As the ALSEP power packs degraded, they eventually dropped below the point
that they
could run the radio and any other instrument. This was about 1977-8.

The radio beacons were left on for any possible Earth-based tracking
experiments, until they too faded -- last
date of reception from any of them, I have no clue, but 1980's-ish sounds
reasonable.

The critical factor was that the ALSEP control room in the Bldg 30 MCC, on
the second floor, was shut down in early 1978
and converted to the 'Skylab Rescue' control room, which soon re-established
contact with the abandoned station and kept
contact through July 1979 when it attacked Australia.