Since the early days of cable television, subscribers have needed converters, small boxes with either buttons or knobs. Converters are provided and installed by the cable company when a subscriber signs up for cable television and they usually sit near or on top of the television set. Turning the knob or punching the buttons changes stations. The channel selector on the television itself remains tuned to the one channel, often Channel 3, that is wired for input from the converter.

Cable-ready televisions were designed to do away with the unsightly box and allow viewers to change channels right on the set or operate them from a distance with remote-control devices. But as cable systems began scrambling signals, owners of cable- ready sets discovered their gadgets no longer functioned as promised.

Some cable systems, including the two in Manhattan and those in many suburban communities, scramble only a few channels - generally Home Box Office, Showtime, SportsChannel or other channels for which subscribers pay a premium over the cost of the basic cable service.

Other systems scramble all their programs, and cable experts say the trend toward scrambling will continue as systems modernize their transmission equipment.

Industry representatives expect cable- ready sales to continue their recent increase - in part because many manufacturers use cable-ready technology in their best televisions. In 1981, according to the Electronic Industries Association, 33 percent of the sets sold were cable-ready. In 1982 the figure jumped to 44 percent, the trade group said, and last year half of the 14 million color televisions sold were cable- ready.

But when faced with a cable-ready set, many consumers are confused.

''My first question was, What does cable- ready mean?'' said Deborah Futter, an editorial assistant at a Manhattan publishing house who was given a cable-ready set last year. ''Does it mean I have to get cable? No. It's a misleading term because it sounds as if you can get cable programs without any hassles when in fact there are the same hassles as with any set, maybe more.''

Other cable subscribers say that retailers misled them into believing that cable-ready sets would work in places where the cable signals are scrambled.

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''The salesman said 'cable-ready' and I thought, How terrific, no more ugly box,'' said Peggy Tagliarino, who owns a Manhattan public relations concern. ''Then I called the cable company. Not until the installer left did I realize that I still had to have that box. If I could get a longer cord I'd bring it to my bedside so I wouldn't have to get up to change channels.''

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Edward W. Dooley, a vice president of the National Cable Television Association, said that cable system operators should not be faulted when cable-ready sets do not work as consumers expect them to.

''The manufacturers are taking advantage of a situation by emphasizing cable- ready,'' he said. ''The basic message that we give is that there's generally no need to buy a cable-ready set and that the responsibility falls to consumers to ask questions before they buy.''

The manufacturers, through the electronic industries group, have been negotiating with the National Cable Television Association on settling the problem, but neither side predicts a speedy solution.

In the meantime the electronic industries group is preparing a fact sheet for consumers. It can be obtained by writing to the Electronic Industries Association's Consumer Electronics Group at 2001 I Street N.W., Washington, D.C. 20006.

The popularity of the new televisions has created what John Gault, the president of Manhattan Cable Television, called ''the myth of the cable-ready set.''

''When you plug in a cable-ready set,'' Mr. Gault said, ''all the over-the-air signals that richochet off buildings can enter the set and intrude on the reception. All the things we went to incredible lengths and spent incredible amounts of money to spare our subscribers show up - ghosts, snow and so on.'' The use of a converter eliminates those reception problems, he said.

The other Manhattan cable operator, Group W Cable TV, says that its system is compatible with cable-ready sets.

One possible solution for viewers with cable-ready sets who subscribe to a system that scrambles signals is to buy a device called a multiple input switcher. It keeps viewers from having to use the converter every time they want to change the channel - even if they subscribe to pay services with scrambled signals - but they will still need a converter.

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Alan Schlosser, a vice president of the electronic industries group, said switchers can cost $25 to $35. Harvey Klorman of New York Video, a Manhattan retailer, said they can be found for as little as $4.

A multiple input switcher will feed signals from the cable directly to both the cable-ready set and to the converter. When the switcher is in place, unscrambled programming will bypass the converter and come straight into the set. The channels can be changed with the buttons on the set or by the remote control if there is one.

To watch channels with scrambled signals the viewer tunes the set to the channel on which the converter operates, then tunes the converter to get an unscrambled signal on the desired premium channel. If the cable company's rules forbid a customer from buying a switcher and attaching it to the set, the company may rent one to the subscriber.