thoroughly cleaned oven. A little pong from there.
Preliminary result was that the surround is very compliant but a little expanded such that the flats at inner and outer are not flat. The roll was a little distorted.
Cleaned off the softened glue from both flats. Behaved like it was contact adhesive which, if so, might be ok in a factory with jigs but which I will not repeat given the forgiving nature of the adhesives currently available.
Now for a long wait to see if enough shrinkage occurs and the flats resume being flat and the hard won compliance remains.
Results with some pics then.
RojPosted 18th June 2017 at 01:30 AM by Roj
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Thought of a more thorough way to recon the outer surround so I removed it completely. That was a bit of a task as the glue was rather hard. Achieved removal with minor damage which will not matter after reinstalling.
Used
1. 2 large ziplock bags. Big enough to allow the surround to sit flat and stay that way during the heating phase. New bags are best as there will be few pin holes or compromised welds
2. ~20mL Oil of Wintergreen (OoW)
3. ~250mL water
4. Paper towels
5. Rubber gloves
6. Large microwave oven
Placed surround, concave up, into first bag.
Taped the bag's corners inwards to prevent fluid from residing there.
Add the OoW and water and seal.
Placed that into second bag as first was leaking a little. Seal it.
Covered the glass turntable with paper towels.
Placed into large microwave oven but on top of a small plastic lid to raise the centre of the bags and force the fluid to the perimeter.
Set 1min High, 3 min Low and 20 min V.Low and Start.
Checked fluid temp a few times by hand. Always hot but did nor appear to boil.
Removed and allowed to cool.
Removed paper towels containing leaked fluid and thoroughly cleaned oven. A little pong from there.
Preliminary result was that the surround is very compliant but a little expanded such that the flats at inner and outer are not flat. The roll was a little distorted.
Cleaned off the softened glue from both flats. Behaved like it was contact adhesive which, if so, might be ok in a factory with jigs but which I will not repeat given the forgiving nature of the adhesives currently available.
Laid out as flat as possible on clean paper.
Now for a long wait to see if enough shrinkage occurs and the flats resume being flat and the hard won compliance remains.
Results with some pics then.
RojPosted 18th June 2017 at 01:34 AM by Roj
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No luck with this technique...so no pics worthy of posting.
The treated surround has shrunk alright but is now smaller than when I started AND its not retained any of its improved compliance. A total bust.
Next:- to experiment with the technique described here:-
https://boomboxery.com/forum/index.php/topic/10126-make-your-own-speaker-surroundbetter-than-foam/[/URL]
Or use the now slightly under-sized, over-hard original surround as a mould and apply many coats of spray PlastiDip or the cheaper (in Australia) Amor All Custom Shieldhttps://www.youtube.com/watch?v=Ne3edUf_xXIPosted 26th June 2017 at 11:11 AM by Roj
