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Repair and Rebuilding an Electrodynamic Speaker Complete Restoration Date August 2013
I was given a American Bosch Model 360 tombstone superheterodyne multiband radio made around 1931-1932. Every electronic component had to be replace. When the radio was tested no sound. It turned out that the speaker voice coil was open. Since the speaker was electrodynamic, meaning it has a large electric magnet that doubles as a filter choke attached to it. I could have kept the coil, mount in somewhere and replace with a modern permanent magnet speaker. However this would be a repair not a restore and it would be visible from the rear of the set. I decided to take on the challenge and build a new voice coil.
1. After finding out that the voice coil was open, I removed speaker cone and unscrewed the coil assembly.
2. The wire was unraveled and the coil form was cleaned
3. Contact Cement was swabbed on the coil form. This will help tack the wire to the form so it won't loosen suddenly.
4. A coil winding jig was setup to maintain tightness when the coil is being wound.
5. The coil is wound. Shown in the first layer. Wire must not have kinks.
6. Coil is completely wound, sprayed with clear gloss to seal out moisture and keep form then installed into electromagnet housing. Coil resistance was measured at 1.5 ohms. Should be around 2 ohms. Reason wire I used is slightly larger.
7. Paper cone reinstalled and bonded to voice coil with paper and tacky glue to maintain flexibility.
8. Voice Coil leads and output transformer leads were soldered back to electromagnet assembly.
8. Speaker was tested with radio and it produce sound. Cone was coated with Armour All to preserve the paper and then painted.
9. Restored Speaker with restored chassis.
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