gpg key problem

I am trying to install trinity for the first time on my stretch system[*], following the instructions at https://wiki.trinitydesktop.org/DebianInstall. I added the Preliminary Stable Builds sources to sources.list. The GPG key was added OK: ---- sudo apt-key adv --keyserver keyserver.quickbuild.io --recv-keys F5CFC95C Executing: /tmp/apt-key-gpghome.STqKpDhXXj/gpg.1.sh --keyserver keyserver.quickbuild.io --recv-keys F5CFC95C gpg: key 96C95152F5CFC95C: public key "QuickBuild Trinity Desktop Environment" imported gpg: Total number processed: 1 gpg: imported: 1 ---- But then: [HN:apt] sudo apt-get update Ign:1 http://ftp.us.debian.org/debian stretch InRelease Hit:2 http://ftp.us.debian.org/debian stretch-updates InRelease Hit:3 http://ftp.us.debian.org/debian stretch Release Get:4 http://security.debian.org stretch/updates InRelease [62.9 kB] Hit:6 http://download.virtualbox.org/virtualbox/debian stretch InRelease Get:7 ftp://ftp.deb-multimedia.org stretch InRelease [33.4 kB] Get:8 http://mirror.xcer.cz/trinity-sb stretch InRelease [46.3 kB] Ign:8 http://mirror.xcer.cz/trinity-sb stretch InRelease Get:9 http://mirror.xcer.cz/trinity-sb stretch/deps-r14 Sources [8,082 B] Get:10 http://mirror.xcer.cz/trinity-sb stretch/main-r14 Sources [58.3 kB] Get:11 http://mirror.xcer.cz/trinity-sb stretch/deps-r14 i386 Packages [17.0 kB] Get:12 http://mirror.xcer.cz/trinity-sb stretch/deps-r14 amd64 Packages [17.0 kB] Get:13 http://mirror.xcer.cz/trinity-sb stretch/deps-r14 amd64 Contents (deb) [9,590 B] Get:14 http://mirror.xcer.cz/trinity-sb stretch/deps-r14 i386 Contents (deb) [9,580 B] Get:15 http://mirror.xcer.cz/trinity-sb stretch/main-r14 i386 Packages [201 kB] Get:16 http://mirror.xcer.cz/trinity-sb stretch/main-r14 amd64 Packages [201 kB] Get:17 http://mirror.xcer.cz/trinity-sb stretch/main-r14 i386 Contents (deb) [655 kB] Get:18 http://mirror.xcer.cz/trinity-sb stretch/main-r14 amd64 Contents (deb) [654 kB] Fetched 1,972 kB in 8s (238 kB/s) Reading package lists... Done W: GPG error: http://mirror.xcer.cz/trinity-sb stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 608F5293A04BE668 W: The repository 'http://mirror.xcer.cz/trinity-sb stretch InRelease' is not signed. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use. N: See apt-secure(8) manpage for repository creation and user configuration details. ---- I don't want to install anything that can't be verified. What do I need to do proceed? Doc [*] Anyone who has tried to use the KDE 5 that is included with stretch will know why I've finally given up on the KDE death-spiral and want to get back to something that works properly. I would have been here long ago if TDE were an official part of debian, but KDE 5 is so painful and bug-ridden that I am now forced to overcome my deep-rooted distrust of software that is not in the official debian repositories. -- Web: http://enginehousebooks.com/drevans