For the chronicle, I investigated why the TD on my 3T77 unit, purchased used on ebay two years ago as "not working" was dead.
The 3T77 does not trigger at all, and that because simply for the fact that one TD was dead.
Incidentally, the TD was the one that its signal was sent ALSO to the TRIGGER OUT BNC on the front panel via a simple pulse transformer.
Very useful but... imagine if you, by mistake, send a "full-bodied, fat" trigger signal (5,10 V or so) TO the scope putting it in the WRONG connector (TRIGGER OUT instead on TRIGGER IN)? The TD surely reacts in a very negative manner!
That was the event made by the previous owner, that destroyed the 1N3129 TD, IMO.