how can i fix my zenith sm3587bt blurry picture?

Focus circuit operates at extremely high voltages and uses safety specified components and requires an engineer.

Loss of focus voltage, possibly bad flyback transformer or dead spot on focus control, tube base socket connection or cathode ray tube.
High voltage arcing sometime quickly points out the fault.

Could be as simple as a dry joint or loose plug costing very little to repair, up to the most expensive repair which is c.r.t. replacement.
Only an engineer can advise you but make sure to ask for an estimate first. Don't attempt to repair it yourself.

Cobweb
January 2006