ReBonjour cher Monsieur José, j'habite à Paris.
0. from the photos de focus and offset trace, most likely open or intermittent connector pin Vert Output to CRT neck pin. Open ckt on one side of CRT deflection plate.
ATTENTION! the CRT neck pins are prone to cracking the Kovar glass seals with any stress or bending!
AVEC GRAN PRECAUTIONS
a . apply contact cleaner to neck pins and V out connectors.
b. At CRT neck, gently rotate and slide the neck pin connector up and down the neck pin,
c. possibly the vertical deflection termination resistors assemblée connections.
See 7904 service manual.
1. second suspect harmonica connectors, micro coax, oxidation, corrosion intermittent contact.
Be very careful about the harmonica conn, easy to break the ancient plastic.
Apply contact cleaner sparingly and gently unseat and reseat the connectors.
2. mystery part in 2ond photo left is old mercury or electrolytic hour meter, perhaps application by previous owner. no relation to the problem.
Bon courage !
Amicalement
Jon
PS how to find French Tektronix vintage group, forum?