Rebuild K-77 Alnico Tweeters

Before disassembly, mark the magnet assembly and the metal diaphragm mount that the wires connect to so they can be oriented correctly for reassembly. A red dot on these two parts on the side that the positive wire connects to is suggested. 

Remove the 4 screws from the back of the magnet assembly.


Separate the magnet from the metal diaphragm mount.  Notice that in this case, the diaphragm and its gasket are stuck to the magnet assembly.  That happens sometimes.  The diaphragm should have stayed with the diaphragm mount on the left.  When the diaphragm stays with the magnet assembly, the lead wires will be pulled off when you separate the magnet assembly.  No problem, you were in here to replace the bad diaphragm anyway.  If you just took it apart to see what was inside, now you need a new diaphragm. 

Anyway, now just carefully peel the diaphragm and it's gasket off the magnet assembly.


If the diaphragm stayed with the diaphragm mount, cut the lead wires to get the diaphragm and its gasket where you can separate them.  Do this carefully.  You may have to use a knife blade to help separate them.  You have to reuse the old gasket, so take your time here and make sure to get the gasket off in good condition.


The next step is to clean out the holes that the old diaphragm's leads are still in.  Here, I prefer to use a "solder sucker". There are a lot of different kinds and I have used them all, however, the kind that uses a spring to quickly develop a vacuum are my favorite. I think the one that Radio Shack stocks is 5 to 10 dollars and will do a good job. Try to work quickly and be sparing with the application of heat since it would be possible to damage the insulation which surrounds the terminal. 

Notice that you do not have to unsolder the crossover wire from the tweeter. 


Clean out the magnet gap.  I like to use a sticky note for this folded so that the sticky stuff is on both sides.


OK, how do we orient the new diaphragm?  Here is the way to tell how this works.  Look at the picture below.  The magnet gap has a notch.  The diaphragm has a red mark.  The red mark is to be lined up with the notch.  The terminal closest to the notch and red mark is the positive terminal.  To be more specific, if the diaphragm is installed like that, a positive voltage applied to positive terminal will result in outward movement of the diaphragm. 


Place the properly oriented diaphragm into the groove in the diaphragm mount.  Allow the copper-beryllium leads to just go through the terminal holes and stick out the other side.  We will solder them later.


Place the gasket you carefully removed earlier on top of the new diaphragm and carefully press it down inside the groove.  It now should hold the diaphragm in place. No glue should be used.


Maintain the orientation for positive you worked out earlier.  Place the screws in the magnet assembly holes to aid in alignment as you put the tweeter back together.



Tighten the screws a bit at a time to gently draw the magnet assembly back into place.  The idea is to keep the magnet assembly straight up and down to keep from damaging the delicate voice coil as it inserts back into the magnet gap.  Get it tilted a bit to one side and you are ready to start over and you need another diaphragm.


Solder the lead wires.  Use as little heat as possible.  If you keep the iron on the terminal very long, you are likely to destroy the insulators on the terminal.


Clip off the excess lead wire after you solder it.  This lead wire is difficult to cut. The scissors on a Swiss Army Knife work better for this than anything else I have found.


You can check to make sure all seems right now if you have a multimeter.  The resistance should read around 6 ohms.






B & K Sound, 1925 East Gum Log Rd. Russellville, AR, 72802 phone-479-967-1542