[Steve D]

Thanks, EdHolland.  I found an old 5lb spool of non ROHS 34AWG magnet wire here at work and we have an old winding machine that is used for prototyping which I can use. I just have to design a mandrel to mount and hold the bobbin. 

I measured and calculated the voice coil gap to be about 0.040" and the diameter is 1", so to center the pole piece when pressing it back in I need to find a circular tube of that thickness and outer diameter. It can be metal, plastic, FR4 or even cardboard as long as it centers the pole piece and doesn't move as I press it it. We have a number of FR4 tubes that we use for bobbins and HV insulators here at work so I will have to search through them and hopefully I'll find a suitable one or one that I can machine to the proper dimensions.

Steve D

[Phlogiston]

One of the biggest issues with rewinding is supporting the coil-form cheeks. So wen you build your mandrel, try to add some large washers to keep the wire from making the coil "fat".

[Steve D]

You are right, Phlogiston, I removed all the winding wire from my coil and the bobbin is intact but very fragile. The cheeks are thin cardboard and will deform when winding.  My next task it to design a mandrill and cheek support.

   

I want the coil to appear original and if the sides are too flimsy I'll make new ones from a different material and cover them with the old cardboard ones.  Some experimenting to do.

Steve D

[klondike98]

Nice work! More than I would take on but you might save this one!

[Steve D]

So I finally got some time to work on the field coil this past weekend.  I made a new bobbin from some 0.030" FR4 sheet and 1.25"OD, 0.062" wall thickness FR4 tube. 

Cutting the bobbin cheeks with a fly cutter:

   
   

Pieces ready for assembly:

   

Next I epoxied the cheeks to the tube (original bobbin shown for reference):

   

Lastly I painted the bobbin flat black to match the original:

   

[Steve D]

Another shot of the new bobbin:

   

The next step is to make a setup to allow unspooling of the wire while maintaining tension and a method to guide the wire across the bobbin. 

Steve D

[Steve D]

Over the past two weeks after a few issues I have been able to wind the field coil.
 
I erroneously assumed that with the relatively stiff .032” FR4 material epoxied to the FR4 tube that I would not need additional side supports. So, I set up an old prototype coil winder using a wooden spool support, and some old Meccano parts to make a hand guide to allow me to move the wire feed back and forth to fill the winding width.  The pipe cleaner in the guide loop adds dampening and some tension.

   

It began winding fine until I got about half way filled (about 5000T) when the side force of the winding broke the epoxy and forced the sides out in a keystone shape. I should have taken Phlogiston's advice. So, I scrapped that winding and decided to re-use the original bobbin after cleaning it up a bit. Now with added side supports, I began again.

   

Below is the bobbin with about 1000T wound:

   

With the side supports the coil wound perfectly and I wound a total of 10,000T. Here is the finished wound coil. It measured 1100Ω which is a tad higher than the original which was 1060Ω but with today’s higher line voltage the small extra drop is beneficial.

   

I coated the winding with a few coats of Glyptal varnish and installed the original covering/termination contacts.  Here is the coil after terminating the magnet wire and lead wires to the contacts:

   

I used the original insulating fish paper over the contacts and then covered the whole coil with friction tape, exposing the original RCA part number.  See next post for finished photo.

Steve D


 

[Steve D]

Continued from the previous post:

Finished coil:

   

The coil fits the pole piece and speaker frame perfectly, so the next step is to press the pole piece back into the frame while maintaining a uniform .032” voice coil gap.
 
After I wound this coil, out of curiosity, I re-glued the FR4 bobbin with JB weld and using the side supports wound another coil.  It also wound perfectly, so now I have a spare coil or a nice inductor for any filtering needs. 

Steve D

[Steve D]

Finishing up the speaker:

I made the centering collar from four strips of 0.010 Teflon material which is what I had on hand and cut each piece successively longer to maintain the same length when wrapped around the pole piece.  This resulted in a thickness of nominally 0.040 and the gap appears to actually be closer to 0.038” and when compressed into the slot, fit nice and evenly tight all around.

   

With this arrangement I was able to easily press the pole piece back in and the resultant gap was beautifully uniform:

   

   

Next I used small strips of .007” paper to center the voice coil while I screwed the spider assembly in and glued the edges of the cone.  I had to remove the cone center cap to access the voice coil to insert the centering strips:

   

Continued in next post.

Steve D

[Steve D]

Lastly, I glued the cone center cap back on and made all the electrical connections:

   

   

Now I just have to finish refurbishing the chassis to power it up.  Hopefully it will play clearly. 
 
This was a lot of work and a good experience and I’m glad I saw it through, but next time perhaps I’ll try harder to find a replacement speaker.

Steve D

[morzh]

Great job!
Laborious process too.

[sdradioman]

I had a replacement speaker the whole time, just never heard from you again....

[Steve D]

Success, I got the chassis working and the speaker sounds great; loud and clear with no rattling, rubbing or distortion. The field coil drops the correct voltage and I Just need to do an alignment and refurb the turntable and cabinet. This was an enjoyable but laborious process which was worthwhile as I gained experience in speaker design but I probably would only do it again for a speaker that is unobtainum.

Steve D