Nikkor AF 24-120/3.5-5.6 D: Slow autofocus at focal length 24 mm (Nikon F4S)

forest bagger said:

When you can see the brush on its way there is no dismantling needed.
Just turn the zoom slowly from 24 to 120 mm and watch carefully the path of the brush.
In most cases the brush looses it connections at one end of the path - in your case on the short end.

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I can see both spots, but only from above at a steep angle. It's too narrow for a mirror.

What I can see looks ok. No deviations.

Andreas Thaler said:

I hate doing this.
...
Michael @forest bagger is a specialist in those.

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Then it would be the easiest way to ship the lens to me.
Usually the repair isn't difficult or expensive.

forest bagger said:

When you can see the brush on its way there is no dismantling needed.
Just turn the zoom slowly from 24 to 120 mm and watch carefully the path of the brush.
In most cases the brush looses it connections at one end of the path - in your case on the short end.

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Michael, since you have experience repairing AF zooms:

Looking at the exploded view above, is this something you would tackle or reason to turn it down as a job?

forest bagger said:

Then it would be the easiest way to ship the lens to me.
Usually the repair isn't difficult or expensive.

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Of course I want to do it myself, maybe you can give me instructions and send me the invoice?

This is a good opportunity to briefly introduce you to Michael @forest bagger and his work.

Michael runs his own repair shop in Germany/Bavaria and has specialized in repairs that others refuse because they are supposedly uneconomical or not possible. Michael thinks of solutions for every case and is successful in doing so.

In the German Nikon photography forum, one of the largest German-speaking photography communities, Michael manages the repair forum, advises the forum participants and presents his current projects (nick „Beuteltier“):


Whenever there is a tricky problem with current or older photo equipment, Michael is the right person to contact and will tell you clearly if a repair is not worthwhile.

Michael and I have known each other for many years, I have sent him several devices for repair and owe him a lot of expert knowledge.


(Site in German, inquiries in English are welcome.)

You have to follow each single pin (I don't know the correct word for these) of the 4 pins of the brush on its way.
In most cases the flexible circuit board has moved on its mounting plate and so at least one of these pins looses contact.

forest bagger said:

You have to follow each single pin (I don't know the correct word for these) of the 4 pins of the brush on its way.
In most cases the flexible circuit board has moved on its mounting plate and so at least one of these pins looses contact.

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Thanks I will check again.

If I am not successful, I will send you the lens for repair.

Andreas Thaler said:

Thanks I will check again.

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And observe the end markings on the flexible circuit board, maybe the brush goes to far over and beyond?

forest bagger said:

And observe the end markings on the flexible circuit board, maybe the brush goes to far over and beyond?

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Viewed from above, everything looks OK.

But I can't see whether the wipers and resistance tracks run parallel. To do that, I would have to be able to expose the part on which the tracks are applied.

Tricky

I opened the lens one last time to check the position of the wipers and the resistance tracks in relation to each other. I couldn't find any deviation.

However, to take a closer look I would have to dismantle the zoom further and I don't think that's a good idea. There is always the risk that the adjustment will no longer be correct afterwards.

In order to be able to use the zoom in MF mode with some comfort - the focus ring is narrow and you can feel the gears of the AF mechanism when turning it - I lubricated the accessible gears and gear rings.



I used my toughest grease, Liqui Moly.

The focus ring now turns a little more smoothly and jerks less.

Repair successful

The solution regarding the stalling autofocus at the short end of the zoom was surprisingly simple after I looked through the repair manual again.

The wiper, which is moved with the zoom ring over the contact field in the tube, sits on a holder (see part #70 in the picture):




This holder is inserted into the zoom ring and can be moved. It is secured with an adhesive strip.

The holder is accessible when you remove the rubber coating from the zoom ring, which is easy to do here because it is not glued.

Here I have already removed the bayonet ring, which would not have been necessary:




Through two openings (green circles in the picture) to the left and right of the holder (yellow arrow), the position of the wiper on the contact track can be observed:




This requires good lighting and a magnifying aid. I used my strongest magnifying glass.

I saw that when the zoom ring is set to the 24 mm stop, the two ends of the wiper are on a non-conductive surface. However, at all other zoom positions the wiper makes contact.

This matched my observation that the autofocus worked again without stalling when I turned the zoom ring a little towards the longer focal lengths.

I also noticed that the 24 mm mark on the zoom ring is a bit below the white alignment line on the tube (red arrow):




Apparently the stop of the zoom ring at 24 mm was no longer correct. And this slight deviation meant that the slider coupled to the zoom ring no longer had contact.

There were three possible ways to correct the deviation:

1. Adjust the glued contact track.
2. Correct the stop of the zoom ring.
3. Adjust the position of the slider.

ad 1: This would only be possible by removing the glue. This is not practical in the tight space, as neither solvent nor heat can be introduced in a controlled manner. In addition, the plastic construction rules out such measures.

ad 2: I could not identify the stop of the zoom ring, so it was not feasible.


ad 3: The solution, obviously also provided by Nikon

I removed the adhesive tape over the holder, levered it up a little with a screwdriver because it was stuck, and then visually adjusted the slider for the 24-millimeter position.

The wiper ends now sit just on the conductive gold contact track (and no longer on the non-conductive black surface underneath):





I fixed this new position of the holder again with adhesive tape:




I checked the position of the wiper at the maximum zoom position of 120 mm through the second opening.

Although the wiper is no longer centered on the contact track, the test showed that it makes perfect contact.

As I said, I could have saved myself the trouble of dismantling the bayonet ring, as it was not necessary for adjustment.

When tightening the bayonet ring, I overtightened one of the two short screws. This happened quickly because this screw engages a plastic thread:




Nevertheless, the bayonet ring is firmly in place, secured by the three other screws.

A test on my Nikon F-501 and Nikon F4 showed that the zoom focuses now without any problems:

---

Conclusion

• What at first and second glance as a repair appears impossible or only possible with great effort and risk can, at third glance, be a relatively simple matter.
• It remains to be seen whether Nikon had anticipated problems like this due to its service-friendly design. In any case, very little effort is required to fix it.
• I cannot understand why this bayonet screw turns in plastic. But perhaps I will find something about it in the repair manual.
• As I adjusted the wiper on suspicion, I cannot guarantee that the values for the zoom position transmitted to the camera are correct. A test film will show whether it is fine.

+++

All information provided without guarantee and use at your own risk.

The question still remains as to why the zoom ring is misaligned

One explanation would be that there is no ring with which the lens can be gripped when screwing it into the camera bayonet.

The wide and rubberized zoom ring is ideal for this.

Since screwing in always occurs against resistance, the zoom ring turns anti-clockwise to the smallest focal length, which is 24 mm.

Due to the force acting on the stop of the zoom ring, it is conceivable that the stop will shift slightly to the left over time. And that is enough to interrupt the slider on the contact track.

When unscrewing, force acts on the stop for the maximum focal length of 120 mm.

Since there is still enough contact track for the slider beyond the stop, a misalignment has no effect.

Wow, fantastic detective work on that lens!

ic-racer said:

Wow, fantastic detective work on that lens!

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Sometimes I wish I had a very fine endoscope so I could examine devices.

Andreas Thaler said:

The question still remains as to why the zoom ring is misaligned

One explanation would be that there is no ring with which the lens can be gripped when screwing it into the camera bayonet.

The wide and rubberized zoom ring is ideal for this.

Since screwing in always occurs against resistance, the zoom ring turns anti-clockwise to the smallest focal length, which is 24 mm.

Due to the force acting on the stop of the zoom ring, it is conceivable that the stop will shift slightly to the left over time. And that is enough to interrupt the slider on the contact track.

When unscrewing, force acts on the stop for the maximum focal length of 120 mm.

Since there is still enough contact track for the slider beyond the stop, a misalignment has no effect.

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Here you can see the deviation of the 24-millimeter mark from the alignment line to the right more clearly:



But you have to look closely.

I assume that the line has to be in the middle of the 24.

This difference from the target corresponds exactly to the small distance between the wiper ends and the start of the contact track.

The wiper is literally at zero.

The AF system obviously depends on detecting the zoom position.

If it doesn't receive any data, the AF jerks towards the focus point.

That must have been the case with this zoom here.

Signal value 0 is obviously not intended.

When set to maximum focal length, the line is in the middle of the 120:



If this is a digital tap, the exact position of the wiper per contact field shouldn't be crucial. The main thing is that it is in the intended range.

It would be different in analogue mode, as the voltage value increases continuously with the resistance value of the resistance track.

This means that I don't have to worry about the wiper here, whether it is just touching the contact surface or in the middle.

However, this is only a guess.

In the German Netzwerk Fotografie (NF) I asked Michael @forest bagger to explain the AF system here:

Michael, can you briefly explain the AF system, i.e. which signals the lens exchanges with the camera?

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Beitrag im Thema 'Nikkor AF 24-120/3.5-5.6 D: langsamer Autofokus bei Brennweite 24 mm (Nikon F4S)'
https://www.nikon-fotografie.de/com...rennweite-24-mm-nikon-f4s.308788/post-2940418

His contribution in German with a hand sketch is worth reading, please use the translation function of your browser or Google Translate.

forest bagger said:

View attachment 368584

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@forest bagger What does this tell, Michael?

You mentioned that is the 4 bit code all Nikkor AF lenses follow?

This 4 bit code is sufficient for the most zooms.
For some longer zooms there is a fifth track.

Update on long-term observation:

Autofocus at the short end of the zoom (repaired in April, 2024) still works perfectly.