During audio's boom days of the 1970s, many companies in the U.S. had a policy of putting aside several samples of their flagship models for review purposes. These were never part of their inventory, and when they sent them out they really didn't expect to see them again. One executive told me that it was much too big a hassle to repackage the equipment once it had been used.

Those days are long gone, but back then I remember visiting a colleague who lived in a farmhouse outside New York. There was a building on his property that had once been a small barn, part of which had now been converted to a garage. The rest was a large storage area jammed with equipment boxes. There was a small pile of them near the door, and when I asked my friend what those were, he said "they're the ones that have to go back".

As for the rest, a few found their way into his permanent system, but the majority ended up in a huge yard sale he had every year. I always imagined audiophiles from all over the state waiting for this affair to pick up some of the latest gear at rock-bottom prices.

Chances are, if you head out to do the garage sales some sunny Saturday, you won't luck into such an event. But audio and video gear does show up in such sales now and again, and if you're careful and know that to look for, you might well end up with a real bargain or two.

It's a long shot, however. Electronic equipment is pretty hardy for the most part, and most people tend to drive it till it dies, even if that means moving it to the rec room or cottage when the main system is upgraded. So when a component turns up on that trestle table, the chances are better than even that it's shot.

That said, there is also the possibility that the vendor either doesn't know what he's got, or simply wants to make a few bucks quickly. In that case, you might go home with something really worthwhile. A few words with the owner will often give you some clue as to whether or not it's a real find.

The price may also give you an idea. If it's a 100-watt amplifier that's going for $10, it's probably a pretty good doorstop and nothing more. On the other hand, it might be worth your while risking the ten on the chance that the thing might work, especially if it's a model or brand you know and doesn't look too beat up. Appearance is not, however, a very reliable guide unless the component looks as though it's been dropped. Lots of scruffy-looking equipment works just fine (including quite a bit of mine).

Ideally, you should try out any piece of audio or video gear before you lay down good money for it, but that may not be practical. Before you investigate that possibility, however, simply asking whether or not the thing works might make the whole matter academic unless you're considering buying a unit for parts. Or the seller may tell you that everything works except the whatnot; if you only intend to use other functions that might be okay.

If the asking price is more than a blowable amount of money, some form of demo is in order if you can swing it. But if you are setting out with the intention of looking at audio gear, you can make the evaluation much easier by preparing in advance.

For instance, the vendor may be selling only one or two components, rather than a whole system. If speakers aren't part of the deal (or if they're truly rotten speakers), you won't be able to do much in the way of satisfactory listening. But most audio electronic components provide headphone jacks, so if you take your own phones, you should be able to audition what's for sale. Take along a tape of your own familiar music as well. And bear in mind that if you use the phones that came with your Walkman, they won't fit the quarter-inch phone jacks of most home equipment without an adaptor, so you should carry one of those as well.

In a perfect world, the seller will have kept the boxes and manuals for the equipment he's offering. That's rarely the case, but you may be the lucky one, so it doesn't hurt to ask; one or the other may be lurking in the garage out of sight. The carton can be useful for transporting the unit home, and its presence is probably pretty good evidence that the seller didn't get the unit at a yard sale himself. Manuals are always good things to have (even if some of us only consult them as a last resort).

By and large, you should avoid buying anything bearing a brand name you've never heard of. A particular brand is never an absolute guarantee of quality, but if nothing else, a familiar name usually means that the company is one you can write to for information on the unit. An unknown brand might mean the unit was put together for a retailer, and the manufacturer is untraceable. Or it could be from before your time, in which case the unit might be so old that it stands little chance of working properly.

Even before you listen to a component, assuming you can, simply working the buttons and knobs can often tell you a fair bit about how the unit has been treated. Loose controls, wiggly buttons or ones that stick, and general ricketiness -- all suggest that the item has had some hard use. When you are actually listening, work all the knobs back and forth to see if they impart loud crackles to the sound; minor noises can often be tidied up with a shot of contact cleaner, but more severe disruptions usually mean that the controls are shot and will have to be replaced.

Aside from these general principles, there are things you should consider when looking at the different types of components:

• Cassette decks: With your trusty headphones and a familiar tape, you can do a reasonable audit of a tape deck, assuming there's somewhere to plug it in. Listen for speed irregularities -- wow and flutter -- and for reasonable output in the high frequencies. If the sound is very muffled, it may simply mean that the machine needs its heads cleaned and the transport demagnetized, which are not very onerous tasks. Or it may indicate more serious problems; that's the risk.
  
  Check that the mechanism loads and unloads the cassette smoothly, that there are no obvious physical noises -- squeals and the like -- when the tape is playing, and that the tape counter works. And if it's a dubbing deck, make sure both transports operate.
  
  
• CD Players: These don't vary much when it comes to sonic characteristics, so anything on sale will probably be just fine -- or it'll be broken. Play a disc (you did bring one, didn't you?) to find out. At the same time, listen for skipping and mechanical noise as the disc spins.
  
  Also, if you're going to want to program the player, you'll really want the manual.
  
  
• Tuners and receivers: These are where you're likely to find problems with crackly controls and wobbly switches, so check those carefully. Also, if it's an older model with a rotary tuning knob and a linear pointer, spin the knob back and forth a couple of times and watch the pointer; if it doesn't move smoothly, or at all, forget it. In older tuners, the various moving parts were connected by a piece of thread that wound around drums and guides to impart the knob's motion to the pointer and the tuning circuit itself. If the string is broken, trust me, you don't want to fix it.
  
  Check for fairly heavy-duty speaker terminals on the back of a receiver (or amplifier); if they're flimsy, or the unit uses little plugs, it's probably pretty cheap. And if the rear panel specifies the speaker impedance you should use, make sure it matches your speakers or any you are contemplating buying.
  
  
• Headphones: Choosing phones is entirely a matter of how they sound, so if you can't listen to them, give them a miss. If you can, and they sound okay, consider how comfortable they are and how secure they are on your head. If you're actively shopping for phones, you might consider taking a portable tape or CD player along, in case the seller has nothing to drive the phones; again, familiar music is the key.
  
  
• Speakers: Always the hardest components to choose, speakers don't get any easier when they've been around the block a few times. As with phones, the overall sound balance will be the determining factor, but unlike headphones, they'll be hard to audition in any meaningful way in a driveway or on a lawn. If you know the brand and model, fine; if not, buying second-hand speakers under these circumstances is a real crap shoot. But if the price is attractive enough, and you're willing to put the speakers in your own yard sale if they don't pan out, it might be worth taking the gamble.
  
  But before you even consider the sound, some physical matters should get your attention. For instance, some older speakers used foam for the woofer surrounds, and it turns out that it disintegrates with age. Take off the grille and look at the surrounds; if they're foam, tap one gently and see if a little cloud of dust is produced. If so, the foam is going (or it may be more obvious: there may be big holes in it). In that case, you may be in for an expensive reconing job, which might well cost more than you pay for the speakers.
  
  If the grille itself is reticulated foam, that suggests the speakers are quite old. That may not be bad in itself, but it does mean the likelihood of damage and deterioration is higher. And twenty-five years ago, speakers -- especially inexpensive ones -- were nowhere near the equal of even modest ones today.
  
  Construction is important too. Rap on the enclosure; if it produces any identifiable "note" rather than a dull thud, the box is flimsy and may resonate. Cardboard or Masonite backs are usually the hallmark of the dinky, as are little speaker wires coming out the back, instead of solid binding posts or spring-loaded connectors.
  
  Finally, even if you can't make critical listening judgments at the sale itself, you should try to get the speakers powered up and listen very carefully to each of the drivers, with your ear up close. Tweeters, and to a certain extent midrange drivers, are very delicate, and lots of speakers have been relegated to the attic (or the sale) for one or more blown drivers. If that's the case, there's no point in pursuing things further.
  
  
• VCRs: As long as they're not malfunctioning, there's not a lot of difference between VCRs -- especially older ones -- when it comes to picture quality. If you get one home and the picture looks unusually grainy, it usually just needs a pass with a head-cleaning cassette. When you are considering purchasing one, even if there's no TV set to hook it up to, you can check that the tape loads smoothly and that it plays with no internal physical noises (buzzes, for instance, mean there's something amiss with the tape-to-head contact, which could ultimately wreck both tape and machine). If you can view the result, check for noise and lines in the picture. If things are clear, the VCR is probably fine.
  
  If you intend to use the VCR as part of a home theater setup, it must have hi-fi sound. Not all machines that feature it say so on the front panel, but all that do mention it have it (but don't be fooled by terms such as "stereo" or "Dolby"; in older machines they don't denote true hi-fi sound). The tip-off is usually line-level stereo inputs and outputs on the back, but it's not foolproof.
  
  
• Television sets: Older TVs tend to suffer from two main illnesses: weakening of the phosphors in the picture tube, and mechanical problems in the tuner.
  
  The first means that the picture gets darker with age, to the point where no amount of boosting the brightness control will correct things. To see how far these have advanced, try to get the set out of the direct sunlight and play with the brightness and contrast controls to see how good a picture you can achieve. You'll probably find a local station that comes in strongly enough for this test; if the set doesn't have rabbit ears, simply holding your finger on one of the antenna terminals will usually do the trick.
  
  Mechanical rotary tuners can develop contact problems on some or all channels that cause the picture to flash or cut out; usually just wiggling the knob a bit will make it happen. If you intend to use the set's tuner regularly, that might be a problem; if you intend to set it on channel 3 and use your VCR's tuner, what happens in the set doesn't matter as long as it pulls in channel 3 without disruption.

An alternative to pounding the pavement in search of electronic bargains is to buy used equipment from a dealer, in which case the process is exactly the same as buying new, but cheaper. Cheaper still is going the garage sale route, and while the risks are greater, they're not insurmountable if you know the pitfalls to watch out for.