When the Soviets first turned on the electricity to a mammoth new American-designed coal strip-mine excavator here two years ago, the lights went out all over town-a grimy boom settlement in eastern Siberia.

Last year, the 6,000-volt excavator itself broke down and has set silent for months while American and Japanese technicians from the companies that designed and built it sought to overcome the difficulties. The Russians had ordered 10 such machines for about $4 million apiece.

Elsewhere, 29 huge American made trucks for hauling coal and rock from the pits stand idle, with nothing heavier dump-bodies. The Russians say defective wheels make it impossible to use the vehicles at full capacity and laterations are necessary. They had ordered 85, at $1.3 million a piece.

Lacking adequate equipment of their own to fill in while these troubles are ironed out, the Societs have been stymied in starting the massive work of removing millions of tons of frozen rock to get down to the rich coal seam locked beneath in the permafrost. The interruption could eventually prove costly, for mining the seam is crucial to a pioneering multimillion-dollar international venture aimed at speeding exploitation of Siberia's long-neglected mineral riches and providing coking coal for Japan's ravenous steel furnaces.

Despite those woes, there is little doubt that the Soviets seventually will overcome them and succeed in extracting the coal, concentrating it in massive mills, and shipping it south and east over the new Baikai-Amur Magistral (BAM) railroad being cut through wilderness taiga to the Pacific coast for sea shipment to Japan. They have little choice: under terms of an agreement with Japanese, the coal must repay loans of $450 million which financed machinery and technology used to develop the Neryungri deposits, among the richest concentrated coal reserves on earth, but isolated within a harsh continental land mass of summer time swamps thick with mosquitoes and deep winter frosts.

Soviet geologist estimate ther are 30 to 40 billion tons of high-quality coal available for strip mining in South Yakutia. About 300 million tons of coking coal and 100 million tons of lower grade coal for heating and electric power plants are in the Neryungri deposit alone. The Russians have contracted to begin shipping 3.5 million tons of coking coal from here to the Japanese beginning in 1983, with deliveries to end in 1998.

Soviet performance on the contract is being closely followed in the West as a possible bellwether of the nation's ability to marshall its notorioulsy clumsy planned economy for efficient exploitation of the vast natural gas, oil and mineral deposits elsewhere in the one million square mile exanse of Yakutia, where roads, rails, and people are in short supply.

Moscow's own keen interest in the Neryungir effort and other major projects in Siberia arises from its desire to infuse new production sources into an economy whose growth rates this decade have stagnated. Perennially shoddy Soviet manufactured goods have found limited markets in the West, and short of economic reform that could require loosened Communist Party control, the Soviets must rely on sales of natural resources to earn the hard currency needed for future development.

While the Russians have achieved much since work began in earnest here five years ago, the troubles with the 180-ton trucks and excavators are only the latest in a string of cost overruns, bad planning and bureaucratic bungling that has plagued Neryungri almost from inception. The saga of the mighty "240-M Superfront" excavator designed by the Marion Division of Dessar Industries of Dallas, Texas, and built under license by Sumitomo of Japan, seems consistent with the short history of this settlement, in the Stanovoy Mountains about 200 miles north of the Soviet-Chinese border.

Signs of high-level Moscow concern abound. In a January newspaper interview, V. Belye, Soviet first deputy coal minister, sharply criticized the project. An original estimate of $1.4 billion to assemble the towns, roads, machinery, machine shops, rail spurs, depots supply dumps, and open the earth for mining has ballooned to $4.5 billion, he asserted.

The principal planning institute, called Siberian Hydro-Project for Mines, he said, had given the inadequate excuse that South Yakutia is an uudeveloped region with severe weather conditions. Harsh conditions here with attendant higher construction costs, are well-known, Belye said. The 13,000 people at the site are too few to both work the mines and build housing needed to expand the work-force. The planners erroneously calculated that 80 percent of the work-force would be single, when in fact 80 percent are married, causing acute housing problems.

Large rubble heaps from the mines were piled in front of planned rail extensions, and plans offor a single central heatin boiler to cost five million rubles and employ 70 were ignored, and now 22 temporary bioilers costing a total of 66 million rubles and needing 800 workers have been installed, Belyed declared.

Asked about these criticism, Neryungri First Deputy Mayor Eduard A.Krasnov conceded reluctantly there had been massive overruns.

"But these will decline and we will try to keep the cost down," he said, after noting that "the estimate of so might be change. The increase is caused by the fact that this was one of the first such construction sites with this difficulty."

He said the latest estimate of $4.5 billion will cover all constrcction costs for an eventual work force of 50,000 by 1983. Of the charge of misplaced rubble heaps, Krasnov replied: "I don't know such facts."

Housing conditiond in the Neryungri complex vary from reasonably attractive log dormitories and apartment blocs to ramshackle shanties jumbled amid the clutter of unfinished concrete slab apartments. Indoor plumbing is a rarity throughout Siberia and Neryungri is no exception.

Although pay rates here average about 500 rubles a month ( $750), double the European Russian average, and there are many Komsomols (young Communists) who come for what they asserted in private interviews is the "romance" of being part of a brave new project, manpower turnover throughout eastern Siberia remains a perennial problem. Daunted by the isolation, privation, and dullness of life, many young families take their savings and depart for more hospitable parts when their three year contracts are up.

With all these problems, it seems hardly fair that the Soviets should have trouble with the American excavating machine, designed to scoop 20 cubic meters of rock or coal at a time, four times as much as present Russian machines. But they've had trouble by the scoopful.

Five machines have been delivered, laboriously brought inland in parts. Three have been assembled, used briefly, then put out of commission by various problems. The chief woe involves the scoop bucket, which broke down while biting at the rock covering the coal seam.

The Soviets had planned to remove 75 million cubin meters of rock this year, toward a total of 240 million that must be claewed away by 1983, to get at the coal. But Soviet mine officials said the delays mean only 20 million cubic meters can now be removed this year, and are worried that the trouble may result in shortfalls in the coal production schedule.

Yuri A. Zakarov, chief of construction at the Neryungri pit, in a long complaint voiced while he stood near the stricken machine in the-30 degrees Farenheit cold of the quiet pit, said the scoop had been misdesigned by Marion, the division of Dresser's. Dresser has been hired by the Soviets to build a $144 million oil drill bit plant to aid oil exploitation in western Siberia. The sale was approved by the White House last year after initially delaying it to study the strategic implications of the sale.

Zakarov said the machine had been badly designed and equipped to withstand intense cold of the site, where it can be below-50 degrees Fahrenheit for days at a time.

But John H. Bailey, a hydraulies specialist working on the machines at Neryungri, said the hydraulics had given no trouble for more than a year of test and use. New teeth for the buckets are scheduled to be installed soon and the machines can get back to work, he said. But he conceded, "the cold has a hell of a lot to do with it." CAPTION: Map, no caption, By Richard Furno-The Washington Post