AUDIOCLINIC (Sept. 1992)

Cards and Trouble Shooting

Q. I have been attempting to repair a multi-track tape recorder. I thought it would be a simple job because I immediately found a couple of open pass transistors in the power supply. Upon replacing these, I expected the ma chine to come alive. It didn't. I now had voltage, but some resistors were running very hot, indicating that some thing was drawing too much current.

My next thought was to remove one card at a time till I found the defective one, but this did not work either. Now what?

-Name withheld

A. You've given me a lot of good information; I hope it's enough.

When I see something like this. I begin by asking myself what components failed first. Here, it was the pass transistors. You're lucky they opened when they failed, or the power supply's full, unregulated voltage could have been fed to many components. Still, the full voltage could have been fed to some or all of the circuits just before the transistors opened.

Since you didn't get lucky by pulling cards one at a time, it's most probable that more than one card has a problem. I'd suggest pulling out all the cards, turning the recorder on again, and checking for any overheating components. This will detect or rule out failure of any components on the main board.

Turn off the recorder, and insert just one card into its slot. (Unless you're sure all cards are identical, right down to their trimpot settings and capacitors, mark each board and slot to make sure you return all cards to their original places.) Let's hope this card does enough things to let you determine, once power is back on, if the circuit associated with it is not operational.

Check for component heating, both on the main board and on the card. If all is well, keep the card in its place, and add another one. Repeat the testing procedure for each card.

When you find a card that does not work or causes component heating, turn off the machine and remove the card. But continue trying the other cards to see if any of them has shorted components. Your original test was not adequate to find multiple problems; the test suggested here covers that possibility.

The annoying task will be to find the defective components on each of the defective cards. You must check be tween the appropriate voltage and ground. Hopefully, there will only be one or two electrolytic capacitors, so finding the one which could have shorted out will be relatively easy. If it's just a capacitor, you're lucky. If a transistor or an IC has shorted out and you only have the recorder manufacturer's part number, tracking down the correct replacement part will be more difficult, especially if the model you have is an old one.

If you can't fix the defects on the cards, you probably can send the cards to the maker of the recorder. It's cheaper than sending him the entire machine. If the company is no longer in business, you must try to find a firm that specializes in the repair of professional recording gear; they just might have the right replacement parts.

Using Limiters

Q. I am prompted to write because of something you mentioned in a response to a question in the July 1991 issue. You referred to a correspondent who had incorporated a limiter in his system, making soft passages as loud as loud ones. With a number of my recordings, I find that if I turn up the volume high enough to hear the soft passages, then the loud passages are much too loud. I have neighbors to consider.

Do you think a limiter would help me? If so, how is it used?

-James Bliss, Bartlesville, Okla.

A. Yes, a limiter can help reduce the dynamic range of your program sources. It can be installed in the tape loop, between the output of a preamplifier and the input of a power amplifier or in the "pre/main" loop of a receiver or integrated amplifier.

Actually, there are two types of de vices to restrict dynamic range, limiters and compressors. A true limiter will pass all dynamics without change until the signal level reaches some critical value. Beyond that point, further increases in input cause no significant increase in output-the output reaches a limit and maintains a constant level until the input signal falls again.

Compressors also pass dynamics unchanged up to a critical point. After that point, output continues to rise as input level does, but at a slower rate (for instance, a 1-dB rise in output for each 2-dB rise in input). Often, compressors are designed to act as limiters too, once some further signal threshold is passed. A compressor should interest you more than a limiter, because it leaves some semblance of dynamic change rather than rendering the music lifeless.

Compressors and limiters are more common as professional equipment than as home audio components, so you may have to try a pro audio dealer or a large musical-instrument store to find one.

Channel Leakage and Balance Controls

Q. After hooking up my new system and checking the balance control, I noticed that when I turned that control to favor the left channel I could still hear some music in the right-channel loudspeaker and vice versa. I checked all wiring and all seems well. Is this a normal situation or could something be wrong with my preamplifier?

-Robert Sherman, Walnut, Cal.

A. Many preamplifiers that I have worked with operate as you have de scribed, so this is probably a normal situation. I cannot conceive of any imbalance condition that could not be corrected by such circuits, despite the slight amount of signal leakage that you have noticed.

Leaving CD Players Turned On

Q. Should I leave my CD player turned on when it is not being used? I average about one hour of listening to CDs daily. Somebody told me to leave it on, because power surges that are present when turning it on and off are worse than just leaving it on. Yet if I leave it on continuously, the player gets fairly warm and I'm concerned that this heat may not be the best thing for it.

-Sam Steinmetz, Dallas, Tex.

A. I really do not believe that the power surges produced when the player is turned on or turned off are significant. I think that the heat which builds up when the player is left on all the time will tend to shorten the lives of electrolytic capacitors more than power surges will. These players require small amounts of power, so the d.c. resistance within the windings of the power transformer should be sufficient to minimize the effects of power surges.

If you play a disc and walk out of the room for a couple of minutes, and then come back to listen some more, leave the player running during your absence. Otherwise, turn it off when it is not in use.

(adapted from Audio magazine, JOSEPH GIOVANELLI, Sept. 1992)

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