So what goes wrong with AF-S sonic motors? - Printable Version

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So what goes wrong with AF-S sonic motors? - davidmanze - 04-09-2018

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!

RE: So what goes wrong with AF-S sonic motors? - davidmanze - 04-10-2018

[Image: 107bc7083ff04c55a87a54c29abf125d]

Here's an image of the rotor/stator being cleaned.....apparently this motor is used in a few Nikkor lenses....

..the AF-S Nikon 28-70 2.8, AF-S Nikon 80-200 2.8.... and the Nikon AF-S 17-35 2.8........which is handy to know.

In general these motors do not go wrong......other than "micro surface corrosion" .....given that the lens hasn't had a fall or a bang...........the owner assured me that the lens hasn't suffered any fall and that motor was slowly dying, then packed up altogether!

RE: So what goes wrong with AF-S sonic motors? - Brightcolours - 04-10-2018

Brave man. Good luck with getting it to AF once again!

RE: So what goes wrong with AF-S sonic motors? - davidmanze - 04-10-2018

(04-10-2018, 10:29 AM)Brightcolours Wrote: Brave man. Good luck with getting it to AF once again!

   Thanks BC!

RE: So what goes wrong with AF-S sonic motors? - davidmanze - 04-18-2018

The Nikkor AF-S 80-200mm ED F2.8D arrives.........looks well used but clean and no scratches or nasties....zoom ring is a little loose and turns easier tha any other lens I've used, optically it looks well centered....

   ....effectively the AF is dead as a dodo!  ....... No broken wires to the switches...

      Optically it looks great, better even than the AF 80-200mm F2.8  push pull....200mm wide open is sharp and easily well good enough for portraits at that FL.

      I checked for AF accuracy......by using the manual focus indicator in the viewfinder.....looks accurate at a first glance.

       I've been soaking up information like a sponge........manuals, utubes you name it,  one in particular popped up......for my AF-S version.....the problem, AF motor dead..."bingo"  ..pity it's in Japanese.

    Still, it shows step by step disassembly, the switch and circuit have to be un-soldered and the ribbons unplugged as you have to get halfway down the lens from the bayonet to get to the motor.  Finally the guys turns the motor by hand we hear the motor squeaking...problem found!
   I feel so lucky to have found that utube video..    Of course I will be filming the process for my benefit.....to re-assemble it..
The video.....so you can see what's in front of me!


     As it's likely to be, firstly, hyper boring and secondly, full of cursing a swearing etc.      I'll spare you guys the "agony" of linking it here. Tongue

RE: So what goes wrong with AF-S sonic motors? - davidmanze - 04-19-2018

Well I'm in down to the motor.....it's a huge thing beautifully made.....

 .....and it's a squeaker!


RE: So what goes wrong with AF-S sonic motors? - Brightcolours - 04-19-2018

Thanks BC?

RE: So what goes wrong with AF-S sonic motors? - davidmanze - 04-19-2018

(04-19-2018, 06:51 PM)Brightcolours Wrote: Thanks BC?

To be continued........

RE: So what goes wrong with AF-S sonic motors? - davidmanze - 04-22-2018

Hi guys,
             I cleaned the motor as advised....it was a pain to access in between the toother stator to remove the dried polish....

  ...the net result is the motor chirrups like a dawn chorus in springtime.. Tongue when turned by hand..not good!

   I could not find how to separate the rotor from the stator as you see next to nothing in respect to the motor surfaces...

In the meantime I found a S/H motor on ebay from a spares company in China 129 euros delivered...cheap!......I ordered it quickly before it disappeared!
 Received info from the forum;  the motor needs a few drops of acetone to soften the thread lock in order to unscrew it.....so I will try that later to see if it's a worn out motor or whether there's still some polish powder gumming up the works and re-clean the surfaces.

   So my masochistic battle for optical satisfaction continues Sad ...

  To be continued.....

RE: So what goes wrong with AF-S sonic motors? - davidmanze - 04-25-2018

Hi guys,
No luck separating the focus motor....acetone didn't work...I made in the lathe a rubber bung which fits perfectly in the motor tube and used a Nikon rubber covered lens mount cap on the other end...I got a huge grip on the threaded parts and really put some muscle into it after adding acetone....nothing!
So, I'll wait for the replacement motor to arrive!

In the meantime the same seller called me with the other lens he has on sale the; AF-S 28-70mm F2.8D again with an AF problem, this time the motor works but sticks at 2.5 mts......apparently it's the ribbon that comes adrift from the front inner barrel and fouls the distance scale blocking the AF from reaching infinity...another cheapie!

Luckily it uses the same motor as the 80-200mm, so when the replacement motor arrives I'll have two lenses and three motors.....

.......hopefully I might be able to take a photo with one of them......that would be nice!