Sorry for the idiot level reply, it just seemed too much of a coincidence that you unplug a timebase and the sweep disappears.
Do you have another scope, also a high voltage DC probe capable of measuring 2kV?
A quick test which should spot a problem in the Z axis / unblanking circuitry is to locate R2135, CRT bias, on the high voltage board. Note its position, turn it slowly in each direction looking for anything on screen, and return it to its original position. I had a 7623A with several storage board problems but they did not affect non-store mode, that is not to say that there might be some issue on the storage board that would affect non-store mode. I would try the various storage modes and also the reduced scan to see if anything interesting happens and also simply to exercise switches which may be a slightly contaminated.