UBM Tech
UBM Tech

If only I had been holding a screwdriver!

John Coha -October 11, 2012

My tale takes place while I was a student at the University of California, Santa Cruz. I had just begun my second year, when I met a first-year student named Julie. She was smart and fun to be around, so we ended up spending a lot of time together. In those days my interest in electronics was driven by a love of music. Since I had an absolute lack of musical talent, I figured I could be involved on the technical end. I had already built a bunch of speaker cabinets and even helped build an oscilloscope in junior high school. Anyway, as time went on, Julie wanted to introduce me to her parents. Before our first meeting, I found out that Julie's dad, David, ran a pretty well-known electronics company! So it was with some nervousness on my part that our introduction eventually took place.  

David was also a music enthusiast, and he sat me down to listen to a stereo system that his chief engineer, Bernard Oliver, had built for him. Barney had made the speakers by gathering some Altec Lansing drivers, building a crossover network, and fabricating a box to house everything. These speakers were a two-way system, with a horn and compression driver to deliver the upper octaves. As I sat listening to the stereo, I noticed there did not seem to be any high frequencies coming from one of the speakers. When I mentioned this observation to David, he routed around in a drawer, pulled out a volt/ohm meter and some tools, and told me to find the problem. Yikes!

I accepted the challenge, and proceeded to tear apart the speaker. I checked for continuity of the compression driver's voice coil, examined the components of the crossover network, and did a visual inspection and resistance testing of the wiring. After much checking and rechecking, however, I could find nothing wrong. I left that night feeling really bad that I had not been able to figure out what was wrong with David's speaker.

A few weeks later, Julie told me her dad had taken the speaker back to Barney, and it was discovered that the magnet in the compression driver had not been magnetized when it was manufactured.  

I've been working now for more than 25 years fabricating and repairing instruments used in scientific research. This work has given me a chance to hone my troubleshooting skills. I sometimes look back, however, and think, if only I had been holding a screwdriver close to the HF driver that day, I might have noticed the lack of attraction.

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