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Electrostatic Tweeter repair and restoration.

This is a German made electrostatic tweeter.  It has failed like so many others of this vintage.  This tweeter is from a 1959-60 Emud Junior 196 six tube AM/FM radio.  There were three choices involved with this speaker, leave it in the unit dead, find a replacement that works or repair the existing speaker. 

If one wiggled the positive contact while playing the radio the sound would cut in and out.   It was a quite subdued sound but present none the less.  So see below the steps to restoring this Electrostatic Tweeter  speaker. 

Note below the three main problems that contributed to the failure of this speaker.  All caused by deteriorating foam.


  IM000216.JPG (101648 bytes)The first step was to remove the speaker from the cabinet.  You can see the two contacts on the tweeter.  One contact is the actual metal perforated grill of the speaker (ground).  The other is a loose and floppy silver contact (positive 170 volts DC through a 100k resistor!  It will bite you, Ouch!).  

With so much DC on this speaker one must ensure cleanliness and the integrity of the internal components do not create a short circuit.  The 100K resistor would protect this particular radio but this is no guarantee in other radios.

 IM000217.JPG (315373 bytes) A  Dremel Moto tool with a sandpaper disk made quick work of the formed plastic retainers that hold the assembly together.   Note:  Leave as much plastic as possible.  It will be re-flared with a hot soldering iron to hold the assembly together.  

An alternative is to drill out the rivets and use tiny machine nuts and bolts to hold the assembly together.  Use Glipt, Loctite or old fingernail polish to lock the nuts to the threads.  This assembly vibrates since it is an electrical to physical transducer to vibrate air waves.

bulletIM000218.JPG (344377 bytes)This picture shows the internal components of the tweeter.  
bulletLower right is the silver contact that has significant corrosion on it.  
bulletThe upper left is the delicate extremely thin plastic like membrane.  It has a conductive coating on one side that is press fit to the silver contact mentioned above. 
bulletThe silver item in the middle is the perforated grill.  It has a deteriorated pieces of foam on the right side that pressed the membrane to the silver positive contact. 
bulletThe lower left brown item has some dry rotted foam that presses the membrane against the perforated grill.  

Problem identified - Deteriorated Foam.

This was the main problem.  Deteriorated foam that causes a poor or no "press fit" connection between the silver contact and the membrane.  

corosion.JPG (293657 bytes)

bullet Seen here is the corroded contact, the deteriorated piece of foam and the cellophane tape that I think insulates the positive contact from the negative perforated grill.  
bulletI have seen this type of foam stain, corrode and ruin what ever it is in contact with including dial scale printing.

This was the second problem with the speaker.  The foam corroded or coated the silver contact with crud.

Burnish and clean the silver positive contact with fine grit sandpaper.

IM000221.JPG (47494 bytes)

This was to a lesser degree, the third problem.  This foam backer was not pressing the membrane close to the negative electrode (the perforated grill).IM000223.JPG (233213 bytes)

Cut out a new replacement piece of foam that holds the membrane against the perforated grill


Reassembly - Put it back together 

IM000222.JPG (316958 bytes)

Assemble in order:

bulletPerforated Grill
bulletElectrical tape
bulletDouble sided sticky foam tape from Radio Shack.
bulletMembrane.  Be careful to maintain the orientation of the conductive side true to the disassembly.
bulletPlace the silver positive contact in the plastic cone.
bulletAnd replace the brown deteriorated foam with your new piece that you cut or fabricated.
bulletPress the assembly together and flair the plastic rivets with a hot soldering iron.
IM000224.JPG (353142 bytes)  This picture shows the assembled speaker.  The new foam piece is the other half of the foam inside the speaker.  It was twice as thick as the original.  I used an X-Acto blade to slice the thickness in half.

Also seen is:

bulletElectrical tape to replace the cellophane tape.
bulletDouble sided sticky foam tape to press the silver contact to the membrane and 
bulletThe deteriorated foam used to press the membrane against the perforated grill.  The brown foam broke apart into powder when I picked it from the speaker structural cone.


IM000226.JPG (226930 bytes)The rebuilt tweeter on the Emud sounding Great!  The output is significantly greater (louder) than before it was rebuilt as compared when wiggling the previously loose positive contact.

I can't properly reproduce the sound on this web page so a hearty "Thumb-up" will have to do.




This is a flat square electrostatic tweeter from a Grundig 3035.  It had a corroded contact that is press fit against the conductive vibrating membrane.  Disassembly and sanding off of the corrosion where the contact touches the membrane is all that was needed.  

1 grinding.JPG (198564 bytes)  Use a hobby grinder to take off the tops of the melted plastic rivets pry a.JPG (187850 bytes)  Test the side with out lead wires by prying up. pry b.JPG (167383 bytes)  You may need a little more grinding to remove the retainer side with the lead wires.  inspect .JPG (189450 bytes) Lay all the part out on your bench.  The white material seems to be wool.  That is used to keep the conductive membrane close to the metal screen.  The wool is much better than the foam rubber.
coroded contact.JPG (183389 bytes)  Identify the corroded contact the is press fit against the membrane and a strip of foil seen on the right of this picture.  Use sand paper or your favorite burnishing technique to remove the corrosion.   assemble.JPG (182386 bytes)  Reassemble the unit and test before melting the rivets.  weld and test .JPG (181982 bytes)  Melt the rivets, use glue or hot melt glue to hold the tweeter together.   test OK.JPG (199147 bytes)  Sounds Great!  


A new Tweeter failure mode

I came across a new failure that I have not seen.  The conductive coating has been ruined.  Almost like eaten away.  This essentially disconnects the conductive film from the positive solder lug rendering the tweeter silent.  Simple replacement of the foam is not the solution.  I had to find a replacement conductive film to fix this problem

After spending a day researching the internet and talking to technical sales people I discovered there is little hope for a simple cost effective source of materials.  The Chinese manufacturers want a minimum order of four metric tons.  Well they can forget about that.  US based distributors have to contract out the coating of Mylar to make one side conductive. But I was not satisfied.  

I came across some Mylar model airplane covering on the internet.  So I headed to my local hobby shop with a micrometer and DVM in hand.  They did not have anything that was conductive on one or both surfaces.  I left defeated.  

On the way out of the parking lot I stopped at a Dollar General for a pop (soda pop).  While inside the Dollar General I spotted the Mylar balloons floating at the ceiling.  A buck fifty for a cartoon character and a buck for a plain silver star.   I bought the star.

At home I let out the gas (did not inhale it) and cut the star open.  The exterior was conductive.  I got around 20 ohm per inch and a half.  The interior was an insulator.  Yreka!  The original conductive layer is near zero ohms.  I figured with such high impedances in this tweeter system what do I have to loose?

I reopened the tweeter with the fresh foam and pulled out the diaphragm.  Traced around it and cut it out.  Once inside the tweeter housing I connected the B&K digital cap meter and heard the familiar little buzzing tone and read about 600 pf.  Connected it up to the radio and WOW.  A dollar fix!

Enjoy the pictures below on how I facilitated this repair.


IMG_6705.JPG (399893 bytes)

Bad foam. 

IMG_6707.JPG (351818 bytes)You can see the deterioration of the conductive film.  The dark part is non conductive.

IMG_6717.JPG (217600 bytes) 

The star of the solution.

IMG_6718.JPG (280214 bytes)

I cut out the Mylar and placed it into the tweeter with a new contact strip. 

IMG_6721.JPG (299614 bytes)

And it WORKS!  But there is no safety margin between the conductive layer and the opposite insulation side.  It would not take much Bias DC to arc over.  

IMG_6730.JPG (247017 bytes)

I came up with this paper insulating ring. 

IMG_6722.JPG (243053 bytes)This is a pattern created in VISIO and printed.  

IMG_6723.JPG (86837 bytes)

Perfect fit. 

IMG_6724.JPG (250258 bytes)

Contact cement.  I did not have rubber paper cement. Contact cement is rather unforgiving.  

IMG_6725.JPG (244545 bytes)There is a margin of paper to increase the distance between the conducive layer with DC bias and the grounded grill. 

IMG_6726.JPG (307378 bytes)

Replace the foam sticky tape pressure pad. 

IMG_6727.JPG (324760 bytes)






Remember after the next Birthday party save some of those Mylar floating balloons.  It is up to you what you do with the helium. 

To see other Tweeter restorations see:

Telefunken page 

Blaupunkt Page.

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