A rotary cutter lifting apparatus comprising a table plate having an opening and an upper surface, a carriage disposed beneath the table plate and adapted for supporting the rotary cutter, and an adjustment mechanism positioned entirely below the table plate upper surface and operable through the opening. The core assembly defines a core hole extending through the core assembly from a core interior to a core outer surface. Resilient means bias the second clamp toward the first clamp. A recording power for which the change in signal quality due to the deteriorated recording conditions is sufficiently small is determined as an optimum recording power capable of providing adequate recording margin.