On June 29th, ITAR-TASS, the Russian state-operated news agency, reported that the separatists had indeed captured an air-defense base:

Self-defence forces of the Donetsk People’s Republic have taken control over a missile defence army unit equipped with Buk missile defence systems, the press service of the Donetsk People’s Republic told Itar-Tass on Sunday.
So far, no details are available about the number and condition of the missile systems taken over by the self-defence forces. The press service refused to comment.

As we've been reporting, however, the only news agencies which have this report are Russian state-operated, and the origin of the report may be TV Zvezda, the news agency for the Russian Ministry of Defense.

The claim that the separatists captured the base, the claim that the base had a Buk, and the claim that the separatists captured the Buk are all unconfirmed.