Hello Alfio,
Welcome to the forum.
I would remove the search unit from the mechanism & then plug back in the long edge connector. make A side selections and check that the correct pins are being pushed out on the rear of the search unit, also test the B side as well.
The normal check is to go up a letter and number each time, A1, C2, E3, G4 etc & B1, D2, F3, H4 etc.
If these do not select correctly then you have issues on the search unit (check for a cracked PCB, continunity on the A & B pin pusher coils, clean the S1 & S2 open relay contacts and R1 & R5 relay contacts on the keyboard.
If the pins do correctly push out then the fault is on the Stop Plate Assembly behind the Search Unit.
Regards
Alan
Alan Hood
ami-man
UK