Audioclinic (Aug. 1979)

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A.C. Adaptors

Q. Can the a.c. adaptors, such as those sold for portable radios, calculators and recorder's, be left plugged in for long periods of time when they are not being used?

-James R. Henderson, Victoria, B.C., Canada

A. It is only in those instances where the adaptor serves something like an intercom, which must be ready for service at any time, that I recommend these adaptors be left plugged in when not actually in use.

These a.c. adaptors are actually complete power supplies and are comprised of a stepdown transformer, a rectifier system, and the necessary filter capacitor. When one of these devices is operated, there is some heat generated by the stepdown transformer which will not be significant unless the filter capacitor should short out.

When this occurs, the transformer will run excessively hot and might cause problems.

Keep in mind that when the device is plugged in, but not feeding a suitable load, the voltage developed across the filter capacitors will be higher than the nominal output voltage of the a.c. adaptor. If the filters are made to operate at or close to their maximum ratings, this high voltage remaining across the capacitor for extended periods of time could cause a short circuit.

"Popping"

Q. My component system develops a "popping" noise in my speakers every five minutes or so. It sounds like a static discharge. It is heard whether my amplifier's mode switch is on FM, Tape, or Turntable.

What do you suggest that I try in the way of troubleshooting? The people in the store where I bought my system said, "let it get worse, and then the technician has a better chance of finding the problem."

-Tuck Krehbiel, Cincinnati, Ohio

A. It will be hard to locate your "popping" problem because of its intermittent character, and that is why your dealer was reluctant to attempt servicing the equipment until such time as the symptoms become more constant. This is probably to your advantage inasmuch as this kind of servicing can be expensive. Where a problem occurs more or less constantly, it is simpler to track down and, therefore, less costly to solve.

You do know something about this problem, however. For instance, you know that the problem is not located in the early phono stages of the equipment. You know this because it occurs even when these stages are switched out. It is located in stages common to the rest of the input functions. You did not state whether or not the condition exists in just one channel. If the condition exists in both, you know that the "popping" is the result of some common component, and power supply or decoupling elements would be suspect. This condition could also indicate that the "popping" is external to your system. Transient voltage changes on the power line often cause this condition. Various filters are available which can suppress such interference.

Poorly soldered connections or minute cracks in the circuit "lands" could also cause the problem. Resolder suspicious joints.

Try signal tracing. Use a second amplifier to enable you to listen to the output from various stages in the defective equipment. Feed the input of this second amplifier via a blocking capacitor. This will allow you to make direct connections to collectors where this is necessary. Work your way from the input selector switch to the output of the defective channel. When the "popping" is heard in both amplifiers, this means that you have reached the output of the defective stage. You can then investigate the components in that particular stage. The ultimate source of the problem can be anything from defective semiconductors to defective capacitors, resistors, or even solder or circuit foil problems.

(Source: Audio magazine, Aug. 1979; Joseph Giovanelli )

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