Field coil is inside this cup on the back of the speaker. Voice coil is held around the post by a centering device. Plenty of irritating white power remains.
Before reassembling the cone and speaker frame I want a close look at that field coil. It measures roughly half as many ohms as it should, perhaps due to a partial short. If I can access the coil maybe I can fix it. To do that I’ll have to free it from its steel containment cup on the back of the speaker. Half a dozen long screws hold it to the frame and they come out easily. As soon as the cup is freed, flakes of white powdery stuff fall out of it, probably the same powdery stuff I saw on the voice coil. It has a talcum-like feel, smooth and almost “soapy”. I do not know what it is but impulsively I blow on it to clear it away. Bad idea! My nose and throat sting as fine powder swirls in the air. Let’s not do that again!
Could this be zinc oxide? Zinc was often applied to ferrous metals as a rust retardant. If this was zinc, perhaps time and moisture have helped it oxidize. Whatever it is, I open some windows and use a vacuum cleaner to suck up as much as I can, not only off my work bench but from the coil cup as well.
The center post looks dirty which encourages my next dumb mistake. I use steel wool to remove residue from the post and a mounting plate (not pictured). It does the job admirably well and at the same time releases more white dust into the air. Will I ever learn? The stuff is nasty. Not only that, I notice zillions of tiny steel slivers now clinging to the slightly magnetized center post. Steel wool is actually made with steel, after all. Sticky tape and a can of compressed air help remove every last particle I can find. I won’t make that mistake ever again.
As much as I try to free the coil from inside its steel cup, its mounting bolt is just too tight. I cannot loosen it. Rather than apply more muscle and risk damaging the bolt (or me!), I decide to leave it as it is. A partial short may be a compromise but the coil will still work. A single added resistor will compensate for the shorted windings and even though the coil may not perform at tip-top efficiency, I decide to make do with it.
Then a new thought hits me. With most of the zinc oxide removed I worry that rust will begin to spread on my newly-shined metal. A couple of thin coats of lacquer sprayed onto the bare metal will protect it. The speaker frame receives a good cleaning and a couple of coats also. After an overnight dry, the speaker is reassembled for testing.
In order to test the repaired speaker I need DC current for the field coil as well as an audio signal. Using voltage and resistance values from Rider’s schematic I can calculate approximately how much current should flow through a good field coil during normal operation, around 70ma at 75 volts. I remember an ARBE-III battery eliminator’s 90v output can supply up to 65ma, which is about right, so I hook it up (and double check the current draw with a DMM, 63ma). Two more clip leads from the voice coil to a nearby radio and shazzam! My speaker roars to life! It sounds clear, no rattles or buzzes, and the cone is dancing proud in response to a rap song that is playing. The field coil may not be operating at full strength but you’d never know it.
Hey, did you know that pudding placed on a vibrating speaker can actually dance? I saw it on the SyFy channel.
A clean machine is a priori combattimento around here; the coating of grunge on everything has got to go.