I was troubleshooting a simple power supply that had power connectors in and out, a four-position switch, a step-down transformer, a fuse holder with a fuse inside, and a pilot light. It came to me for fixing because there was simply no output from the power supply.
First, I checked the switch, and it seemed to work. The transformer also worked. There was power going to the transformer and the fuse was OK. Everything inside the power supply seemed to work, but the power was simply not there.
I checked everything again with a VOM Simple analog meter. The result was no load on the power supply; no apparent problem. I applied a load, just a 6V, 4 amp incandescent lamp, which is what the power supply was supposed to light. I looked for a voltage drop across every point in the power supply -- there were not very many.
I found a 7.5V potential across the fuse holder, but I previously checked the fuse and it was OK. I checked the fuse again, and it was still OK. The only thing left was the fuse holder. Sure enough, the fuse holder failed. There were no cracks or breakage, but there was no contact to the fuse either.
Now a fuse holder is what I call a passive component. It doesn’t do much in the way of affecting the circuit. This particular fuse holder was made in Switzerland and contains German components. Only the Germans can make a fuse holder that fails. There was nothing complicated about it, just a screw-in-post type fuse holder.
I replaced the fuse holder with an American one that fits in the same hole and has terminals in the same place. I applied the 60 cycle smoke test -- 60 Hertz to some folks. All is now well.
This entry was submitted by Howard Gorin and edited by Rob Spiegel.
Howard Gorin repairs ophthalmic instruments. He is also a second source for repair parts for the equipment he services. He is connected with the Charles River Museum of Industry and Innovation.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.