There are times when the Russians are eager to convince everyone -- and perhaps themselves -- that they are joining the modern world. One of those moments came in 1980, just before the Olympic games that were held in Moscow, when the Soviet Union introduced direct dial telephone links with the rest of the world.

Two years later, this cautious experiment in direct communication across the Iron Curtain was chocked off in an apparent attempt to keep the nation isolated from the capitalist world. (Links with socialist countries continue uninterrupted).

At the same time, the political police of the committee for state security, the KGB, have moved decisively against the few remaining political dissidents and Jewish activists, who have been warned in no uncertain terms that outspoken opposition would no longer be tolerated.

By a coincidence that was, as Western analysts here say, no coincidence, the authorities have renewed pressure on foreigners who maintain contact with Russians. The harrassments range from punctured tires to the detention in Zagorsk of two Russians traveling in a Canadian diplomat's car; the diplomat was allowed to return to Moscow while his Russian friends were detained in Zagorsk, a city about 60 miles from here.

None of this is illogical in a society whose government seeks to control all channels of contact with the West. Without direct dialing, the KGB's technicians will have much less difficulty monitoring inernational calls. Closer monitoring will allow the KGB to squeeze the chanels between Soviet citizens, westerners and Soviet emigr,es in the West through which dissident ideas have reached Europe and America, then to be beamed back to the Russian people via Western radio stations.

Perhaps, as periodically happens, the Russians have decided to stop pretending that they want to become full-fledged members of the modern world.

It is puzzling, though, that the authorities have taken a decidedly harsher stand at a time when it is increasiglgy difficult to speak of a dissident movement here. What was called in the early 1970s the democratic movement, a loose coalition of several hundred intellectuals, does not exist any more. Its members are exiled, dispersed, jailed or demoralized. Samizdat, the informal circulation of typed, dissident manuscripts that once flourished, is now difficult to find.

In the absence of any explanation for this abrupt tightening up, speculation focused on the new KGB chief, Vitaly Fedorchuk. A no-nonsense professional who reportedly served as a KGB operative in Vienna in the early 1950s before moving to senior positions in Moscow and than in the Ukraine, Fedorchuk is believed to want to make personal impact on the vast organization he has been heading since last May. (He got his new job when Yuri Andropov was promoted from the KGB to a senior position in the Communist Party apparatus.)

According to this interpretation, Fedorchuk's appointment coincided with President Reagan's ideological challenge, especially his London speech in which he called for a woldwide crusade against communism.

As head of the Ukrainian KGB, Fedorchuk had been concerned with the turmoil in neighboring Poland and had warned in an article he wrote last year against Western "ideological subversion." His article catalogued Western subversive activities aimed at the "spiritual decomposition" of the Soviet people. As a hard-line career officer, Fedorchuk is believed to reflect the KGB's concern over East-West relations and to encourage the organization to combat all nonconformist trends that the hard-liners blame for the events in Poland.

But this analysis is described by Western diplomats as only partially satisfying. No one official, and particularly not a political outsider like Fedorchuk, is able to take such an important decision.

Another possible explanation advanced here is that the authorities have decided to finally stamp out political dissent altogether. While in the past Moscow's harshness toward deviation at home may have been somewhat inhibited by Western public opinion and foreign policy considerations, the state of Soviet-American relations has reached such a low point that Western disapproval could have little leverage here, some speculate.

It is now 18 years since Leonid Brezhnev and his colleagues deposed Nikita Khrushchev. Brezhnev appears to have bounced back from his grave illness of last spring, but nevertheless, the ruling Communist Party seems to be preoccupied with the succession question. The two principal contenders are Andropov and Konstantin Chernenko, a Brezhnev proteg,e who ascended to the ruling politburo relatively recently.

According to Soviet sources, both men are engaged in inside maneuvering without endangering the delicate balance of power at the top. Both apparently see continued Brezhnev stewardship as advancing their personal positions within the party.

Western diplomats here say the appointment of Fedorchuk to head the security aparatus may have been motivated by a common desire in the Politburo not to give any politician control over the KGB during the transition period.

Nobody here professes to know just how long the transition period is going to last. But Western specialists believe signs of internal tightening suggest precautionary measures for internal stresses and strains that eventually are bound to come when Brezhnev departs the political scene.