Killing a Nikkor AF-D lens cpu: whats behind a Micro-Nikkor 105mm f/2.8 AF-D internals?

This is a short Dunning-Kruger tale about an attempted repair of a Nikkor AF-D lens that was misreporting its max aperture and somehow killing the CPU - the dead CPU means that the DSLR body no longer recognises a CPU enabled lens which means no autofocusing (even on motor-enabled bodies), no metadata and no body controlled aperture - only dumb manual focus operations available.



How did it happen? I don't know and it shouldn't have happened as I've worked/repaired with fragile electronics for over 3 decades but yet here we are. A story of interest and what makes up the mechnical-CPU enabled internals of a Nikkor AF-D lens.

It goes without saying, dragons and demons ahead.

Kenneth Olsen has the following video where he also disassembles this lens as a point of video reference:



If you need to service the optics of this Micro 105mm f/2.8 AF-D lens, you do not need to be here - go here instead.

Stripping Down

To start, strip down the front of the lens and its various retaining rings until the focus ring slides off. Following this, remove the rear baffle and rear mount to get to this stage:



Looking at the rear you can see the 3x black screws that secure the distance scale housing to the internal structure. Remove these, but they are tight.



The screws holding the focus-screw gear mechanism need to come out but don't attempt to remove gear just yet - its easier to remove the screws and half cresent brass ring that holds the rear contact block ribbon in place.



There are 3x grub screws on the dinstance scale housing that need to be loosened and then the ring can be spun off - this can sit tight.



Next set the lens to MFD so theres a bit more space in the internal barrel and you can pick out the focus-screw gear - you need to dislodge it from the fork that holds it before picking it out.


^{ignore the fork that will come out shortly - notice the gap between the brass which is where the fork needs to sit when reassembling}

It's a little painful pulling off the distance scale housing - I set the A-M to A to be able to lift this off. When this housing comes off it exposes the resistor track and the dangers of killing your lens CPU.

The Resistor Track

As the lens focus distance changes, this track helps communicate information about the focus position via the brushes - at infinity the brushes align with the edge of the tracks and just before the orange ribbon.







As you spin the lens around you see more of the internal circuit board and CPU.



I don't know if exposing this clumsy handling is what killed the CPU in my lens - this was confirmed when moutning the lens to a DSLR but also confirmed with lack of continuity from pin 5 (left) to pin 1 which differed from the other pins 2-4.

Resassembling

You must secure the focus-screw drive gear first and then attach the fork - this fork is what engages/disengages the focus-screw mechansim by virtue of being pulled down the the distance scale housing's A-M mechanical switch. The fork needs to sit in the rounded cross shape of the free-rotating collar.





With the fork in position rotate the lens so the fork is facing up (so gravity doesn't disloge it) and with the bump on the distance scale housing (the bump is to accomdoate the focus-screw gear) slide the housing over the fork and it will seat itself in the notch within the housing. This can be fiddly.



Once in place pull the housing down to seat it onto the pins and then align the housing with the screw holes of the internal structure and resecure. After this is in place, reassembly of the other parts of the lens/rear mount is simple.




Aside from professionaly trained and experiences technicians I don't know of any reason why anyone would need to strip down this lens to this state. Leave it alone and save yourself a murder charge on your AF-D lens.