WARNING: thread drift
http://history.nasa.gov/SP-4219/Chapter11.html
In late November 1977, while the two Voyagers were still on route to Jupiter, one of Voyager 2's two duplicate radio transmitters began to degrade. It was switched to low-power mode to nurse it along. Something was wrong, but there was no way to know exactly what. Months later, in April 1978, the Voyager team discovered that Voyager 2's backup receiver had failed to detect signals sent from Earth because of a shorted capacitor. The primary radio receiver suddenly failed completely, as well. Voyager 2 was silent. Continuing to Uranus and Neptune was no longer possible, unless a way could be found to communicate with the backup receiver. Moreover, the failure of the Voyager 2 primary radio system had potential repercussions beyond the Voyager project. Its radio equipment was very similar to that on Pioneer Venus, which was launched the following month, in May 1978. 78
Normally, the radio receiver automatically compensated for the Doppler shift of signals transmitted from Earth. The changing velocity and direction of the spacecraft relative to Earth caused this Doppler shift. Without the ability to compensate for the Doppler shift, the Voyager 2 radio system could not detect any signals sent to it. The solution to Voyager 2's radio problems came from NASA Deep Space Network engineers. They prepared computer tapes that slowly varied the frequency of the radio signals transmitted from Earth in order to compensate for the expected Doppler shift. The Deep Space Network station outside Madrid transmitted the first test signals on April 13, 1978. Fifty-three minutes later, Voyager 2's acknowledgement returned. The trick worked. As a backup measure, in October 1978, Voyager 2's memory banks were loaded to the brim with commands that would provide