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;
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;