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Remagnetizing Early Horn and Paper Speakers
From Colorado Radio Collector's "The Flash!!"
Larry Weide11/94

Back Hi... all you CRCers! Well, while the "boys of summer" are out on the picket line, I've been fortunate enough to find someone who was willing to step up to the plate - so to speak. What I'm mumbling about is that while I was carping to Dave Gonshor one day about having a tough time coming up with a 'Box' article for this month, he mentioned that his daughter had given him a 1935 book on radio maintenance. In this book, amongst a lot of other very interesting information, there is an article on how to recognize depleted permanent magnets in old style horns and speakers AND how to re-magnetize them.

So, with an apology to Alfred Ghirardi and his book "Modern Radio Servicing", and a lot of thanks to Dave and his daughter, I'd like to pass this information on to all of you folks. As you'll see, Dave has not only used this information, but he's upgraded some of it to take advantage of currently available resources.

As many of you know, the early speakers, particularly the horn type, were really nothing more than re-fashioned earphones. That is, as with an earphone, there was a permanent magnet closely associated with a finely wound coil of wire. Adjacent to the magnet/coil assembly was a metal diaphragm. When voice and music signals passed through the coil they would induce a variance of the magnet's effect on the diaphragm causing the diaphragm to vibrate in time to the program material. These vibrations are what we then hear as sound. The main difference between an earphone and the horn speaker was, of course, the horn itself. It was designed and formed to focus the sound so that more than one person at a time could hear the radio.

There were a number of different types of assemblies used. Most of these were derivations of what were called balanced-armature and iron-diaphragm type speakers. In the later derivations of these assemblies, paper cones (such as in the popular Atwater Kent early speakers) were attached to moveable pole pieces by a mechanical linkage instead of having a metal diaphragm. In any case, they all required a fairly strong permanent magnet in order to give useable volume.

Back in the old days, they didn't have magnets that used steel alloys like Alnico nor did they have ceramic type magnets - all of which are nearly impervious to demagnetizing. So, mechanical shock, inadvertant AC voltage on the coil and "aging" could cause the speaker's permanent magnet to weaken in strength. The typical test for a good magnet was to touch the magnet face or pole piece with a screw driver. As Mr. Ghirardi says, a tenacious "pull" should be felt. If the attraction is weak then the magnet needs re-magnetizing.

Be aware that there are any number of reasons why these speakers may work poorly or not at all. Be sure that you've checked out the speaker for other faults before going to the trouble of re- magnetizing. Reasons for failures that I've personally run into are;

  • An open coil or lead wire. Its not uncommon to have the lead wire separate from the coil right where they're soldered together. Its a touchy job, but not impossible to fix this situation.

  • Metal particles and/or flakes of old coil varnish collected in the airspace between the magnet and the diaphragm or moveable pole piece. It may take a disassembly to take care of this problem.

  • Some of these speakers had an "operator accessible" adjustment that optimized the attainable volume. Sometimes rust/corrosion and/or mis-adjustment caused problems.

  • The speakers that have paper cones are commonly subject to tears, distortions due to dampness, and separated glue seams and joints. (Look for a future 'Box article on how to repair/replace these cones)

The basic process of re-magnetizing involves placing the depleted magnet core through a coil of wire which has a momentarily strong DC current passing through it. The coil, as described in the book, is made on a form consisting of a 1.25" diameter round dowel fitted with flat end plates - similar in appearance to a typewriter ribbon or film reel. The width of the form, or the distance between the end plates is 7/8". At least one of the end plates needs to be removable so that the finished coil can be removed. The book suggests that strips of cotton or other insulating material be used to cover the form before the wire is wound on.

The coil itself is made up of 196 turns of #16 AWG enameled covered wire laid down in 14 layers of 14 close wound turns each. This amounts to a pound of wire - a unit of measure in which the wire can be bought. When you are finished with the windings, you need to continue with the taping so that the coil will not fray and will stand up to use. Perhaps you might want to attached sturdier multi-strand lead-in wires such as you find on a power supply transformer. Just make sure that the "hole" in the coil is kept clear for its intended use.

This coil, as designed, should draw about 12 amps on a 6 volt car battery. Guess what? Dave couldn't find a 6 volt car battery! So, being the resourceful rocket engineer that he is, he amazingly discovered that he had a 12 volt battery in his old heap, right out there in the garage (would you ride on a rocket designed by this guy?). In order to accommodate the higher voltage, Dave changed the wire size to 22 AWG and increased the winding to about 200 turns.

A safety point, that Dave made more than once, is that you must be VERY cautious and aware that either voltage version of this coil will heat up quickly due to the fairly high current draw. However, as indicated below, only a momentary application of voltage to the coil is necessary.

The actual re-magnetizing process goes like this;

  • The magnet is slipped through the coil. Position is not important so long as some part of the magnet is ALL the way through the coil.

  • A "keeper" of soft steel MUST be in place across the pole faces before the coil voltage is applied. this is very important in order to properly conduct the magnetic lines of force through the magnet core. The keeper must lie smooth and flat on the pole faces - just like they did on those toy magnets that we had when we were kids.

  • The coil is momentarily placed across the battery voltage - for no more than a second. Again, be VERY careful at this point concerning coil heating. You could, of course, use a power supply instead of a battery, but you'll need one that can supply about 15 Amps.

  • With your third hand(!?), and while the coil is connected to the voltage, strike the magnet with a sharp blow with a small hammer. Presumably this helps the steel molecules to align magnetically.

So that's it. Careful re-assembly should have your speaker as good as new. By-the-way, I understand that by the time Dave finally got around to buying some really good speakers, he was so old that they didn't sound any better to him than his old horn speakers did!

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