NOT MANY years ago, when he was well nigh on 30 years of age, Robert Krughoff had one of those illuminating experiences that have laid low so many of us so often.

"Driving away from an auto repair shop for about the third time in a period of a couple of weeks, I stopped. I realized I would soon be back for a fourth time pretty soon.

"An idea dawned on me," says Krughoff, "and I had the mistaken belief it was original."

The idea was for a magazine that would tell you where to go for an honest repair job. A magazine that would compare service at every shop -- or at least most of them -- and steer you toward the best.

Four years later (1976) the magazine was a reality. Today 20,000 Washington-area residents subscribe to Washington Consumers' Checkbook.

Checkbook is put out by a nonprofit parent organization called the Washington Center for the Study of Services, of which Krughoff, formerly a GS-15-level administrator with the U.S. Department of Health, Education and Welfare, is founder and president. Since the first issue on health care services, in 1976, the center has published four others. The results of these studies, including comparisons of Washington-area home-maintenance firms, such as heating and air-conditioning contractors and roofers, TV, stereo and appliance repair firms, and lending institutions, have been reported widely.

The initial car repair article, in 1976, for instance, found that costs for the same repairs vary greatly in the Washington area and that independent companies often give superior service, at lower costs, than established dealerships. A comparison of home-maintenance services, about a year ago, revealed that taking down a tree can cost anywhere from $325 to $765, depending on who you call. a repair job one roofer said would cost $925, another said he could do for $285.

In comparing firms in the TV, stereo and major appliance repair fields, the fall 1978 Checkbook found, "On specific repair jobs, prices at some firms were more than double the prices of their competitors. As is so often the case in the fields we have studied, some of the lowest priced firms rate at the top on quality."

"We're not trying to show the extent of a problem," said Krughoff. "Our intent is not to document the need for legislation. Our priority is to tell a consumer how he can go about helping himself."

But his methods do not exclude "rigging a car and taking it to auto diagnostic centers." In doing so, Checkbook received eight different lists of things wrong with the car and repair estimates ranging from $40 to nearly $500. It also found that the eight diagnostic centers visited were somehow connected to repair shops that might have something to gain from the diagnosis.

Checkbook, which Krughoff believes to be the only publication of its kind in the country, performs these studies on an annual budget of around $260,000 with six full-time employes, a host of volunteers and some financial help from area foundations. For much of its information, however, the publication relies on a hardy band of some 20,000 regular readers -- and local subscribers to Consumer Reports magazine -- who relate their own experiences with Washington-area firms.

It works this way.

When Checkbook decides upon a service, or a group of services, it wants to compare, Krughoff and the staff call readers and experts in the field to find out which areas should be covered. The staff then draws up a questionnaire they send to Checkbook readers. Consumer Reports -- the publication of Comsumers Union, an Arly financial backer of the center -- sends the questionnaire to its Washington subscribers as well without charge to Checkbook.

Checkbook questionnaires are not simple. In fact, they are downright imposing. One asks, "If in the last year or so you have used any of the six types of stores listed below, please rate those you have used most recently in the three spaces provided, by circling the numbers which best describe each store's performance."

The person filling out the form must enter a code for the type of store, then the store's name, phone number and address. On the right are different performance areas to be rated "inferior," "adequate," "superior" or "can't rate." The ratings are applied to such questions as "advice on choice and use of products and related questions," "promptness of service," "staff attitudes, atomsphere," "quality of products," "variety of product types (and brands)," "prices," "fixing right on first try," "completing work when promised," "letting you know early how much work would cost" and "overall performance."

"We depend on these people to be conscientious," says Drughoff. About 15 to 20 percent, he says, fill in the forms and return them.

After the staff selects the firms rated most often (there may be hundreds), they are on the phones again -- or sending out more questionnaires -- asking individual firms about warranties, estimates on the time needed to do a certain job, average price andcharges for estimates. They also call local consumer affairs offices and better business bureaus for complaints registered against the firms.

The staff compares information received from the companies with that supplied by readers to check whether firms actually deliver on their promises.

It may sound a little complicated -- because it is. Krughoff acknowledges that "one of the disadvantages we have is that the tables (accompanying the stories) have a lot of information in them and are rather burdensome to get through."

One table, laid out with company names running up and down and questions left to right, ran over three pages. ("I'll never do that again," Krughoff says.) The table on TV, stereo and appliance shops listed some 22 pieces of information on each firm, including the number of respondents for each company and the "overall performance" rating by readers, given as a percentage. (For television repair firms, the figures ranged from 33 to 100 percent satisfied.)

Short of doing complete follow-up surveys, it would be impossible to tell how companies have reacted to Checkbook studies and whether any of the more poorly rated among them have mended their ways.

"We want to have a major impact on the market," says Drughoff. "We want these firms to shape up."

Following up the car maintenance issue, Checkbook did send out another query to readers asking what, if anything, they had done about selecting an auto repair shop. Of 3,500 who responede, 41 percent said they had selected or planned to select a shop based on the article. An additional 23 percent, said Krughoff, confirmed their original choice.

"When we have surveyed firms," said Krughoff, "one shop said it had received 1,000 calls for appointments after a TV station followed up our article." And with mixed feelings he points to a misprinted phone number accompanying a hearting/air-conditioning shop readers rated 100 percent. As last report, callers were driving the holder of that errant number up a wall.

"One plumber," said Krughoff, referring to the same home-maintenance article, "had to cut back his service area."

Many people who volunteer their services have contributed to Checkbook's success, from student interns to lawyers to designers. Some readers give research time, artwork and photographs.

(Readers have stuck with the magazine -- and the $14 subscription price -- even though Checkbook hasn't come close to meeting its original claims as a quarterly publication. There have been only five issues in three years. The $14 is now good for four issues. Individual copies cost $4.95.)

Krughoff himself lends much to the idea that Checkbook is a publication for true believers. Before assuming the task of building the magazine up from scratch, he was the 28-year-old director, under Elliot Richardson, of HEW's $500-million investment in research and evaluation planning. But he did have a reputation as a Don Quixote, sort of. His co-workers, he says, "probably through this was just one more windmill I was going off to battle.

"I didn't know fully what I was going into," he says. "It's a bit of an adjustment going from having two secretaries waiting to take dictation from you to having to carry your own mailbags to the post office." But, "I felt that I wanted to settle down and do something to completion. Even at HEW I'd gone from one job to another and, like a lot of other people in the Office of the Secretary, felt like I was just passing through, having no permanent imprint on anything.

"There aren't too many things you can find that haven't already been done and that are sensible.... It seemed like it would be tremendous fun to create a shole new social invention."

Checkbook is currently putting out an issue on food services and another on more home-maintenance companies. For more information, write to the Washington Center for the Study of Services, 4th floor, 1518 K St. NW, Washington D.C., 20005.