Estou jogando meu /etc/apt/śources.list . Ele contém apenas repositórios que vêm com o Ubuntu. Alguns deles estão desativados na configuração padrão, mas não sei quais. De qualquer forma, essa lista não deve lhe dar problemas. Você também deve esvaziar a pasta /etc/apt/sources.list.d onde todos os ppas que você adicionou são salvos.
Então, vem a lista:
# deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Beta amd64 (20120328)]/ dists/precise/main/binary-i386/
# deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Beta amd64 (20120328)]/ dists/precise/restricted/binary-i386/
# deb cdrom:[Ubuntu 12.04 LTS _Precise Pangolin_ - Beta amd64 (20120328)]/ precise main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu precise main restricted
# deb-src http://de.archive.ubuntu.com/ubuntu/ precise main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu precise-updates main restricted
# deb-src http://de.archive.ubuntu.com/ubuntu/ precise-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu precise universe
# deb-src http://de.archive.ubuntu.com/ubuntu/ precise universe
deb http://archive.ubuntu.com/ubuntu precise-updates universe
# deb-src http://de.archive.ubuntu.com/ubuntu/ precise-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://archive.ubuntu.com/ubuntu precise multiverse
# deb-src http://de.archive.ubuntu.com/ubuntu/ precise multiverse
deb http://archive.ubuntu.com/ubuntu precise-updates multiverse
# deb-src http://de.archive.ubuntu.com/ubuntu/ precise-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu precise-backports main restricted universe multiverse
# deb-src http://de.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu precise-security main restricted
# deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
deb http://archive.ubuntu.com/ubuntu precise-security universe
# deb-src http://security.ubuntu.com/ubuntu precise-security universe
deb http://archive.ubuntu.com/ubuntu precise-security multiverse
# deb-src http://security.ubuntu.com/ubuntu precise-security multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu precise partner
# deb-src http://archive.canonical.com/ubuntu precise partner
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu precise main
# deb-src http://extras.ubuntu.com/ubuntu precise main