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Replacing foam speaker surrounds . . . just follow this step-by-step procedure Perished foam speaker surrounds are a common problem for hifi enthusiasts. Unfortunately, replacement drivers are often unavailable or are expensive. Here’s a simple step-by-step fix to get things going again. By BILL HENDRY This article is in response to a letter published on page 91 of the August 1997 issue, concerning the replacement of per­ished foam loudspeaker surrounds. Although the answer includes appropriate references to the perils of DIY repairs, the process certainly isn’t as onerous or as mysterious as one might infer. I’ve been repairing speakers suffering from this complaint for six or 14  Silicon Chip seven years now. Although it’s not my professional area, I’ve developed techniques that return the speakers – from 4-inch midranges to 15-inch woofers – to virtually as-new condi­ tion. None has failed in this time and owners I’ve spoken to since have reported normal operation. As has now become common know­ l edge, foam-type surrounds, even if “tropic proofed,” deteriorate quite quickly. Some become a soggy mess in just two years, particularly in my area, Mackay, on the Central Queensland coast. The most durable compounds ap­pear to be rubber (1215 years) and impregnated cloth, the latter used on some good Australian speakers. My technique is well suited to the dedicated home enthu­ siast. It’s inexpensive but does require patience and dexterity. It can be used with all cone materials and the assembly appears to retain its fundamental free-air resonance. However, the four to six hours of work required may not be justified for low-end, plug-in replacement drivers. The step-by-step procedure is as follows: Step 1: check the speaker care- fully to ascertain that the cone is in good condition and that the voice-coil is intact. You should also check that there’s nothing in the air-gap to impede normal movement and that the rear suspension “spider” is in good shape. There’s no need to cut away the dust cap – in fact, doing so could damage the (now unstable) voice-coil assembly. Step 2: clean the chassis and cone edge thoroughly, remov­ing all traces of the perished material and any glue. Some models may have dress rings or segments which can be carefully removed and later reinstalled if necessary (although they are usually only cosmetic). I use a hobby knife to scrape away the material but I haven’t experimented with sol­vents. On a pair of AR 25s that I recently refurbished, the ring of surround material remaining on the edge of the cardboard cone was best removed by carefully pulling it away from the surface towards the edge. This has the effect of also removing a thin layer from the cone material which can later be stabilised by the application of a painted layer of PVA glue (eg, Aquad­ here). Other situations may require the careful scraping away of decomposed residue. Inspect the underside of the cone edge to confirm a stable surface that’s necessary for the next step. All traces of the perished material have been removed from the chassis and cone edge of this driver, which is now ready to accept its new felt roll surround. Note the four dress ring segments which have been removed intact – these can later be replaced if desired. Step 3:: obtain an appropriate piece of (black) felt from a haberdashery shop. You may not be able to specify density or thickness but the operation doesn’t appear to be grade specific. At this point, determine whether you’d prefer to mount the felt surround replacement on the underside or the upper (visible) side of the cone. (On my last job, a 6-inch Bose Studiocraft midrange, the rear of the chassis was fully enclosed, necessitating attach­ment of the surround to the upper side). Generally, underside mounting results in a more presentable appearance, allowing for a very neat “dressing” of the join. The felt rings have been glued to the cones of the drivers shown here but not yet to the chassis. Note that, in each case, the felt has been glued to the underside of the cone but it can also be glued to the top of the cone if necessary. overlap around the edge of the cone when the felt is in place. The outside of the felt should overlap the edge of the speaker chassis by about 10mm (this will be trimmed later). You can use a template such as a saucepan lid or a bowl to mark out the felt, which can then be cut using a hobby knife or a pair of scissors. Step 4: (delete if you choose Step Step 5: run a bead of PVA glue 7). If the edge of the felt is going to be visible (ie; attached to the upper side of the cone), cut a hole in the felt that’s 10-12mm less than the diameter of the cone. This will provide a 5-6mm around the edge of the cone and smooth it neatly to produce a band 5-6mm wide. This done, do the same to the inside edge of the felt ring out to the point of overlap. Step 6: place the felt ring central- ly onto the cone and press it carefully onto the surface. Work the leading edge so that the felt appears “chamfered” at the line of contact with the cone. Paper wedges (eg, loosely crumpled tissues) placed between the cone and the basket will help to keep the cone stable during this procedure. Step 7: (delete if you chose Step 4): if the edge of the felt is to be attached on the underside, cut out a circle in the felt using a compass, a dressmaker’s white pencil and November 1997  15 not allow runs to dribble down the sup­ports towards the spider during this procedure. Step 11: this step is critical. Gen- Once the felt has been glued to the chassis, it can be trimmed by running a sharp hobby knife around the inside of the lip. scissors. Neatness isn’t so important in this case, and you won’t need to spend time hunting for a template of the right dimension. Apply glue to the appropriate edges as described in Step 5 above. It can be frustrating working through the rear of the chassis, so an artist’s brush can be a help. Step 8: carefully manoeuvre the felt ring over the cone and bring the mating surfaces together. Now, working from the back, make sure that the felt ring is centrally located and work the contact area to optimise adhesion. 16  Silicon Chip Step 9: run a thin bead of glue around the cone at its junction with the felt ring. At this stage leave the PVA to dry completely. Step 10: lay the speaker on its magnet. You now have the inner edge of the felt ring attached to the cone and the outer edge overlapping the chassis by about 10mm. Lift the skirt of the felt ring to expose the flat area on the outer edge of the basket and, using a small brush, paint a generous film of glue onto the entire flat surface. Do tly place an upturned glass centrally on the cone (over the dustcap), with sufficient weight to depress the cone to its maximum backward excursion position. This will be indicated by either the former contacting the back of the magnet assembly or the rear suspension spider being stretched to its limit (be careful not to deform it perma­nently, though). Carefully wiggle the glass up and down and from side to side to make absolutely sure that the voice-coil is centred in the air-gap. It may be necessary to “play” with the assembly to become aware of the tolerances. Step 12: gently move your hands around the edge of the speaker, working the felt evenly onto the glued surface. This done, use a blunt table knife to create a sharp corner in the felt at the point where the lip is flanged forward. Step 13: remove the glass and check that the assembly moves freely Below: the lefthand speaker in this photo has been finished, except for the mounting holes and the optional dress ring segments. Note the “roll” in the felt between the edge of the cone and where the felt attaches to the frame. to its natural rest position. In so doing, the felt will bulge or dome to form the roll necessary for normal cone travel. Allow the glue to dry. Step 14: run a hobby knife around the inside of the lip to create a neat invisible edge at the flange. This done, gently lift the cone by applying equal equal pressure to both sides and check that the whole assembly is axially free but radially secure; ie, the cone should move backwards and forwards easily but should not move from side to side. Step 15: make mounting holes in the felt to align with the holes in the chassis flange. This can be neatly achieved by first gently pushing a hot soldering iron tip right through the felt from the rear at each flange hole position to create a pilot hole. The holes can then be finished by pushing the soldering iron tip through from the front. Step 16: at this stage, it’s time to decide whether you want to replace the dress ring segments. The finished speaker looks quite acceptable without them and, in any case, they may have been damaged during removal. If you do decide to fit them, glue them onto the surface of the felt using a thin layer of PVA but don’t let the glue contact the roll. Turn the speaker face down so that its weight is on the dress segments during drying. Step 17: apply a generous quantity of grease to the felt. Do not use engine grease; instead, use a high-temperature, waterproof compound (eg, Bel-Ray marine grade). This is applied to the felt surround with a fairly stiff-bristled brush, so that the grease is worked well into the fibres (do the whole surround if there’s no dress ring). This has the necessary effect of clogging the air-gaps between the fibres but allows the surround to retain its flexibility. It also discourag­es creatures from making a meal of the felt. If some deformation of the roll occurs during this process, reform it by gently running the handle end of the brush around the underside of the felt. The result is an attractive, fairly New Foam Surrounds For AR Speakers On page 91 in the August 1997 issue, G. E. of Armidale, NSW asks about new foam surrounds for AR speakers. I cannot entirely agree with your answer. I have had more than 20 speakers fitted with new surrounds, some over 20 years old, and in no instance has there ever been a problem with the cones themselves. More than half my repaired units have been AR (I am a huge fan) and I still own and use five AR pairs. The best pair are 24-year-old AR3a’s which are quite superb and compare easily with anything costing up to $5000. In New Zealand, a cone surround job for a pair of 12-inch drivers costs about $NZ120 (approximately $A100), while new sur­rounds for 8-inch drivers cost about $NZ85 ($A70). For that cost, the units are inspected (cones, suspension and chassis), fitted with new surrounds and the voice coil as- uniform surface that looks quite professional. The speaker is now ready for use. It’s a good idea to feed a very low-frequency sinewave (say 10-20Hz) at low voltage into the voice-coil to check that the cone moves freely before the system is reassembled and played at high volume levels. Final notes A few final points are worth noting: (1) You may feel inclined to paint the entire surface of the (cardboard) cone with PVA to: (a) freshen the appearance, (b) stiffen it, and (c) minimise “grease-creep” across the cone. I don’t normally do this to a diaphragm in good condition, in case it significantly alters the cone’s mass. (2) You might consider coating the surround of a new speaker with the abovementioned grease, even if it has been tropic-proofed. I did this with a 10-inch Etone subwoofer foam surround and there’s been little change in its appearance after seven years. (3) Although I’ve never noticed a problem, you might feel more confident using a non-water based glue, thereby obviating poten­tial corrosion of the metal parts. During the devel- sembly checked for cor­rect alignment. They are also fitted with a new spider suspension and dust cap if necessary, tested and guaranteed. This is excel­ lent value and the situation is probably similar in Australia. AR have always been masters of acoustic suspension speak­ ers, so cabinet size, cone size and air tightness are critical. Your correspondent should stress this to the repairer. Of course, if he can buy new drivers at reasonable cost, then that is a simpler solution. Depending on age, correct drivers may be difficult to obtain. The Australian agent for AR is WC Wedderspoon Pty Ltd, 3 Ford St, Greenacre, NSW 2190. Phone (02) 9642 3993. If your correspondent requires any help or would like to communicate with me I would be happy to oblige, as an AR enthu­siast. J. Calkin, Takapuna, NZ. opmental stages of this technique, I used contact glue but there’s no room for error – the mating surfaces have to be positioned exactly. (4) Often the gasket that seals the speaker to the baffle board is damaged, disintegrated or missing. To overcome this problem, I use the Bel-Ray grease to form a continuous ridge at the edge of the baffle-board cutout. The refitted speaker then provides an automatic seal which, if necessary, can easily be broken if the driver needs to be removed. Finally, please note that although the technique described here generally gives good results, it doesn’t restore a driver to its exact original specifications. That’s because the compliance of the felt used to make the repair will differ from the compliance of the original foam surround. The method of attachment will also have some effect on the free-air resonance of the repaired speak­ er, although its sensitivity will probably be much the same as before. In the end, it’s up to you. If you don’t want to fork out big dollars for new drivers, then you’ve got nothing to loose and you will probably be quite happy with the end result. SC November 1997  17