What the thread found

• A clip-on matte box can increase mechanical resistance on the lens focusing mechanism enough to trigger the MDR safety power limit, which shuts down the motor to prevent overheating.
• Motor freezing may occur during static focus periods rather than during active focusing moves.
• Calibration can succeed normally even when the motor later freezes during a shot.
• Matte box mounting pressure should be minimized to avoid binding internal lens elements, though this alone may not solve all freezing issues.
• Misalignment between the motor mounting rod and gear assembly can cause binding that resembles a stiff lens problem.
• Faulty motor cables or shock damage to the MDR unit could cause intermittent motor failures.
• Hand-barrel rotation difficulty is a reliable indicator that a Preston motor will struggle with that lens configuration.
• Recalibrating without the matte box and then reattaching it may reduce available focus throw.
• Cold weather can require manual assistance during calibration.
• Firmware adjustment to the safety power limit may provide additional operational margin.

Depends on

• The original poster's specific lens stiffness assessment is subjective and may not match technical measurements.
• No resolution was confirmed—the poster indicates Preston would adjust firmware settings but does not report final results.
• Multiple potential causes were discussed without systematic elimination testing shown.
• Cold weather calibration issues mentioned in early replies but not confirmed as related to the main freezing problem.
• No documentation of whether the issue persisted after firmware adjustment.

Gear in this thread

MDR

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