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 Post subject: field coil speaker repair and fundamentals of operation
PostPosted: Mar Mon 20, 2017 2:05 pm 
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Location: Mineral Wells, tx 76067
Gentlemen,
( I hope this is the right place for this topic). I am looking for information on antique speaker repair and how they operate. I am very interested in the engineering and design of these devices and how they actually work.
I have seen the on-line repair videos and for me , they didn't offer enough information. I am looking for technical manuals and/or principles of operation, etc. These things fascinate me and I would like to know more about them .
Thanks for your help,
walt


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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Mon 20, 2017 2:27 pm 
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A speaker is an electromagnet attached to something that will move air when the magnet is energized.

An electromagnet will produce a force on any permeable material, but if it pushes against another magnet, the force is much stronger. In modern speakers, the voice coil works against a permanent magnet.

In vintage radios, a field coil is used to generate a magnetic field for the voice coil to work against. I don't know what drove this initial design choice, but one benefit is that the field coil also acts as a filter choke for the B+.**

I suspect that early speaker designs were mostly empirical.....through trial and error, people found the best way to make cones, voice coils, field coils, etc.


**one possible design driver is that permanent magnets in the 20s and 30s were not as strong as we have now.

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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Mon 20, 2017 2:32 pm 
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Go to the top of this page and click on Archives. Click the third book in the, "Elements of Radio Servicing: Chapters 9-12" series. This should help you.

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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Mon 20, 2017 2:49 pm 
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Location: 13 Critchley Avenue, PO Box 36, Monteith Ont, P0K 1P0
waltdavis777 wrote:
Gentlemen,
( I hope this is the right place for this topic). I am looking for information on antique speaker repair and how they operate. I am very interested in the engineering and design of these devices and how they actually work.
I have seen the on-line repair videos and for me , they didn't offer enough information. I am looking for technical manuals and/or principles of operation, etc. These things fascinate me and I would like to know more about them .
Thanks for your help,
walt



The term you need to search is "Electrodynamic Speaker" https://www.google.ca/search?q=Electrod ... jwTl7K_ABQ

This search should give you more than enough resources to get your head around how they work.

cheers

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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Tue 21, 2017 1:25 am 
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Joined: May Tue 31, 2016 9:49 pm
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Location: Mineral Wells, tx 76067
THANKS so much for the reference material . I will get busy with that . I also wonder where someone might get the cones in case i might try to repair one myself.
walt


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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Tue 21, 2017 4:06 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Merrick,NY,USA
My understanding is that electromagnetic field coils were used in early speakers because permanent magnet technology was not up to snuff. To make a permanent magnet with enough "pull" it had to be massive in size for speaker use. There are some early 30's RCA radios that used permanent magnet speakers and the magnets on these things have got to weigh at least 20 pounds! Field coils were a better solution for supplying ample magnetic force without resorting to using 20 or 30 pounds of iron. They also conveniently doubled as chokes in the power supply.


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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Tue 21, 2017 9:11 pm 
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Location: Mineral Wells, tx 76067
Very interesting . Those old radios weighed enough with just the power transformer and wood cabinet , without adding a large magnet for the speaker.


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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Wed 22, 2017 11:56 pm 
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In most radios, field coils were a burden, not a benefit. Depending on size, the coils consumed anywhere from four to 20 watts of power, so power transformers had to be bigger, higher voltages had to be developed (due to the fact that higher current rectifiers were more costly), and the radios ran hotter. Owing to the need for a gap in the pole for the voice coil, they did not offer much inductance to assist with filtering. Radio manufacturers couldn't wait to ditch field coil speakers as soon as good alnico permanent magnet ones became available after WW-2.

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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Thu 23, 2017 1:41 am 
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Joined: May Tue 31, 2016 9:49 pm
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Location: Mineral Wells, tx 76067
So now I see why radios became lighter and more compact after permanent magnet speakers were developed. I just opened up a 1952 Coronado that I have and determined that it has a nice little 4" PM speaker that sounds great .

I am thinking about trying my hand at re -coning a 1941 Philco 6" oval speaker that I have , that is all patched up, but I have no idea where to get the cones. It's gets pretty expensive to get that done by a professional when you pay the fee plus shipping both ways . I'll probably mess that up and have to get it done by a pro anyway but I think it would be fun to try. After all ,that's how we learn stuff.
Anyone have any ideas?? thanks
walt


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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Thu 23, 2017 2:38 am 
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The source list here at ARF has lots of speaker links, but most are focussed on repair
viewtopic.php?f=19&t=201537

I'm starting to think that E-Bay might be the best source---eg this store:
http://stores.ebay.com/Circuit-Shop-Aud ... 34.c0.m322

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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Thu 23, 2017 2:50 am 
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Location: S. Dartmouth MA 02748-1225 USA
waltdavis777 wrote:
So now I see why radios became lighter and more compact after permanent magnet speakers were developed. I just opened up a 1952 Coronado that I have and determined that it has a nice little 4" PM speaker that sounds great .

I am thinking about trying my hand at re -coning a 1941 Philco 6" oval speaker that I have , that is all patched up, but I have no idea where to get the cones. It's gets pretty expensive to get that done by a professional when you pay the fee plus shipping both ways . I'll probably mess that up and have to get it done by a pro anyway but I think it would be fun to try. After all ,that's how we learn stuff.
Anyone have any ideas?? thanks
walt
It may be difficult to find replacement oval cones other than 6 x 9. Look for an inexpensive speaker with a small magnet, new. Salvage the cone from the new speaker to repair the electromagnetic speaker. Toluene is the solvent for low end speaker assembly. Using a plastic eyedropper and feed solvent around glue lines in a few minutes the joint will loosen and the salvage cone can be removed. Do the spider too. Disassemble further and assemble to the old voice coil. Use new speaker cement your choice but a solvent based cement allows for correcting a mistake. The voice coil will need to be shimmed equally as it is assembled. Cut shims from thin stiff plastic or metal shim stock.

GL

Chas

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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Thu 23, 2017 12:51 pm 
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Joined: May Tue 31, 2016 9:49 pm
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Location: Mineral Wells, tx 76067
Thanks . Those are some great tips on how to recone my speaker. I was wondering what kind of cement to use with which to Glue it all back together. I suspect super glue would not be a good choice !!
walt


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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Thu 23, 2017 1:52 pm 
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For mounting the cone to the frame, I used black rubber cement from Parts Express. They sell it as " speaker glue", but I'm not sure how it might be different from generic rubber cement.

Voice coil to cone:
At least for large speakers handling significant power, the advice I found was to use epoxy. I did this successfully on a pair of 12" speakers.

In researching this, I'm finding fewer sources for cones than maybe a year ago. EBay is looking better by the minute.

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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Thu 23, 2017 4:23 pm 
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Joined: May Tue 31, 2016 9:49 pm
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Location: Mineral Wells, tx 76067
Thanks for all the advice. I'm taking lots of notes and getting anxious to try this reconing process .
walt


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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Thu 23, 2017 10:18 pm 
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Joined: May Tue 31, 2016 9:49 pm
Posts: 120
Location: Mineral Wells, tx 76067
Chas,
You specified getting a speaker with a small magnet. Is that because the hole in the cone will be smaller? I saw some that looked like the magnet might be 2" in diameter. Is that too big?? thanks
walt


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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Thu 23, 2017 10:42 pm 
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waltdavis777 wrote:
Chas,
You specified getting a speaker with a small magnet. Is that because the hole in the cone will be smaller? I saw some that looked like the magnet might be 2" in diameter. Is that too big?? thanks
walt
Yes, a light duty, cheap oval speaker will have a small magnet structure and a small O.D. voice coil.
However a 2" voice coil is quite large and indicates a heavy magnet for high wattage input, also expensive just to take apart for a cone.

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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Fri 24, 2017 1:07 pm 
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Joined: May Tue 31, 2016 9:49 pm
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Location: Mineral Wells, tx 76067
Thanks so much.
Best regards,
waly


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 Post subject: Re: field coil speaker repair and fundamentals of operation
PostPosted: Mar Fri 24, 2017 4:29 pm 
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Location: Lafayette, CO
So my question is, what exactly happened to make alnico magnets cheap and versatile enough for use in radios? Most of my early televisions still have field coil speakers. All are postwar sets. Craig


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