AUDIOCLINIC (Dec. 1991)

VCR Mystery Mono

I take keyboard in hand to offer another possible solution to the "No Surround Sound" problem as related by Steve Metz in your column in the January issue.

It has been my experience that very few Hi-Fi VCRs (actually, none that I can think of) include multiplexed stereo as part of their modulated-r.f. output. They only feed stereo from their audio output jacks. Further, most salespeople and many technicians are unaware of this.

It has also been my experience that many people upgrading to a stereo video system have simply replaced their older, monophonic VCR and followed tradition by connecting the VCR as part of the r.f. signal loop (i.e., feeding the r.f. signal from the antenna or cable box to the VCR's r.f. input, then feeding the VCR's r.f. output to the TV receiver's r.f. input). When you're not playing a tape, the VCR will usually just pass through whatever r.f. broadcast or cable signal it receives, and the TV (which I assume to be stereo-capable) then decodes any stereo signals and provides stereo audio to the sound system. During tape playback, however, the VCR's modulator creates an r.f. output signal (usually on VHF channels 3 or 4), with monophonic audio (sometimes obtained from the linear audio track). This signal is then presented to the TV receiver, which can then send only monophonic sound to the preamplifier/decoder.

The solution to this can take many forms, depending on the capabilities of the TV receiver. If the TV has separate A/V inputs and outputs, and input selection of either r.f. or line sources, then the video (either composite or "S") and left and right audio outputs of the VCR should be routed to the video and audio inputs of the TV. The TV's A/V outputs should then be routed to the sound system. To view the broadcast or cable signals, the TV receiver's input switch is set to "TV" (antenna/r.f. or whatever). To view a videotape, you switch the TV to the appropriate input (which might be labeled "Video," "Line," "VCR," "A/V," or the like). If your TV has no such inputs, you can feed your VCR's audio outputs directly to any unused line inputs on your audio system.

There are many other possibilities depending on the equipment being used. The key is to abandon the r.f. link in favor of the separate audio and video connections. This should produce a cleaner picture than using the VCR's r.f. modulator.

-Clifford I. Knight; Plymouth, Mass.

Using Tape Loops

Q. My preamp has only one tape loop. I have a Hi-Fi VCR, a cassette recorder, and some signal processors. How can I set up this conglomeration?

-Gordon R. Taylor; Bowling Green, Ohio

A. If all you had was one recorder and one signal processor, you could probably hook the processor to the tape loop and the recorder to that, because most processors now have tape loops of their own. You might even be able to daisy-chain another processor or two if each unit has a true bypass switch. Otherwise, you might pick up noise and other problems from such a connection.

In your case, you'll need some kind of external switchbox to which all your gear can be connected. An article in our June 1989 issue covered such switchers from Canton, DB Systems, Niles Audio, QED, Radio Shack, Russound/FMP, and Vanco. Sony also makes switchboxes, and dbx used to make a fancy unit that had indicator lights to show which components were in-circuit. Most switchboxes include tape-dubbing switches that allow you to feed the output of one tape deck to the input of another; these switches can also be used to feed a processed signal to a tape deck during recording or to process the tape deck's output during playback.

Output Tubes Glow

Q. I recently purchased a receiver, vintage 1968 or earlier, and I liked it so much that I also got a Fisher Model 800C receiver for its excellent AM section. The output tubes of both receivers glow a blue or purple color--some not at all, some only a little, and some quite a bit. Is this bad? Is it the fault of the receiver or the tubes themselves? All of the tubes test out fine. When should output tubes be replaced? How much power do I lose as they age?

-William Luginbuhl; Bluffton, Ohio

A. Glowing output tubes can be the result of various factors. The tubes themselves could be gassy. There may be insufficient bias applied to the grids. This can be caused by problems with the bias itself or by leaky coupling capacitors feeding the grids. That could even make the grids positive with respect to cathode. If such an extreme condition exists, the tubes will be destroyed relatively quickly. The sound will be very distorted as well.

I don't have an answer as to when to replace output tubes. Much depends on how often the gear is used. Certainly if you notice that you can no longer obtain proper output without audible distortion, change the tubes. Deteriorating driver stages can also produce some of these same symptoms.

If you cannot bring the plate current to specification, even when the bias is set properly, the tubes are probably bad and should be replaced.

CD Deterioration

Q. I have read several articles regarding the deterioration of CDs over time. Is there any truth to these allegations, or is it a controversial question without a definite answer? If CDs do have a limited life and will not last forever, will there ever be a medium that will?

-Joe Jarocki, Franklin park, Ill.

A. So far as I know, there is no problem of CD deterioration; I have not had such problems, nor have I received even one letter from a reader indicating that they have.

As for what medium is truly permanent, nothing on earth is permanent. Magnetic tapes can be erased, or their plastics can lose plasticizer. Vinyl phonograph discs can wear out. Perhaps the DAT system offers at least some hope of being archival-not because the tapes can't be erased, but because the information can be copied from one generation to the next by digital means. This is a virtually lossless process as long as the copies are made before the master has had time to deteriorate.

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(Source: Audio magazine, Dec. 1991, JOSEPH GIOVANELLI)

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