Audioclinic (Nov. 1978)

Turntable Repair

In response to your answer on "Record Changer Repair" in the March, 1978, issue of Audio about slip page in an old automatic changer, I have been working as a professional with turntables and other audio gear for some six years now and would like to add something to your answer.

Instead of using alcohol on the idler wheel, use a common solvent, such as chloroethylene or "Chloro-Kleen" (a product of G.C. Electronics), made specifically for rubber drives. It re moves most dirt and old, loose rubber particles from the driving surface of the idler wheel. It is very effective in restoring the "grabbing" surface of the wheel. It is also excellent for cleaning the inner rim of the platter and motor pulley, providing it is not made of plastic.

Using sandpaper on the idler surface sometimes works, however it usually produces an uneven surface which contributes to varying speed and in creased rumble. If all that I have suggested doesn't do the trick, the only alternative (and sometimes the easiest) is to find a replacement idler wheel. EV and G.C. Walsco have excellent lines of replacement idler wheels for old and new equipment, which are available from local electronics parts distributors.

-Glen Whatley; Denver, Colo.

Cable System Antennas

Q. In my area, cable TV is now avail able. Is it possible to use the cable as an antenna for my tuner?

- Mike Neuman; Deer Park, N.Y.

A. The cable used for TV distribution in various parts of the country is not a long antenna, but rather shielded coaxial cable, and the signals which appear at your end of the cable are those intentionally supplied by the operator of the cable system. Many such systems do not supply the signals on the FM band. However, if your cable system does supply the signals you wish to hear, they would be stronger than those picked up by your conventional antenna.

Before attempting any connections to the cable system, other than those originally provided for when the sys tem was installed, check with the cable company as additions to a cable installation can result in radiation from the system which can interfere with the reception of TV and FM signals by those who are using regular antenna systems rather than being on cable.

Power Output

Q. How is it possible for an amplifier rated as low as 2.5 watts per channel to work with a magnetic phono cartridge?

- Scott T. Baker; Winston Salem, N.C.

A. In order for an amplifier to handle the tiny signal produced by a magnetic phono cartridge, the amplifier must possess sufficient voltage gain to bring the signal up to approximately the level of signal produced by a tuner, tape recorder, etc. Also, this amplifier must have the necessary equalization so that the cartridge will have a flat frequency response. Once these conditions have been met, the phono signal can be fed into the later stages of the amplifier which drive the speakers or headphones. Thus, the actual output, in this case 2.5 watts, isn't really relevant to whether the amp can do the job with the tiny signal from a magnetic cartridge.

Shielding Against A.C.

Q. I would like to know which metal would be best for shielding equipment against a.c. power lines and power supplies?

- Michael D. Snyder; Pittsburgh, Pa.

Probably the best shielding against a.c. pickup is the use of bell foil cable.

This cable works out well because it provides much better shielding than the spiral-wrapped cables we usually find. However, it has a higher capacitance per foot than other kinds of cable, which may be a consideration in such applications as phonograph installations.

Be sure to route all a.c. lines away from audio cables. Good shielded cables, as well as careful wire routing, may well cure any hum condition you have.

Amplifier Hiss

Q. I have a problem with my receiver. It produces an audible hissing sound when in either AUX, Tape Monitor, or the Phono mode. The hiss is present with the speakers, the cassette deck, the Dolby Noise Reduction, the turntable, everything except the FM position. When I took my receiver to a technician, he said that his speakers showed no sign of hiss at normal listening levels. He said that he could hear a hiss when headphones were used, but this was not uncommon with an integrated amplifier. He added that I shouldn't be able to hear hissing in my speakers unless I listened at high output levels. I understand that there will be noise in even the best of systems, but this is a very audible hiss.

-Dale C. Allen; Roanoke, Va.

A. Probably your service shop used a speaker with less efficiency than your speakers, and this would eliminate the problem as far as he was concerned.

Presumably the hiss of which you speak is present even when the volume is turned all the way down. This means that it occurs in the audio stages after the volume control. This hiss may be inherent in the design of the amplifier.

Your letter has a statement in it which puzzles me somewhat. You state that if you listen to FM, you do not hear this hiss. Perhaps there is some kind of gain reduction switched into the circuit, resulting in elimination of the hiss. Also you may have one of those circuits in which signal is introduced into a stage before the volume control and FM signals are the loudest, producing more voltage at the input of such stages. This means that, when listening to FM, the volume control must be lowered more than for listening to other program sources and this would prevent hiss from being audible. If this is the case, then the hiss is generated between the input to the high-level sources and the volume control.

(Source: Audio magazine, Nov. 1978; Joseph Giovanelli)

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