When Joellen Cassidy opened her computer repair shop in 1983, she got a special discount from the Maryland Yellow Pages because her store was one of the first ever listed under "Computers -- Service & Repair."

Now, only two years later, the computer repair business has exploded, yielding more than 50 computer service sources in the Washington area. As the use of personal computers increases, more machines are being bought by users lacking technical training who run to computer doctors whenever their machines act up.

"The computer repair market nationwide is potentially astronomical," said Cassidy, whose American Computer Repair Inc. in Arlington uses the motto, "We Keep You Up And Running."

Market surveys show the D.C. metropolitan area ranks second only to San Francisco in the number of personal computers per capita, making it a prime market for computer repairs.

"Business here is incredibly booming," said George D. Green of Computer Doctors, an independent repair shop in College Park.

Computer repair services are offered by firms as diverse as nationwide service companies, such as TRW Inc., Sorbus Inc. and Xerox Corp.; franchised computer dealers, such as ComputerLand, and "mom and pop" local outfits like Computer Doctors, which is not affiliated with the Computer Doctor Inc. franchise in New York.

Television repair shops, record stores and video outlets also are trying to get a piece of the action, along with firms such as Eastern Business Machines in Greenbelt, which made its name repairing typewriters and calculators but has turned to fixing computers.

"It only makes sense," said Cassidy. "It's a recession-proof industry. People will spend money to get the computers they already have repaired rather than buy new ones. And with sometimes over $1,000 invested in a computer, it's worth it for a customer to spend the average $150 in repairs and routine maintenance."

Estimated to be a $11.6 billion-a-year business now, the computer repair market is expected to soar to $25.4 billion by the end of the 1980s, according to Input, a market research firm in Mountain View, Calif.

"There was a fallacy at first that computers would never break down," said Mark S. Blood of Eastern Businsss Machines. "But they certainly do."

A computer system is, all in all, more reliable than many other machines, computer buffs say. "It's said that if the integrated circuits in your computer work for the first 90 minutes, they'll work for years without failure," said Joel Makower, local computer buff and co-author of "Everybody's Computer Fix-It Book."

Most computer troubles involve moving parts. "There are relatively few moving parts in a computer," Makower noted. "Where parts do move -- disk drives and printers, for example -- is where a computer system is most vulnerable."

Printers are the biggest problem, say computer technicians. "They're very mechanically oriented and they wear down, as opposed to the other parts that run by electricity going around inside the chip," explained David Everard, a local technician and manager for The Computer Store.

Computer repair technicians, who earn about $13,000 to $30,000 a year, frequently have received their electronic training in the military. Others are trained at technical schools such as Control Data Institute or Computer Tech Institute, which offer four- to eight-month courses.

Some service technicians have only a high school background in computer programming. "A computer repair technician is the kind of kid who would take apart his parent's watch and put it back together," said Everard. Or, as Makower succinctly put it, "Once upon a time, they were the prototypical nerds."

But, as demand for technicians has exceeded the supply, many with diverse backgrounds have realized it is a lucrative profession. The U.S. Bureau of Labor Statistics predicts that jobs for computer service technicians will nearly double by the end of the 1980s, making it one of the fastest-growing job markets in the country.

Makower insists that a good computer technician doesn't need to know much about electronics. "In fact, you may be better off with a degree in psychology or history," he said. "Computer repair is not a matter of electronic engineering, it's a matter of identifying problems and solving them."

If you go to a neighborhood TV repair shop, Makower said, you are likely to find technicians more skilled than those working at a local computer store. "You'll see the TV technicians playing with voltmeters, oscilloscopes and wiring diagrams," he said. "Computer technicians may have these tools also, but chances are they are armed also with a diagnostic disk that automatically determines a problem and spells out exactly what to do about it," he said.

Computer service technicians have been reduced to being "board swappers," ripping out suspected defective circuit boards and replacing them with new ones, Makower said. "If something is wrong, they insert a diagnostic disk in your machine and run it through its paces," Makower explained. "At some point, the machine tells them what to replace. Got a problem with the disk drive? Replace the drive. Something wrong with the monitor? Replace it. A flaky chip? Put in a new one."

Service technicians agree that a computer generally is not difficult to fix because of its "modularity" -- its replaceable "cards," or expansion boards. A computer, rather than being an integrated piece, is made up of many smaller pieces that can be replaced.

"If a refrigerator motor breaks, the whole thing goes. You don't just pop another motor in," explained Everard of The Computer Store. "But with a computer, you pop the top off the computer, and you change the cards."

When Apple computers are repaired, for instance, the parts that are removed are sent to a central service center. There, the cards are tested and fixed either by service people or robots. The advantage of this type of repair is that the average turnaround time is about one day, Everard said. The disadvantage, with some systems, is the cost of replacing the whole part.

For years, the computer service market has been dominated by giants such as International Business Machines Corp. that serviced the computers and related equipment they sold. IBM traditionally worked only on its own equipment, which caused problems for individuals and companies using an IBM machine with supporting equipment -- such as a printer -- from another maker.

Up until about a year and a half ago, computer manufacturers generally didn't want responsibility for "mixed systems," such as an IBM computer used with a memory expansion board made by Quadram Corp. When it came to maintenance, users with hybrid systems frequently were left on their own. Typically, each computer maker would assert that the problem lay in the other manufacturer's equipment.

To fill that void, smaller independent firms -- called third-party maintenance companies -- jumped into the business of repairing computers. The appeal of independent repair shops, computer users say, is that they provide a single source of maintenance for equipment from different manufacturers.

Spurring the growth of independent repair shops -- which account for about 10 percent of the service market -- is the boon in the use of personal computers, or microcomputers. Independent repair shops took in an estimated $1.1 billion last year.

"There has been an explosion of sales and shipments of microcomputer equipment," said Ronald J. Shugan, an industry analyst at International Data Corp. in Framingham, Mass. "And it obviously requires maintenance."

Independent repair services claim they often charge less than the manufacturers' or dealers' repair services. And the more equipment a computer-using company buys, the more likely it is to look for the lower-priced third-party servicers, the independents say.

A standard warranty for a computer is 90 days for parts and labor. "Consumers should be careful not to be constricted by the warranty to bring the machine back to the shop where they bought it," said Everard.

Almost all the area computer services try to sell customers a "maintenance contract" with monthly, annual or biannual fees depending on the type or the number of computers and additional equipment. Computer stores say these agreements -- like an insurance policy -- provide savings in the long run. And hiring an independent computer technician for on-site repairs costs anywhere from $60 to $100 per hour.

"These maintenance contracts are generally not a good deal for the average computer owner," he said. "No matter what it's called, it's an insurance policy. And an insurance policy, when you come right down to it, is a gamble.

"The amount of money you spend on them is not worth it," Makower added. "Once a computer has been up and running for 90 days, it could well work for a couple of years without problems." On the other hand, if a computer is used for business, its "down time" will be a key concern, so businesses may want to consider the extended contracts, Makower said. But they might also opt for monthly and weekly rentals or loaners instead.

Maintenance contract prices and terms vary from one service shop to another. For an IBM XT with 256K memory, the manufacturer charges $653 for a one-year, on-site maintenance contract, excluding monitor repairs. Ken Marx of American Computer Repair said his company charges $425 for a similar contract. Eastern Business Machines charges $585. Each firm maintains that it offers a special package of services, which could include rapid turnaround time, unlimited service calls and the use of loaners.

Independent computer repair services have promoted themselves by promising better service in addition to lower prices. Last year, drawing a contrast between itself and IBM, ComputerLand -- with 780 franchise outlets worldwide -- promised customers that its repair people would make house calls, as doctors once did.

Soon after, in June, IBM launched its first advertising campaign for its service operations. And by last August, it announced that for the first time it would start servicing other manufacturers' equipment that is attached to its personal computer.

In the past three years, dozens of major corporations, such as Bell & Howell Co., Xerox Corp., Western Union Corp. and ITT, have entered or expanded into the microcomputer service business. Control Data Corp. has added a new microcomputer service division to service IBM PCs. RCA Corp. won a contract from Apple to service its products.

Xerox has signed up about 400 independent retail shops under its "Americare" service program. The company sends its technicians to the computer stores so the stores' employes can concentrate on selling rather than servicing, according to Arthur Zuckerman, spokesman for Xerox. "Many dealers were pushing computer products as indestructable," Zuckerman said. "They thought that talking service was an error."

The Xerox service program allows small computer stores to use the company's name to attract customers. The firm also services many large corporations, Zuckerman said.

Bell Atlantic Corp., the local telephone company, which also is branching out into other communications areas, last year acquired the Sorbus Service Division. Sorbus, which had been in the mainframe computer business for years, has moved into the personal computer market. "Approximately 10 percent of our business is now related to personal or home computer use," said George Hatzfeld, a Sorbus spokesman. Sorbus is now the No. 2 third-party computer maintenance firm, ranked just below TRW Inc.'s Customer Service Division.

The growth in the local computer repair business is promising, area service people say. "The future looks excellent," said Cassidy of American Computer Repair. "More and more people are buying computers for home and business and becoming more dependent on their systems."

Even if the design of computers improves, it won't hurt the computer repair business, the experts say.

"You can't improve humans," said Cassidy. Much of her business comes from human error or the forces of nature. "Users may drop a Pepsi on a printer, put their plug in the wrong port and blow up the machine or drop the printer down the stairs," she explained. "Business always picks up when there's an electrical storm and lightning has zapped a computer or two."