K-55V Diaphragm Replacement

To begin with, I would say that in most cases, if the K-55V is working, it is good and needs no repair.  Seems though, as time goes by, I see more cases where the diaphragms are in need of replacement even though they still work.  After all, many of these are over 30 years old and some are over 40 years old.  I have seen a few cases where the K-55V worked but was performing poorly at the frequencies below 1 khz.  In those cases, replacing the diaphragm put them back into spec.  

Typically though, when the K-55V needs work, we find the voice coil completely open like the one in this picture.

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This K-55V is the single piece phase plug type with the push pin terminals for connecting wires to the driver.  The two piece phase plug version can be identified from the outside by the fact that it has solder terminals for the wire connection.  Both are simply labeled as K-55V.  Diaphragm replacement works the same on both types.

Remove the 6 screws that hold the driver together.

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Try just pulling the top part off the magnet assembly.  You may find that you have to do some gentle prying with a flat tipped screwdriver around the circumference.  Sometimes the two locator pins are a bit stubborn coming out.

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Here is the way the driver parts separate.  I hope your K-55V looks better than this one.  I think this one must have been under water at some time.

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Magnet assembly, diaphragm mount and housing gasket.

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Remove the old plastic diaphragm gasket by prying it off.  If it breaks, no problem, you have a new one with the new diaphragm and you just need to get the old one out of the way so you can unsolder the diaphragm leads.

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Unsolder the diaphragm leads.

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Remove the old diaphragm.

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Here is a good place to talk about cleaning the magnet gap.  That is the gap that the voice coil fits into.  I use sticky note pads folded with the sticky stuff to the outside to run around the gap cleaning out any dirt, grime or particles from inside the gap.  Keep doing that until the sticky note comes out clean.  This one was really bad.  

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Here is a picture of the diaphragm mount after the diaphragm was removed.  This part in the center is called the phase plug.  Notice the two locator pins that stick out of the diaphragm mount.  These pins fit into holes in the diaphragm and it's plastic gasket and later they will align everything when the driver parts are re-assembled. 

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Here is the new diaphragm.  This is the voice coil side.  The second picture is the other side.

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Now we need to determine the voice coil polarity so we can connect the positive side of the diaphragm to the positive terminal of the driver.  Notice the red paint mark near one of the diaphragm leads.  This is the positive lead and you connect the positive lead to the positive terminal of the driver.  Notice that the positive terminal is also marked with red.

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Slip the new diaphragm down on the locator pins and into position inside the groove of the diaphragm mount and solder both leads.

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Cut off excess lead length after soldering.

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Put the rubber housing gasket back into place.

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Line up the locator pins in the diaphragm mount with the corresponding holes in the magnet assembly and slip the parts back into place.  Put the 6 screws back into their holes.  Push gently on each screw to break through the thin plastic sheet that covers the holes.  Then, tighten the six screws a bit at a time to gently bring the parts together compressing the gasket.  Try to draw these parts up pretty evenly since you are inserting the voice coil into the small magnet gap as you tighten the screws.  Good to keep things pretty straight to avoid rubbing the voice coil on the side of the magnet gap.

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One more test.  This reads 10.5 ohms and is good to go.

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