It was solar powered, as you may have read from the Wikipedia article.
The traveling wave tube remains the best available wideband microwave amplifier,
although the bands used by Telstar (4-6 GHz) could easily be covered with modern
SS devices. Some recent Moon survey missions used fairly high speed data links
with TWT microwave transmitters:
http://www.grc.nasa.gov/WWW/RT/2007/...RCE-force.html