Aqui está um padrão, mas se seus pacotes estão de alguma forma bagunçados, mudar seu sources.list
pode não ser suficiente.
deb http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
# deb http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ precise-proposed main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu trusty-security main restricted universe multiverse
# deb-src http://security.ubuntu.com/ubuntu precise-security main restricted universe multiverse
deb http://archive.canonical.com/ubuntu trusty partner
# deb-src http://archive.canonical.com/ubuntu precise partner
deb http://extras.ubuntu.com/ubuntu trusty main
# deb-src http://extras.ubuntu.com/ubuntu precise main