The foam on my RM-1 woofers finally rotted out, and what was originally intended to be a simple refoam turned out to be a bit more than that. I called up Brian and asked what the 8" megawoofers cost, along with new passive radiators, and the circa $500 price was a bit steep for my current funding level. I just got through installing new speakers and a few more upgrades on a couple of Marshall guitar amps, so I was audio poor with a new reloading bench to set up this winter (note my username, Mauser 98 action hunting rifle fans), so I have to think in that direction as well. I decided to refoam, but I also decided to upgrade from foam to rubber surrounds.
I had to buy a 4-driver surround kit from Speakerworks for around $50 delivered, and another post tells what I did with the passive radiators. Upon opening the kit, the instructions said the supplied PVA glue would not work on rubber surrounds, but there it was anyway. Sure enough, the PVA glue was worthless on rubber and polyethylene. I'll spare you the details of the glue search, but I found that special two-part cyanoacrylate (super glue, CA) would get a good hold on the polycone AND rubber. You will pay $5 to $8 for the Loctite brand that can barely do the 4 woofers. You MUST use the stuff made for polyethylene and polypropylene (and it is very toxic). I had to use 2 other glues for rubber to styrene plastic and styrene plastic to metal. Aren't we having fun finding all this out? Then there was J-B Weld epoxy as well, when all else failed. Brian knows all about the glue hassles, which is why they don't do this stuff at VMPS, and the toxicity issues as well. For occasional use, the toxicity issues aren't all that great, but would be a major health hazard in production work.
First you pop off the styrene plastic trim rings. Be careful, they tend to break at the screw holes. After pulling off the old foam, the old adhesive was removed with acetone (buy the quart can, you may use a lot of it). The phase plugs are held on by an adhesive pad and easily pull off. The polycone is very tough, but stuff some tissue paper in the voice coil to keep out bits of trash. Sand the edges of the polycone where the rubber surround will attach. You COULD leave the old adhesive in place on the polycone and glue to that, possibly saving you a lot of trouble. You decide. The rubber surrounds were attached to the trim rings because that centers them and makes them SO much more easy to handle. I used Devcon WeldIt and applied glue to BOTH the trim ring and the rubber. This is fast dry stuff and you can't apply to just one piece and expect it to get a grip. Now, I noticed what seemed to be polyurethane bond (Gorilla Glue) used by the factory to attach the trim ring to the basket frame. I upgraded to the white fast dry Gorilla PU glue and used it to bond the trim ring to the frame and the rubber surround to the frame. After putting the trim ring in place, I weighted the assembly with several books and set aside to cure overnight. PU expands when curing, so do NOT use too much or you will have white foam glue all over the place that will need to be trimmed. Experiment first if you are not familiar with how much to use.
You can now glue on the phase plugs using J-B Weld. I tried thick CA, and it held at first, then the phase plugs popped right off after a while. It should cure overnight, unless using the faster J-B Quick.
The polycone had already been primed with a thin layer of CA. Use the activator on the polycone then QUICKLY spread a THIN layer of CA all around on the rubber surround mating surface. The CA will get very hot when it comes into contact with the activator so you have to work fast as it sets up fast. Experiment first if not familiar with it. The trim ring is attached to the frame now, and so is the rubber outer surround. Next you must attach the inner surround to the polycone. Spread activator on the polycone and a THIN layer of CA on the mating surface of the inner surround. I next used a flat plastic circular tool cut from a plastic storage container lid that just touched the inner edge of the rubber roll with a hole the size of the phase plug exactly in the middle. This will center the surround on the polycone. Also, and this is important, you must have inserted three or four evenly spaced paper spacers between the voice coil and pole piece to keep it centered while attaching the surround. They need to have a friction drag fit, so find the right thickness of paper to use. With the polycone and surround properly coated with their respective glue components, the tool goes on and weight is added such as a heavy glass beer mug or smething hollow in the middle that will not contact the phase plug. Press up on the polycone inner surface to make good contact along the entire glue joint. Like me, you will probably have to leak a bit of CA into places that didn't glue down properly then press and hold a while longer in that particular place. This is a critical step that you need a good idea of what you will be doing, work fast, and be precise as possible.
I had the ceiling fan going, windows open, and crossflow fan in operation while doing all the chemical based operations. I was sick for at least a day afterward in each case. Almost nothing attacks polyethylene, which is why hazardous chemicals are often stored in polyethylene. You had better respect ANYTHING that will attack and get hold of PE.
With everything stuck down and cured out, it was time to finish up. The mortite that was painted with a magic marker was painted with flat black paint. The lightly painted phase plugs got a coat of flat black paint. The sanded edges that stuck out beyond the rubber surrounds got a coat of flat black paint, to match the rubber surrounds. The woofers look better than original.
I swept the speakers with a signal generator, and resonance was about 35 Hz. Also, one of the speakers had a rubbing voice coil. It originally had this problem as well, so it wasn't all my doing. Here is how you fix a rubbing voice coil. Take 600 grit wet or dry sandpaper (the black kind) and cut strips about an inch wide and about 4 inches long. Insert the sandpaper between the voice coil former and pole piece, rough side to the aluminum former, in the loosest part of the gap. Use a signal generator (CD player, Stereophile Test CD, and power amp will work) and drive the speaker in the bass range near maximum excursion while you work the sandpaper from the loose side toward the tight side. Aluminum dust will stick to the sandpaper, so you change it out when it clogs up. In a few minutes, you should be able to freely move the sandpaper in a circle around the pole piece while the voice coil is pumping back and forth. You should have a voice coil that isn't rubbing and fairly evenly spaced. If a speaker has a dust cap, it has to be removed for refoaming anyway, but a phase plugged driver should be able to use this fix at any time, even with the driver installed in the loudspeaker.
A phase plugged speaker without a dust cap will have an air leak between the pole piece and voice coil, which is a kind of resistive flow vent. A big air leak is bad, but a small one can tighten up your bass. The resistive flow vent is a neat trick that hasn't gotten the attention it deserves. Like shock absorbers on an automobile, the right amount of damping improves the bass.
After the modded woofers were installed, I got bass response readings of about flat at 40 Hz, relative to correlated pink noise, -3dB at 31.5Hz and -6dB at 25Hz in my room at my listening position. My Vandersteen speakers were flat at 40Hz, -6dB at 31.5Hz and fell flat on their face at 25Hz. Same speaker positions, amps, everything.
On actual music material, the speakers have very tight bass, tighter than the Vandersteens.
All my drivers are mounted with black oxide coated panhead screws I bought from Parts Express, about $3 per hundred pack of #6 and #8 screw sizes in 1" length. The bugle head screws supplied with the kit were only used for internal mounting of the crossover boards to the cabinet, after I countersunk them. Bugle head screws are made for drywall and softwood use where the head countersinks itself. You want a flat mating surface for holding down metal and plastic components, unless the device being held down is countersunk and all mounting holes are precisely aligned and centered on the countersunk hole. But, a bugle head screw forms a curvilinear cone rather than the straight cone of a normal flathead screw intended for countersunk use. All panhead screws used in my cabinets cost me about $2 and the black finish is unobtrusive. I have not seen bugle head screws used in photos of the factory mounted drivers, so I set aside the ones I got in the kit for drywall use or other projects around the house.
Yes, if you are short of cash and need to rebuild those RM-1 woofers, it can be done. If you have the cash, spend it on the megawoofers and save yourself a lot of hassle.
