aquila2010 Posted September 12, 2005 Report Share Posted September 12, 2005 In an erlier post about replacing the surround you said: "It is absolutely unnecessary to shim the voice coil on this woofer when replacing the surround; in fact, it is usually better *not* to shim it, because you can cause a slight deformation of the spider if the coil is perfectly centered with shims and then the surround is glued in place. Rarely is the voice coil perfectly centered even when manufactured. The spider can bind slightly, and eventually it might tear". As English is not my native language I can't understand exactly the sentence "The spider can bind slightly". What do you mean exactly? what the spider can bind slightly to? Thank you in advance for any answer and sorry for my English Link to comment Share on other sites More sharing options...
aquila2010 Posted September 12, 2005 Author Report Share Posted September 12, 2005 Exuse me, another question: When they manufactured these woofers did they shim them to center the voice coil? Link to comment Share on other sites More sharing options...
tysontom Posted September 12, 2005 Report Share Posted September 12, 2005 >In an erlier post about replacing the surround you said:>>"It is absolutely unnecessary to shim the voice coil on this>woofer when replacing the surround; in fact, it is usually>better *not* to shim it, because you can cause a slight>deformation of the spider if the coil is perfectly centered>with shims and then the surround is glued in place. Rarely is>the voice coil perfectly centered even when manufactured. The>spider can bind slightly, and eventually it might tear".>>As I don' t speak English perfectly ( I am Italian) I can't>understand exactly the sentence "The spider can bind>slightly". What do you mean exactly? what the spider can bind>slightly to?>Aquila,The word "bind" might be a bit strong, but what is meant is that a shimmed voice-coil assembly is, in some cases, being forced into another position in which it was not originally placed. The voice coil resides within the "magnetic gap" with small tolerances on both inside and outside the pole piece and top plate. This tolerance varies slightly from woofer-to-woofer, because the voice-coil/cone assembly was placed into position with a "jig" at the factory. Even though this was automated to some degree, there were still slight tolerance variations. The spider was then glued into place once the coil assembly is "centered" with the jig, and each woofer would then be checked for rubbing and distortion, etc.... When you shim the voice coil, you are forcing the coil assembly to be "centered" by wedging the coil against the pole piece all around the center. Then you would put on a replacement surround while the cone is in the "forced" center position, then remove the shims and put on a new dust cap, etc. The surround is glued into position relative to the shimmed voice-coil position, and when the shims are removed, the spider and voice coil want to return to their original "centering." The surround is now trying to hold the cone in a slightly different position from where it was originally. I must admit that I have *never* observed an AR woofer damaged from being shimmed, but I have seen one or two Allison 10-inch woofers with torn spiders because the woofer was shimmed. I should also say here that the Allison woofer did not have tolerances as close as the AR woofer, and the spider assembly is much lighter and thinner compared to the AR 12-inch woofer.In any event, by "bouncing" the cone from the center dust cap, and then using slow-drying glue, the cone can be restored to its original setting pretty easily because the spider will always try to put it there. You are not forcing it, but allowing it to find its own center position. Rarely will the woofer have a rubbing voice coil if done patiently this way. You are also not disturbing the original dust cap. Removing it is somewhat hazardous because you can accidentally cut the tinsel leads from the voice coil.I'm not saying that shimming should not be done. I'm just saying that it is unnecessary and probably not the best way to put on a new surround. I am sure that a lot of people will argue with that view point, and many have had good results with shimming and fast-drying glue, etc.--Tom Tyson Link to comment Share on other sites More sharing options...
tysontom Posted September 12, 2005 Report Share Posted September 12, 2005 >>Exuse me, another question: >When they manufactured these woofers did they shim them to>center the voice coil?>>In the very earliest days of production, shims were probably used. Once in full production, special jigs were used that would place the cone in the correct, centered position with respect to the magnetic assembly and speaker frame. This would hold fairly close tolerances, but even it was not perfect.--Tom Tyson Link to comment Share on other sites More sharing options...
Guest cober Posted September 12, 2005 Report Share Posted September 12, 2005 I never really understood the purpose of shimming for the foam surround replacement. The spider centers the voice coil in this plane. One hopes to get the surround on to the cone in a concentric manner relative to the spider movement. If the frame is bent then this is another matter.During the refoaming process I have used a variable power supply, operating through a DPDT switch to change polarity across the speaker. Using this one can carefully listen for voice coil rubbing and one can also do the cone movement electrically that Tom Tyson describes, just before, during, and after applying the surround. I actually bring the cone up electrically to make first contact with the surround. With a variable power supply one can also take the cone excursion in either direction to a reasonable extent to check for rubbing after the glue has set. Link to comment Share on other sites More sharing options...
dynaco_dan Posted September 13, 2005 Report Share Posted September 13, 2005 Hi thereAnother 2 1/2 cents worth from me on this topic.Approximately 12 years ago I did two of my AR 12" woofers for the very first time.The first one took about 1 1/2 hours and the second about 10 minutes.Previously when I worked at the local warantee depot, I saw JBL or Altec type speakers re-coned by the owner.We're talking 12" or 15" diameters, which I only saw being re-coned from a distance. Shims were provided with these kits, the gap was quite a bit narrower than AR type speakers.Even with care they sometimes required several attempts with just tacking the edges of the surround, letting the glue set and running a low level signal to check for buzzing.I bought a kit from the US, as this was the only source I could find.Here in Vancouver the only source I could find was a store that wanted to re-foam them for the labour charge plus the kit only. The kit I bought contained 2 syringes of white glue, an instruction sheet and 2 foam surrounds for the AR 12" woofer (special size).I peeled off the remaining foam from the speaker cone and gently scraped the excess from the edges.I saw that the cone could wobble off axis slightly so I connectd an amp and signal generator (test record with repeat of selected frequency at really, really, low volume level will also work), after I had tack glued the surround to the cone in a few spots.Just before tacking the surround to the frame I meve the entire surround and cone in each direction to hear the buzz. It doesn't take a lot of movement to hear the buzz in any direction and after a few attempts you can almost see where you are in centering the surround.I let the glue set and then gently pressed the cone towards the magnet checking for rubbing.After a few pumps I would move about 20 degrees and try again, this should have shown if I was too much out of alignment.Then, I slowly increased the volume of the amp and watched and listened to the woofer, which is out of the enclosure, sitting on my padded bench, facing upwards.I didn't overdrive it because the restoring force of the cabinet isn't there.Again, I'll try really low level music, I mean real low level, and by this time I stand back and just listen for a few minutes.I pat myself on the back, and finish running the full bead of glue around the perimeter of the surround and frame.One final test and re-install it back in the enclosure, to be tested again with the restoring force.Done.Good luck. >I never really understood the purpose of shimming for the>foam surround replacement. The spider centers the voice coil>in this plane. One hopes to get the surround on to the cone>in a concentric manner relative to the spider movement. If the>frame is bent then this is another matter.>>During the refoaming process I have used a variable power>supply, operating through a DPDT switch to change polarity>across the speaker. Using this one can carefully listen for>voice coil rubbing and one can also do the cone movement>electrically that Tom Tyson describes, just before, during,>and after applying the surround. I actually bring the cone up>electrically to make first contact with the surround. With a>variable power supply one can also take the cone excursion in>either direction to a reasonable extent to check for rubbing>after the glue has set.>> Link to comment Share on other sites More sharing options...
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