Scanned Secret Service documents dealing with the installation
and Maintenance of Nixon's recording system are available here.
They were declassified by the Nixon Library in June 2009.
[Extract from John Powers, "The History of Presidential Audio
Recordings and the Archival Issues Surrounding Their Use"
(1996). Used with permission.] From February 16, 1971, through
July 12, 1973, President Richard Nixon secretly recorded over
3,700 hours of his meetings and conversations. Microphones were
installed in the Oval Office, in Nixon's office in the Old
Executive Office Building (EOB), in the Cabinet Room, and in
Aspen Lodge at the presidential retreat at Camp David, Maryland.
In addition, microphones were placed in telephones in the Oval
Office, the EOB office, and the Lincoln Sitting Room in the
residence section of the mansion. There were two separate
systems for the telephones in Aspen Lodge at Camp David.  During
the transition period, President Johnson offered Nixon the use
of the White House Communications Ce