K-77M Rebuild

Rebuilding EV T-35A and Klipsch K-77M tweeters (square magnet)

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

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

Separate the horn from the plastic diaphragm mount.

Pry the plastic diaphragm mount off the front of the magnet. Use the two indentations on the plastic diaphragm mount for this. Use a jewelers screwdriver or other small pointed device to alternately pry on the two sides until it slips off the magnet.

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Now you will have exposed the diaphragm. You want to observe the round paper or fiber gasket. It may be stuck to either the magnet assembly or (more likely) the diaphragm. Since you have to reuse the gasket, this is a good time to remove it and put it in a safe place.

Now since you are in here to replace the diaphragm, you can just cut the two leads which are soldered to the terminals in the plastic diaphragm mount to get the diaphragm out of the way.

The next step is to clean out the holes the old 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 use a spring to quickly create a vacuum are my favorite. I think the one that radio shack stocks is about 5 to 10 dollars and will do a good job. Speed is of the essence when removing the solder (and putting it back for that matter) because the diaphragm mount is plastic and it is penetrated by the terminal you are heating. The plastic will become soft at just about the same time solder melts. Work quickly and if you need to heat a second time, allow things to cool off good between applications of heat.

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After both holes that the leads go through are open, place the new diaphragm in position while guiding the leads through the holes. The new diaphragm has a red mark near the positive lead. (note, this mark will line up with the "notch" in the magnet assembly...even if that does not match the marks you made in step 1) Make sure this lead goes into the side you previously marked as positive. (Just before putting this in you might want to go ahead and measure the resistance of the voice coil just to make sure you have a good one to install.)

Note: If needed, you can use a couple of tiny dots of super glue to hold the diaphragm in place.  Also, you may need to put a couple of tiny dots of glue to hold the gasket in place.

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Now is a good time to clean out the magnet gap. I use a strip of business card about 1/2 inch wide with a similar size piece of scotch tape (sticky side out) folded over the strip of business card. While holding the tape in place with my fingers, I run a corner of this "cleaning tool" around the magnet gap until I no longer get anything out of it. The clearance between the voice coil and the walls of the magnet gap is tiny so I think it pays to spend a bit of time getting everything out. 

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Now, time to start reassembly. No I did not forget to solder the new diaphragm leads in place. I prefer to do that after the tweeter is reassembled.

Place the diaphragm mount on the horn. Remember to orient the marks you placed on it earlier.

Place the gasket on top of the new diaphragm. Position it very carefully and make sure it stays in position during the following steps.

Note: You may want to use a couple of tiny dots of super glue to hold the gasket in place.

Hold the magnet assembly, back facing up and oriented with the mark you placed earlier lined up with the mark on the plastic diaphragm mount. 

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Drop the 4 screws back into their holes in the magnet. Carefully line up the screws with their holes in the plastic diaphragm mount and make sure that they all drop into the holes as you slowly lower the magnet down until it makes contact with the plastic. Run the screws up a bit at a time until they are all in contact with the magnet. Alternately tighten the screws to pull the magnet down into its seat in the plastic diaphragm mount. Remember that small gap we cleaned earlier? Now you are inserting the voice coil into that small gap. Try to keep everything very straight.

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Now it is time to solder the new leads from the diaphragm. You are now working with a tiny piece of flat wire that is extending out of the two holes you cleaned out earlier. Again, you need to work quickly and try to prevent the plastic from getting too hot. Make sure that your soldering iron is fully up to temperature and that everything you need is at hand.  Try to very quickly solder it where it leaves the hole in the terminal.

OK, we're done! Let's hear them tweets! Not so fast. Let us see if everything looks all right. Measure the resistance. Should measure around 6 ohms for the 8 ohm diaphragm. Resist the temptation to just hook them up to an amplifier to see how they sound. These tweeters can tolerate very little power so hook them back up to the crossover and then enjoy the sound.


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