Voice Coil Repair, Adaptation of Dome Tinsel Repair Thread

[Charivari]

This particular thread is basically a photo documentary of the techniques detailed in this thread being adapted to simple voice coil repair.

The driver repaired here is a 1" textile dome tweeter as found in the Design Acoustics PS-10 that was measured as being open or no continuity.

- JP

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[Charivari]

The first step is the simplest, you need only to disassemble the driver. For most domes, this can be typically accomplished by removing the four screws (not the mounting screws) located near the dome that attach the face plate to the magnet. The particulars of which screw goes into which hole are unimportant, however, be sure to mark the faceplate and magnet in some way such that you may be able to line the same faceplate/magnet holes once again. While the screw holes are supposed to be perfectly symmetrical, they typically are not. So, without a matching of the holes once again, there is the risk that the voice coil will not be centered and so will rub.

Most better domes drivers tend to have the dome and voice coil attached directly to the face plate. For these, you need to carefully pull the faceplate straight off, not at an angle, to prevent rubbing the VC against the magnet and forcing the wires out of position and creating more work than you bargained for. Some of the less expensive drivers sometimes have the dome/VC attached to a separate piece sandwiched between the magnet and the faceplate. Such is the case with this particular tweeter.

- JP

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[Charivari]

The secondary plate with the attached dome removed. Larger dome drivers tend to have a hole in the pole piece of the magnet half that is filled with foam or felt to absorb the backwave. Smaller units, such as this tweeter, sometimes add only the shaped material atop the pole piece to both absorb the backwave and to maintain the shape of the dome.

Once the VC is removed from the gap, take particular pains to keep any metal fragments (as can be seen barely in this picture stuck to the magnet) from your work surface or that remained from manufacture, from slipping into the gap. Once there they are nearly impossible to remove and can either force VC rub or completely jam the former against the magnet or pole piece creating a junk driver.

- JP

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[Charivari]

Once you've taken everything apart, look closely for any breaks in the magnet wire or tinsels. Typically, the breaks are found in the tinsels, those sections of wire that lead from the connections to the VC, as these tend to be free hanging and experience much motion during driver operation. A second common place is found where the voice coil former is glued to the dome and such where the tinsel must run into the VC. The wire found on the larger domes is small enough, with a tweeter dome such as this, a strong magnifying glass is recommended.

- JP

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[Charivari]

Here's a close up of the break where the wire leads to the VC. You can see where a portion of wire is completely missing and the sharp bend indicates a fracture that will soon let go. This entire section is unrepairable and should be removed very carefully for replacement. Take your knife, make sure it's as sharp as you can get it, and very carefully scrape away some of the glue that may block the replacement wire.

Mind you, with dome drivers, the voice coil is the same size as the dome, so a 1" dome has a 1" VC thus making these pictures not reveal just how small all this is.

- JP

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[Charivari]

It's time to get your repair tools in order and ready at the bench. Fancy tools aren't necessary and a sharp swiss army knife can be sufficient if your hands are steady. While I don't have one, a swing arm magnifying lamp is highly recommended else you'll have to work via brief glimpses through the magnifying glass and memory (not the easiest thing to do). Be sure to clamp the plate in some manner to give yourself an easy angle at the section of the VC needing repair. As you can see here, the 15 watt soldering iron has the finest tip I had installed, but it was insufficient. So, I filed it down to a finer point and smoothed it out on the whet stone to ease the solder flow. When accomplishing the repair, keep a very tiny drop of silver solder just on the very tip to make the joint else the VC may be shorted or clogged with solder going where it shouldn't.

To accomplish the repair, just refer to the instructions outlined in the other thread. Be very careful with fine magnet wire such as this else it's easy to break it while scraping the coating away for solder. The bridge over the break was made with a single ultrafine strand from some scrap Sound King speaker cable and tinned with the silver solder to make the task easier.

With the location of this particular break, I didn't need to worry about keeping it flat enough to fit in the magnet gap (sits just above it). Other repairs down in the true VC would need especial care to keep the wire from standing above the rest too much else VC rub will occur as well as possible shorting.

- JP

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[Charivari]

The nearly completed repair. You can see the long tail thus far left on the replacement wire. This gives the repairer something to hold onto the wire with and hold it in place while attempting to solder the new with the old. Very carefully take a pair of sharp, fine tipped sideways cutters and remove the excess as close to the old wire as possible to prevent rub, snags, and the associated breaks.

- JP

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[Charivari]

Final visual inspection of the repair. Shortly afterwards, a regular old Simpson 206 multi-meter was used to verify continuity of the repair between the connections. Oft times the soldering iron's heat will melt the glue holding the VC wire and make the replacement wire seem to stick, but there is no electrical connection. With no electrical connection, the driver simply won't work.

- JP

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[Charivari]

After reassembling everything (with the proper screw holes aligned), it's time for final testing to make sure something didn't break or short when reassembled. The slight voltage from the meter will be sufficient to indicate whether it's working and oft times the sound will indicate if there's VC rub or not. With larger domes, it's possible to gently press straight down at the edge (where the VC is attached) gently and listen for the very quiet rasp of VC rub.

When all checks out, stick the driver back in the speaker and use the finest tool of all, the Mark I Ear to verify all's sounding as it should. If so, pat yourself on the back and call it a job well-done.

- JP

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[Charivari]

Recently, I had the opportunity to attempt a similar repair on a Philips AD5060/W8 5" Midbass Cone. These are fairly common units used in a great many better designs from the 1970s, such as the Infinity Quantum series and the Dahlquist DQ-10. While these units seemingly abound, when one is in need, they cannot be found. This particular unit came from one of Paul's (thedelihaus) new DQ-10s and the time table to make his birthday meant either sending him speakers with an inappropriate replacement or attempting the repair of this. The symptoms were that it mostly didn't work and measured open for the most part, but would mysteriously emit a bit of noise or measure properly very intermittently.

First thing off, everything looked alright from what was visible. Measuring continuity with the audible alarm on my second meter engaged, I did a tap test on the various parts until I discovered a soldered joint on the cone where the tinsels meet the voice coil wire had lost most of its conductivity. A little bit of scraping with a knife and a quick hit with a soldering iron fixed this. Unfortunately, this wasn't the only problem and surgery was required to get at the VC and inspect that.

Close up of the repair on the right hand lead.

- JP

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[Charivari]

This particular driver model came with butyl surrounds. Sometimes when left in direct sunlight or stored in poor environmental conditions, these surrounds can crack. Regnar offers a complete rebuild service that includes new surrounds. With this driver, the surround was in good condition overall and time prevented sending it out for rebuilding.

For a butyl surround, it's best that it be reused so any efforts to remove the cone will require very carefully cutting the surround from the frame. Taking the trusty workbench Swiss army knife, sharpened at the tip, carefully work up a little section of the edge trying to minimize the amount of rubber sliced from the surround. Once enough of a space for the blade is created, come in at an angle as shown in the picture, and carefully start cutting along the rubber-metal seam. This approach, while a bit more difficult to accomplish, allows the blade to cut at a flat angle to the frame thus minimizing damage. Before or after the surround is cut free, take a soldering iron and unsolder the tinsels from the connectors to free the cone. Then, taking the knife again, carefully cut the cloth spider as tightly against the frame as possible. The spider will cut very easily, so be careful to keep good control of the blade lest it cut the wrong things.

- JP

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[Charivari]

Hope you've not changed your mind by this point. If so, you're a little late.

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[Charivari]

Ok, I got a little side-tracked for a bit there. Time for the end.

Using the techniques outlined and linked to above for dome repair, inspect the voice coil, and check for any breaks. In this case, one of the tinsels had a break right at the joint where the voice coil former meets the cone, which was easily fixed with just a quick touch of the soldering iron with a bit of silver solder on the tip.

Now has come the time for reassembly.

You've the cone in hand and the frame/magnet assembly in the other, just how the heck do they go back together? Well, hopefully you've paid attention to how the cone came out of the frame and can put it back exactly the same. If you haven't, just line the tinsels up with the connectors as the tinsel holes in the cone will be straight across from the terminals typically. Carefully put the voice coil back into the gap, solder the tinsels back to their terminals, but don't glue yet. When refoaming woofers, the spider does most of the work keeping the VC centered such that it does not rub. With both the surround and spider cut, the VC will want to sit off to one side or the other and just gluing it so will result in an unusable driver.

The trick here is to use a 1.5 volt battery, be it AA up to size D, and connect it to the driver's terminals in reverse polarity such that the cone is seemingly sucked into the magnet. The electromagnetic field created within the permanent magnet field will center the voice coil and the reversed polarity will not only hold the VC tightest in the field (and thus be most accurately centered), but will also hold the free edge of the spider against the basket where it was cut away earlier making for an easier time gluing it back in place and ensuring a better result. Connecting the batter + to + and so forth will kick the VC completely out of the gap and just be a general pain to work with.

When the juice is applied and all is ready, take your glue and run a bead around where spider meets the basket. In my brief experience, regular surround glue seems to work, but a polyurethane based product may be better. When all looks good, walk away and enjoy music in the next room or something that'll keep you from playing with the driver before the glue has set. A few hours later when all seems well, disconnect the battery. The observant will notice right away that the cone rests at a position depressed from where it had been before hand. Reconnect the battery + to + and the like (not reversed) and the cone should push out just past the normal rest position. Run a bead of glue around where the surround is to go, press the surround edge into place, and if necessary find something to clip it down just as you normally would refoaming a driver. Check back regularly and press down spots that may attempt to rise. The battery trick here helps keep the VC centered while fixing the depressed rest position of the cone.

When the glue is dry, check if the VC is rubbing by gently pressing it straight in linearly. Double check with the meter to make sure the DC resistance still measures correctly. Return the driver to its speaker and run your favorite tune at first at a lower level to make sure it does work. Then, enjoy.

- JP

[clint e.]

Great job.
Tanx.

[Charivari]

For another slight derivative, I also affected this repair a little while on a 2" Peerless cone tweeter. This particular model of AlNiCo tweeter was a popular option with a great many speakers in the early 1970s and is perhaps the smoothest cone tweeter that found widespread use. Unfortunately, these little tweeters are a touch fragile and being tweeters have a tendency to be blown in speakers being underpowered and overdrive. The problem arises in that it's been decades since they've been made and proper replacements are becoming all the more difficult to locate.

I'll briefly cover how to accomplish the above repairs on one of these tweeters taken from a Design Acoustics D-6.

- JP

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