Bonjour,
Have used and fixed Tektronix scopes back to 1968, the first solid state portable 454, 453. Amazing these 50 years old scopes can still work at all. Most need a complete examination for tant and lytic caps,dirty switches and controls, and basic PS, CRT and HV check and cal.
Even if you fix the imm¨¦diat issues, expect the HV transformer, HV mult to fail with time.
So, reading this thread, it's a very an ineffective process, I suspect OP needs the full service manual, follow procedure and diagrams for a logical troubleshooting plan.
Bon chance
Jon
Jean-Paul