AUDIOCLINIC (Jan. 1992)

Station Search Problem

Q. My tuner is very sensitive. (I'm afraid that's true in more ways than one.) It will enter the "Search" mode, it will stop on a strong station, and there will be two or three stations that I can hear very loud and clear, all at the same time.

Also, when the tuner is searching, the search system is supposed to pass the weak stations, stopping only at the strong ones. Instead, it stops at virtually every station, even those that are unclear or have nothing except distortion. When I unhook my dipole antenna, the search behaves as I would hope it would. Of course, then I only receive five or six of the very strongest stations in my area.

I don't remember having this problem when I first purchased my tuner. After a tuner has been broken in, is it possible that it might have to be readjusted?

-Joseph Barbera, Cheektowaga, N.Y.

A. My first thought is that your tuner is overloaded by at least one very strong signal. A sign of that is hearing several stations all at one time; the search system will not operate properly when circuits are overloaded. It will stop because of what it perceives to be usable signals, even though the signal in the i.f. passband is nothing but "garbage." The search system behaves as it should when the antenna is disconnected because the overload is no longer present.

We still have to account for the fact that you were not plagued by this problem when you first owned your tuner. It may be that, since that time, a new station has come on the air and is located close enough to you to overload your tuner.

I assume that this "dipole" antenna is the indoor type. You will gain some improvement by using just a short piece of wire connected to one antenna terminal. You will hear more signals than when no antenna is connected, and hopefully you will still not experience the overload problem.

There are a number of tunable r.f. amplifiers designed to be placed between an indoor antenna and the input of your tuner. You can tune in a weak signal, then adjust the tuning of the amplifier for a maximum amount of signal. Not only will this signal be boosted in strength, but all other signals will be attenuated. This may help. You probably can't use the "Search" feature because the amplifier must be tuned to the particular signal you desire. Still, it may solve your problem. Of course, you won't ever want to tune in the signal causing overload when using the amplifier, for the overload problem will be greatly increased.

If I am wrong in that no new stations have come on the air, it still may be that one of those already on the air has increased its transmitter power, causing the problem.

If none of this is true, then you should have your tuner checked. Sometimes tuners do go out of alignment with age. Internal components can also fail.

Noisy Volume Control

Q. My integrated amplifier makes a very annoying "popping" sound that is audible through my speakers when I adjust the volume. It is especially noticeable on quiet passages, and it occurs on all sources. I have found that if I turn off the speakers and turn the volume control all the way up and down a few times, the noise is "flushed out" temporarily.

Could this sound damage my speakers? Is the amplifier repairable? Your thoughts would be appreciated.

-Jay Kassavian; Lafayette, Cal.

A. What you have described is a classic case of a volume control that either needs cleaning or needs to be replaced. It won't be too many years before this scenario will be unknown because modern equipment will make more and more use of electronic controls rather than mechanical ones.

A brief review of the action of a conventional, mechanical volume control will help you understand what is happening. The device used to control the volume is what is called a "potentiometer." Whether it makes use of a rotating knob or a sliding knob, the principles behind the operation of this device are the same. There are three terminals.

Two of these are attached to the two ends of a resistor. The third terminal is attached to a sliding contact that moves between the two ends of the resistor. When the slider is at either end of its travel, there is virtually zero resistance between it and the nearer fixed terminal of the resistor, and the resistance to the opposite terminal is about equal to the potentiometer's total rated resistance. As the slider travels along the fixed resistor, the resistance between it and one terminal increases while the resistance to the opposite terminal decreases.

It is essential for the resistance change I have described to occur smoothy. If the control has become oxidized between the slider and the resistance element, there will be jumps in the action of the control. These erratic variations in resistance result in the noise you have mentioned.

When "flushing out" the noise, I strongly recommend that you turn off the amp altogether and not just the loudspeakers. I suppose you can get away with just turning off the speakers, but there is some danger, especially if you leave the control at full volume when you again turn the speakers on.

Usually, cleaning the volume control will cure the problem. To do it, see if there is space around the various terminals on the control. You'll probably find six terminals and not the three I mentioned. This is because there are actually two separate controls, operated by a common shaft. Squirt suitable contact cleaner into the openings that I hope you'll find. If this area is sealed, your last hope is to remove the knob and squirt the cleaner down the shaft.

Some may find its way into the control, especially if you place the component on its back so that gravity assists you.

In either case, move the control shaft back and forth several times after squirting in the fluid, to help distribute it more evenly.

If cleaning does not work, the control must be replaced with one having appropriate mounting arrangements and the same rated wattage, resistance, and "taper" (the relationship between change of slider position and change or resistance-usually logarithmic, in audio pots) as the original control.

Hopefully, you can obtain an exact replacement from the maker, assuming the equipment is relatively new.

(Source: Audio magazine, JOSEPH GIOVANELLI, Jan. 1992)

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