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Blog posts of '2014' 'May'

Motorboating?

Q. I have a book on tube-type amplifiers that talks about “motorboating.” What is this?

A. In tube-type amplifiers, when the filter capacitors get old they lose some of their capacitance and develop leakage between their plates. They no longer do as good a job of bypassing AC on the positive side of the power supply to ground. As a result, the supply voltage can sag when the output stages draw current. When they stop conducting the power supply voltage comes back up.

These changes in supply voltage are coupled back to the sensitive voltage amplifiers in the input stages of the amplifier. The result is a kind of oscillation that comes in bursts. In a speaker connected to the amplifier these bursts make a “putt-putt” noise that sounds very much like an old-time motorboat.

Magnet Mistery

Magnet Mistery
Q. There is something I don’t understand about magnets. If you wrap a length of wire around a piece of steel and pass current through the coil, the steel will become magnetized. But with a permanent magnet, there is no coil or current. How can this be?

A. Iron atoms, and those of a few other chemical elements, act as little permanent magnets. This is due to the particular arrangement of protons in the nuclei and electrons going around the nuclei. Normally, the little atom-magnets are not aligned with each other. They point in random directions, so their magnetic fields cancel each other out. But when the iron (or steel) is magnetized, some of these atom-magnets become aligned, and point in the same direction. Then their magnetic fields aid, and together they form a magnet.

Repairing Auto Electronics

Repairing Auto Electronics

Q. I work in an automotive garage. A car came in one day with an electrical problem; a fuse kept popping. My supervisor put a pair of needle-nose pliers into the fuse slot. When the smoke from the wires came out of the car he said, “There is the short.” It took days of replacing burned wire inside the car what a mess! I wouldn’t want to own it. When a fuse does blow is there a reasonable way to or trace it?

A. One thing you can do is solder a couple of wires to a single filament tail light. Then connect the tail light across the empty fuse receptacle. If the short exists the lamp will light. The lamp will limit the current to about one Amp, which will not harm the wires in the harness. Since any current-carrying conductor produces a magnetic field, you can sometimes use a compass to trace the wire through body panels, etc. One amp of current should give a strong enough magnetic field.

There are also pulse generators called “buzz boxes” that can be used to trace wires through bundles and harnesses. The buzz box produces pulses coming at an audible frequency. A detector consisting of a pickup loop, audio amplifier, and speaker or headphones is used to "sniff" or trace out the wire. When you’re on the wire itself you’ll hear a loud tone. If you’re farther away or on another wire, you’ll hear a fainter tone. Telephone techs have used these for years.

One word of caution is in order, though. First, don’t try to use an auto buzz box on AC power lines. Secondly, other devices are often called “buzz boxes,” too. For example, l2VDC-to-120VAC inverters are often called “buzz boxes,” too. If you decide to buy one, make sure that the person you buy it from understands what you want.