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So what goes wrong with AF-S sonic motors?
#1
So when your expensive Nikkor AF-S lens suddenly stops focusing......

.....what's gone wrong, the motor itself, the circuit or just a mechanical breakage?

 I've been reading up on that very subject........I have repaired a sticky focus system on the AF-S 70-200mm VR F4.5-5.6 VRII ..........here it was just mechanical glitch that dis-assembling and remounting the motor and cleaning all around, rectified the problem.
  Mechanical binding is a common source of issues.....on the AF-S 80-200mm F2.8D lens, it is common for the distance sensor ribbon connector to become unattached from the lens barrel and foul the distance scale......this prevents smooth focusing often stopping at mid distance, needing coups of AF to reach the end stop.
  Squeaking motors:

   Everyone's heard the famous Nikkor squeal, while the lens may focus fine this squeak often increases to the point where the focus becomes haphazard and can slow...eventually stopping altogether.
 So, what causes it?
   Sonic motors are driven by a high frequency AC signal.....in fact there are two frequencies that are out of phase by 90°, depending on which phase is ahead of the other determines the direction of the motor's travel. 
 Ring motors are essentially a crystal rotor working against a metal toothed stator,powered by this bi-waveform AC signal which is generated by the motors electronic circuit. 
   The best way to imagine the functioning of the motors stator is by equating it to a snake travelling across sand....the snakes body touches at say three points, these points are essentially a type of non rotating screw which flow down it's body..... these points of contact flowing down it's body propelling it along.
  This is the same with a sonic drive motor..crystals are deformed by voltage.. this signal (waveform) deforms the crystal disc slightly and creates a waves on it's surface......these waves propel the stator pushing against felt discs attached to the lens body, in the same way as the ubiquitous snake moves across the desert. 

   Yeah , yeah....But why the squeak?
   Essentially these motors are very reliable (in themselves) having only one moving part...  and the deformation of these crystal stators under power is very tiny indeed, thus their rotation per wavelength is also very tiny, but the frequency is very high and enough tiny movements make the stator rotate quite rapidly.

  But why the squeak?

    Often, corrosion/ dust/ dirt etc. (often corrosion) over the years can form a thin film over the stators surface.... the stator's surface wave/ deformation is of very small magnitude and the motor relies on precise tolerances.......once this coating is of sufficient thickness it interferes with the ability of the motor to drive the appropriate mechanics and it sets about squeaking, often becoming quieter with further use........some lenses start squeaking early in their life and live to ripe old age before succumbing to problems.......others breaking down after a shorter period of time depending on climatic variations.
   What can you do about it?
  Well it's perfectly repairable......and what's more cheap! 
 The lens has to be disassembled down to the motor of course and the stator can then be cleaned until any sign of corrosion etc. has gone leaving a nice clean stator, metal polish is an ideal cleaning product in this case. Once cleaned they work fine again.

  So what's all this pre-amble all about anyway? .....

 Well of course I've come across the Nikkor AF-S 80-200mm F2.8, the last of that series of lenses.......with the fast sonic motor, a tripod lens mount and the later optical layout of 18 elements in 14 groups......... at a bargain price......

    .......and you've guessed it........the AF doesn't work......so I'm going to try and fix it!
         Wish me luck!
Dave's clichés
  


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So what goes wrong with AF-S sonic motors? - by davidmanze - 04-09-2018, 01:31 PM

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