BEC Posted March 23, 2005 Share Posted March 23, 2005 OK, so you take your K-77M or T-35A ceramic magnet tweeter apart and find the situation is much more serious than just a diaphragm replacement. On a good number of these tweeters you will find that the pole piece has become displaced and there is no longer a concentric gap to insert a voice coil into. I have been told that Klipsch and EV both always considered these to be not repairable when this occurs, but with these tweeters getting older and more expensive, repair of this problem may be good. Don't mess up a good one trying this. Only do it if you have the case where the magnet gap has closed up and the tweeter will be junk unless this can be fixed. Bob Crites Edit: 12/11/06 See page 6 for new revised simplified method which eliminates several steps. Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 The problem. Notice that the gap is closed up in the upper left quadrant. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Here might be a good place to talk about what caused this to happen, The pole piece is centered in the hole in the top round plate forming the magnet gap. In the older alnico magnet tweeters, the top round plate had a welded ring on the back of it that the pole piece fit tightly through to center it. In the case of the newer ceramic magnet tweeters, I guess a manufacturing step was saved by instead of welding a ring on the back of the plate, the pole piece is centered by a plastic washer that slides up on the pole piece and fits in a groove in the top round plate. The bottom of the pole piece is held in place by a glue dot. As the plastic washer ages, it tends to loosen a bit on the pole piece and moves down to the point that it eventually leaves the top of the pole piece without anything to center it. It is in an extreme magnetic field and after a slight jolt, the glue dot at the bottom breaks and the pole piece moves with great force to one side trapping the voice coil and destroying it. Now we disassemble the magnet assembly. Place the back plate against a hard edge surface and give the magnet a hit with the heel of your hand. Try not to use a tool to do this. The ceramic magnet is fragile and would shatter like glass if struck with a hard object. Once you break the glue holding the back plate to the magnet, it will just slide off to the side. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Partly removed. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Pic showing match marks I put on the tweeter parts earlier so it all goes back together right. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 After the back plate is off, take pole piece and top round plate off the magnet by holding the magnet assembly by the edges and pushing a thumb through the center of the magnet. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Complete the disassembly by sliding the top rectangular piece off the magnet. This shows all the pieces of the magnet assembly. The "T" I put on the magnet marks the top. Extra credit question: Why would that be important? Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Here is the back side of the top round plate, the pole piece and the famous plastic washer that brought us to this point. This is also the point where you might ponder the amount of money they saved using the plastic washer instead of the welded washer attached to the top round plate that was used in earlier versions. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Hold the top round plate in your hand and fit the pole piece and its stupid centering washer into the groove in the top round plate. Seat the washer all the way into its groove. Thumbnails seem to work for me doing this job. Just make sure that it goes all the way in and is in evenly all the way around. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Now, while holding pressure on the plastic washer, position the pole piece such that the top of it is flush with the top of the top round plate. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Slide the back plate back on the magnet. Be sure to match it up like it came off and also center it on the magnet making sure the screw holes line up with the grooves in the magnet. After you are happy with the lineup, put a bit of superglue around the inside to hold the plate in place on the magnet. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Make sure the pole piece is still flush with the top and the silly plastic washer is still in its groove, then glue the washer to the top round plate and to the pole piece. Allow the glue to dry. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Slide the top plate back on the magnet and line it up using the screws that held the tweeter together. Put a bit of glue around the inside of the center hole in the top plate to hold it in place on the magnet. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Ok, now the glue is all dry and the pieces are as shown below. You will at this point be using the screws as alignment dowels to lower the top round plate and pole piece back into the magnet. Just be ready for how strong the magnet is. When you lower it down the screws, it will become almost impossible to hold at some point. That is where you just hope you have got it pretty straight and just let the laws of magnetism take over. Use the tips of your fingers on the sides of the top round plate and just allow the magnet to take it away from you when the time comes. Do not try to hold it with fingernails for instance. That magnet is a lot stronger than fingernails. When you are ready to do it, make sure your alignment marks you put on earlier are right. Place a dot of superglue in the center of the hole on the back plate and go for it. Bob Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 If things went right, it looks like below now. If not repeat all the above steps as necessary until it does. OR: The thing makes a nice paperweight. Bob Crites Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 One more picture, Here is the way they should have made the top round plate. Machined neck replaces dumb plastic washer. Bob Crites Quote Link to comment Share on other sites More sharing options...
Deang Posted March 23, 2005 Share Posted March 23, 2005 Excellent Bob. I only have one question: Is it "stupid" plastic washer, "silly" plastic washer, or "dumb" plastic washer. We need something solid here for this. Nice job with the posts and pics too. Quote Link to comment Share on other sites More sharing options...
BEC Posted March 23, 2005 Author Share Posted March 23, 2005 Dean, As I am fixing one of these things I use several names sort of quietly at first and then somewhat louder as the job progresses. The first one of these I did took me several hours. I have many more names from that experience. Bob Quote Link to comment Share on other sites More sharing options...
Olorin Posted March 24, 2005 Share Posted March 24, 2005 Excellent post, Bob. Quote Link to comment Share on other sites More sharing options...
DrWho Posted March 24, 2005 Share Posted March 24, 2005 ---------------- On 3/23/2005 10:53:30 PM BEC wrote: Extra credit question: Why would that be important? ---------------- Gotta keep the magnetic field going the right way to keep the tweeter in phase. Do I get a cookie now? Quote Link to comment Share on other sites More sharing options...
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