First job is to test the voice coils. In the case of this image, I am testing the LF voicecoil which should measure about 5.4 Ohms +/- 0.2 Ohms DCR. Little point in buying new foam surrounds if the voicecoil has been damaged. Once the cone is removed a more thorough inspection of the voicecoil can be made.
Next job is to remove the metal gaskets holding the edges of the foam surrounds in place. First, the foam infill pieces are carefully removed by simply lifting out and put somewhere safe. The metal gaskets are removed by taking 3nr small screws out per gasket (there are 8 of these gaskets) which hold them in from the rear, then using bent nosed pliers very carefully applying leverage to the gasket and lifting each out. Continue until all are removed and put them to one side for cleaning up.
After removing the cone, place to one side and inspect the voicecoil gap and horn for any obvious signs of dust or debris ingress. You may need to use compressed air from a can to give a quick burst to clean the gap out. In this case, I simply used some tape wrapped around a squashed cotton wool bud stick and move it around the gap to remove any fine debris…of which there was a little. Once this is done it is very important that the voicecoil gap and the horn mouth are completely and securely covered to prevent any debris ingress. Covering with masking tape is fine.
The voice coil for the low frequency woofer can now be inspected. What you are looking for is any signs of scraping. In this case, the voice coil tested healthy and there were no signs that it had been damaged through rubbing nor has the pole piece end been rounded over from over-excursion so this one looks factory fresh. If any scraping had been noticed, I would have painted over with a very thin coating of clear lacquer at this point, the water based stuff sold in small bottles for lacquering fishing rod eye-bindings.
The next job is to bond the surrounds into place. NOTE: THESE MUST BE FITTED FROM THE REAR! I have seen many attempts at DIY re-foams done from the front where the person doing it is either too lazy to do a proper job or not capable and understandably trying to save cash by a DIY solution. However, you run the risk of holding the cone too low in the suspension (slightly lower when done) and this can risk the end of the voice coil bottoming out sooner as well as altering the compliance slightly due to a loading difference to the standard. It must be done from the back, with the domed part of the surround facing the front, and the dished part to the rear. You must take care to line the voice coil wires up so that when replaced, the cone lines up properly with the solder points on the basket. Contrary to popular belief, no special glue is needed. The best glue to use is the Evo-Stick blue bottle PVA wood glue and this should be used undiluted. The stuff supplied in little white bottles with re-foam kits is just the same as this glue but watered down. Watering down will not affect the properties of the set glue, just make the layers thinner, and mean that grab times are extended to the point that it becomes difficult to effectively hold down the foam to the cone…it’s a faff. My advice is don’t dilute it and don’t use dilute glue supplied with kits. Just apply it thinly. Any over-spill can be cleaned up with a damp finger or rag.
Typical HPD condition after 50 years (in the case of this one!) with original foam surrounds in place. If used in this condition, system efficiency falls to the point that you get no real woofer output and just get the HF from the horn, putting the woofer driver voicecoil in danger from both rubbing and over driving which heats the voicecoil up. You can overdrive the HF to get any meaningful output and cause that to fail so the message is once you notice a deterioration in the foam surround, don’t use the speakers until they are re-foamed!
This is one of a pair of HPD385 (15 inch) drive units destined for a new pair of cabinets for a client using a newly developed cabinet design, similar to the RFC Canterbury but enlarged for the 15 inch HPD.
Here we have (shown above) the HF voicecoil being tested. This one should read 10.6 to 10.9 Ohms not forgetting to deduct the DCR of the test leads (in this case 0.4 Ohms).
Once a modelling knife or similar has been used to cut round the edge of the surround by the basket, the low frequency voice coil connection wires must be de-soldered at the junction by the input socket on the rear of the drive unit basket. The spider assembly retaining nuts must be loosened and carefully removed. Note that there is a locking washer and plain washer under each of the four nuts. The nuts must be carefully loosened off using long nosed pliers, if you don’t have a small imperial spanner that fits. The gasket can require some persuasion to free it up and you have to take great care not to damage anything whilst you are trying to remove this. Once loosened off, lift the whole cone out vertically taking care not to scrape the voicecoil.
Voice coil gap taped up and secured from any ingress of debris.
The cone can now be carefully and thoroughly cleaned up. It is not good enough just to remove the worst of the old foam and glue…all or as much of the residue as possible must be removed. This is a delicate and very time consuming job. Allow several hours for this, using meths or acetone to assist with the clean up and using a modelling or craft knife to gently scrape away the old foam and glue residue. The underside should be clean and you’ll need to clean from under the end of the girdercoustic ribs at the back since the new foam needs to be fitted under these. Note the use of an old spray paint plastic lid fitted over the voice coil to protect it during work. I fitted some foam off-cuts between the lid and the voice coil to protect the windings and allow the lid to be used as handle.
View from the underside showing “Girdercoustic” ribs. Backs of cone need to be as clean as this one before attempting to glue the new surrounds in place.
Gluing the foam into place takes patience and is best done with the cone supported on something, allowing the surround to hang from the edges so it doesn’t get squidged up or displaced. You may have to place little weights on it to hold it in place and I find that using the metal gaskets (once cleaned up) works well. You have to check, before the glue has grabbed (which takes an hour or so but longer to set…preferably overnight) that the foam is properly pressed in at the ribs and that it hasn’t come away from the cone.
The main drive unit basket can now be cleaned up ready to receive the new surround. Once the surround is bonded to the cone, the cone can be re-set into the voicecoil gap and the washers and nuts replaced which hold the spider suspension in place. The nuts should be left loose at this point and you should feel for the voicecoil gap until roughly centred. Next, apply a thin smear of PVA to the basket shelf and press the foam gently into place, gently tapping the cone using equal pressure each side of the dust cap to check that the voice coil is not rubbing/. Once happy, the gasket pieces and be replaced and loosely screwed into place before checking the cone again before doing up firmly. The cone is now left overnight. When you come back to it, the voicecoil should be checked and adjusted at the spider as required to centre it and then the bolts done up a little at a time, tightening them in the sequence N-S then E-W then N-S, E-W and keep going one turn at a time, checking that nothing has moved until they are firmly done up but not overtightened.
I have tested this example on a rub and buzz tester and as shown, it was almost perfectly centred upon completion.
Notice that the Free Air Resonance is now about 15.5Hz, some 7 Hz under factory fresh due to the softening of the suspension with age and use. The cabinet tuning will have to use this figure and not published figures when designing a new cabinet around these.
Please contact me using the “Contact Us” page if you don’t have the confidence to do this work yourself as I am happy to provide this service for you.