[Steve D]

The field coil in my RCA V-210 speaker is open and I know how to electrically replace it if I had to. What I want to do is actually remove the coil and rewind it.  I have access to industrial coil winding equipment so that is not an issue but I wanted to get some opinions on removal and re-installation on the speaker frame.

The speaker magnetic assembly is pressed and riveted together and the pole piece is pressed in.  I see that I can remove the cone and voice coil relatively easily and am considering using my automobile 20 ton press to press the pole piece out allowing me to remove the field coil for rewinding. However, I am concerned about how to ensure proper centering and a uniform coil gap when I press the pole piece back in. Perhaps a sleeve of the correct diameter and thickness can be use to shim it up as I press it it?

Has anyone done something like this?  Any ideas are welcome.

Thanks,

Steve D

[Steve Davis]

I have not done an RCA speaker but I have done a couple of Philco speakers. The Philco speakers have step machined on the pole piece, so when pressed back in, it will self center. I suspect the RCA will be the same.

Steve

[Steve D]

            Steve, 

Thanks for the tip.  I'll find out if RCA uses a similar arrangement when I take it apart. I have attached a few photos of the RCA speaker.

Steve D

[Arran]

To unglue the cone, try lacquer thinner or acetone, if it isn't coming apart already. I think that pressing out the pole piece will work, it does not look welded in like in some speakers, see if you can find an arbor or a hydraulic press to do the job, don't use a hammer and drift. Before you go that far though, see if it's just a broken wire near one end, you will have unwrap the field coil anyhow to rewind it.
Regards
Arran

[tbone]

I have heard of some technicians charging up a high value 450 V electrolytic and connecting it across the field coil in an attempt to weld the break together. In theory it could work. Since it’s alread open, what have you got to loose.
I suppose if there is no corrosion in the windings it could last.

[morzh]

You could try.
I highly doubt that a simple breakage could occur in an immobile coil like the field coil.
It is either a burn or corrosion. Corrosion won't heal by a discharge, and if by some God's miracle it does, it won't be reliable, and burn won't heal, period.

Also, the field coil being an inductance, the spark may not occur / or might occur not there and then where and when you expect it to.

I'm not discouraging you from trying.....but not a good way by any stretch even if it seems to have worked.

[tbone]

Yes, not the preferred method. I’ve rewound all of the bad ones I’ve had, but they also all unbolted from the frame.

[Steve D]

I opened up the coil covering as far as I could in the frame and the open is definitely not at the terminations. So it appears that I have three options: 

1. Buy a modern perm. magnet replacement and add a chassis choke and resistor - not very original and a last resort.
2. Find an OEM replacement on Ebay - A preliminary search was not successful.
3. Try to press out the pole piece and rewind the coil - I'll try this first because what have I got to lose?  If it it doesn't work I'll go with one of the other two options.

I have a 20 ton press in my garage and can use some pipe as an arbor. I think you guys are right that it should be pressed out from the rear and it might have a centering step. I'll know more when I get the cone out. The voice coil and spider assembly are held in with two screws so all I have to do it loosen the cone edge from the frame with some acetone and I think I can get the cone and voice coil out without damage. I am a engineer with a high voltage power supply company and have facilities to rewind the coil at work so that should not be a problem.

I want a permanent fix and I don't think that fusing the open wire by zapping it with HV will be satisfactory. I'll post photos and updates of my progress with this project.

Thanks again for all the responses.

Steve D

[Arran]

Steve;
  What I have done in the past to replace the field in a field coil speaker was to mount the replacement choke on the basket of the P.M dynamic. In this case it was a Sparton console from around 1935, and the output transformer was mounted under the chassis, but in many cases there are extra holes on the speaker basket where you could mound a choke on the side opposite the output transformer, and even if there were not you could drill a pair in a convenient flat spot. Repairing the original is still preferable, but the Sparton's speaker was missing, which is a shame since it was probably an old Magnavox electrodynamic, or a Rolla.
Regards
Arran

[sdradioman]

Hey Steve, I have an RL70L6 speaker out of an RCA V-210 if you need one... I aquired a couple of that model through a big lot purchase and gutted them because the cabinets where junk... If you need it send me a PM and we can set something up...

[Phlogiston]

Some of the Philco speakers have the pole pressed in and then peened. I have ground down the peened end and then pressed out the pole - not fast but not hard either. I would definitely try it on this speaker.

[morzh]

Also (I have not pressed out any poles, but I think I say that) when pressing the pole out, the support should be applied against the part that is the coil holder, not against the outer rim of the basket, or it may warp.

[Steve D]

So, I this past weekend I took the speaker apart and was able to press the pole piece out. Here are some photos of the speaker cone:

   
   

Motor assembly:

   
   

I use some cast iron pipe as an arbor to press the pole piece out:

   

Continued in next post since I can't post more than 5 photos in one reply.

Steve D 

[Steve D]

It took quite a bit of force and when it released it suddenly broke free and popped out but came out undamaged:

   

Here is the hum bucking coil and a copper annular ring that was sandwiched between the hum coil and the field:

   
   

The field coil is wrapped in cardboard and covered in black insulating paper.  The pole piece is a bit rusty and does not have a step at the pressed-in end to center it so I will have to make a fixture to properly center it when I press it back in.

   
   

The coil read open directly at the magnet wire ends so the break is internal, and I will have to rewind it. I took some measurements and the wire is 0.008” dia. over the insulation and 0.0065” over the bare wire where I stripped it, so it appears to be 34AWG with heavy Formvar insulation.  The coil ID is 1.1”, the OD is 2.3” and the winding width is 1.1” and appears to be random wound.
 
The specified resistance is 1060Ω and 34AWG is 0.265Ω/ft. so the wire overall length is 1060/0.265 = 4000ft. The MLT is (2.3+1.1)/2 Xπ = 5.4”. The number of turns = 4000 X (5.4/12) = 8900T.
 
Using an 85% fill factor the #turns/layer = 1.1/0.008 X 0.85 = 120T.  Using a 90% winding factor for the winding build gives a build of 8900/(120 X0.9) = 0.65”.  This is close to the measured build of (2.3-1.1)/2 = 0.6”.
 
So now I have to figure out how to make a mandrel and guides to strip out the existing winding and then to wind 9000T of 34AWG wire. This may take a while.

Steve D

[EdHolland]

Nicely documented Steve. I also like your calculations for the wire build.