N_Cook wrote:

Telequipment was taken over by Tektronix about that time,
maybe this repair tip is useful

Well, yes, it at least gives me an idea on the SPS5286 substitution.
I'm still very curious to understand if the fault I'm observing can be
caused by an electronic problem or if it's definitely a bad CRT, that
changes very much the time I'm going to spend fixing this scope.
Thanks and best regards.

Frank IZ8DWF